Clinical Bacteriology Enhancement Exam Review Flashcards

COMPARISON BETWEEN EUKARYOTIC AND PROKARYOTIC ORGANISMS

Microorganisms are broadly categorized based on their cellular structure, organization, and biological characteristics:

• Bacteria: Single-cell prokaryotic microorganisms.

• Fungi: Single-cell or multicellular eukaryotic organisms.

• Yeast: Unicellular eukaryotic organisms.

• Parasites: Single-cell or multicellular organisms.

• Viruses: Dependent on host cells for survival and are therefore not considered cellular organisms, but rather infectious agents.

• Prions: Naked pieces of infectious proteins devoid of DNA or RNA.

Structural Comparison Table

IMPORTANT CHARACTERISTICS OF BACTERIA

• Prokaryotic structure lacking membrane-bound organelles.

• Lack histones (proteins required for mitotic division).

• Cell Wall: Contains peptidoglycan, except for Mycoplasma and Ureaplasma.

• Cytoplasmic Membrane: Fluid phospholipid bilayer embedded with proteins.

• Site of Energy Production: Cytoplasmic membrane.

• Site of Protein Synthesis: Free ribosomes.

• Reproduction: Multiplies via binary fission.

• Size: Most bacteria range from $1\text{--}3\text{ }\beta\text{m}$.

  • Smallest living organism: Mycoplasma.

Bacterial Morphologies

• Rods: Bacilli

• Spherical/Oval: Cocci

• Very Short Bacilli: Coccobacilli

• Tapered End: Fusiform

• Club-Shaped / Curved: Vibrio

• Helical / Spiral: Spirochetes (Treponema, Leptospira, Borrelia, and Brachyspira)

Cellular Arrangements

Bacteria can be arranged singly, in pairs, in tetrads, in clusters, or in chains.

BACTERIAL CELL COMPONENTS

Cell Envelope

The cell envelope is the outermost structure of the bacterial cell, composed of layers (cell wall and cell membrane) surrounding the cytoplasm. All bacteria contain a cell membrane, but not all possess a cell wall.

Cell Wall (Murein Layer / Peptidoglycan)

• Defines the shape and mechanical strength of the bacteria.

• Serves as the point of anchorage for flagella, as well as a site for attachment and antibiotic action.

• Composition: Peptidoglycan consisting of glycan chains of alternating N-acetyl-D-glucosamine (NAG) and N-acetyl-D-muramic acid (NAM).

• Mycobacterium spp. Cell Wall: Contains N-glycolylmuramic acid instead of N-acetylmuramic acid and features an extremely high lipid content, creating a hydrophobic permeable barrier.

Virulence-Associated Cell Wall Components

1. M-protein (Streptococcus pyogenes): Major virulence factor that prevents phagocytosis.

2. Mycolic Acid: Responsible for acid-fastness in Mycobacterium spp. and prevents intracellular digestion during phagocytosis.

3. Poly-D-glutamic Acid: Found in Bacillus spp.

4. Chitin: Found in fungi.

Organisms Lacking Cell Walls

• Mycoplasma and Ureaplasma lack peptidoglycan cell walls.

• Their cell membranes contain sterols to maintain structural integrity.

• They display pleomorphic morphology.

• Gram Reaction: Gram-negative, but they lack endotoxins.

Comparison of Gram-Positive and Gram-Negative Cell Walls

Gram-Positive Cell Wall

• Composed of a very thick, protective peptidoglycan layer.

• Consists of alternating NAG and NAM glycan chains.

• Teichoic Acid: Acts as an antigenic determinant; crucial for species identification.

Gram-Negative Cell Wall

• Contains a thin peptidoglycan layer.

• Contains porins that regulate the transport of nutrients and solutes.

• Periplasmic Space: Located between the cytoplasmic membrane and the outer membrane; functions in nutrient capture, macromolecular degradation, and solute detoxification.

• Outer Membrane Composition:

  1. Lipid A: Endotoxin component.

  2. Core Polysaccharide

  3. O Polysaccharide: Outermost antigenic component.

L-Forms

Gram-positive and Gram-negative cells that lose their cell walls can survive and grow as L-forms in media supplemented with serum or sugar to stabilize osmotic pressure.

Bacteria Containing Mycolic Acid

1. Nocardia

2. Rhodococcus

3. Gordonia

4. Tsukamurella

5. Mycobacterium

6. Corynebacterium

Structural Property Differences Table

Cell Membrane (Cytoplasmic / Plasma Membrane)

• Deepest layer of the cell envelope.

• Consists of a phospholipid bilayer embedded with proteins and lipoproteins.

• Functions as the primary site of energy production (analogous to eukaryotic mitochondria).

Cytoplasmic Region

Ribosomes

• Sites of protein biosynthesis.

• Impart a granular appearance to cytoplasm under electron microscopy.

• Size: $\text{70S}$, separating into $\text{50S}$ and $\text{30S}$ subunits.

  • "$\text{S}$" represents Svedberg units (a measure of sedimentation rate during centrifugation).

Plasmids

• Extrachromosomal, double-stranded circular DNA elements associated with virulence.

• Carry genes encoding antibiotic resistance and toxin production.

Inclusion Bodies

• Function as intracellular storage sites for nutrient reserves and energy.

Bacteria with Specific Inclusion Bodies or Granules

• Corynebacterium: Babes-Ernst granules (metachromatic granules).

• Mycobacterium: Much granules.

• Nocardia and Actinomycetes: Sulfur granules.

• Pasteurella and Bordetella: Bipolar bodies.

Endospores

• Resting, metabolically inactive structures highly resistant to desiccation, heat, and chemical agents.

• Destruction Method: Autoclaving.

• Chemical Composition: Dipicolinic acid combined with calcium (Calcium dipicolinate).

• Spore-Forming Genera: Bacillus (aerobic) and Clostridium (anaerobic).

Spore Morphology and Localization

• Terminal Spore: Clostridium tetani

• Subterminal Spore: Clostridium botulinum

• Central Spore: Bacillus anthracis

Ethanol / Heat Shock Spore Tests

• Used for the selective recovery and identification of Clostridium species from mixed microbial populations.

Transposable Elements (Jumping Genes)

• DNA sequences capable of moving from one genomic locus to another (within a chromosome or between a chromosome and a plasmid).

• Play a critical role in the dissemination of antibiotic resistance genes.

Cell Appendages

Glycocalyx

An extracellular layer of polysaccharides coating the bacterial surface, facilitating adherence to host tissues, medical devices, or solid surfaces. Appears as either a capsule or a slime layer.

Capsule

• Organized matrix firmly attached to the bacterial cell wall.

• Protects cells from human immune defenses by resisting phagocytosis and desiccation.

• Composition: Polysaccharides (Exception: Bacillus anthracis capsule is composed of a polymer of D-glutamic acid).

• Microscopic Detection: Quellung reaction (capsular swelling).

• Demonstration Media: Media supplemented with milk or serum.

Slime Layer

• Unorganized material loosely associated with the cell wall.

• Inhibits phagocytosis or assists in bacterial adherence to host tissue and implants.

• Serves as the initial stage of biofilm formation.

Biofilm & Quorum Sensing

• Biofilm: Accumulation of microorganisms embedded within an extracellular polymeric polysaccharide matrix, mediating adherence to medical devices and prosthetic hardware.

• Quorum Sensing: Chemical signaling communication within biofilms.

  • Gram-Negative Signalling: Acyl-homoserine lactones.

  • Gram-Positive Signalling: Oligopeptides.

Flagella

• Exterior helical protein filaments (composed of flagellin) that rotate to confer motility.

• Critical for bacterial survivability and pathogenesis.

• Periplasmic Flagella: Axial filaments / endoflagella (found in spirochetes).

Flagellar Arrangements

• Atrichous: Lacking flagella.

• Monotrichous: Single flagellum at one end.

• Amphitrichous: Single flagellum at both ends.

• Lophotrichous: Tuft or group of flagella at one or both ends.

• Peritrichous: Flagella distributed over the entire cell surface.

Characteristic Motility Patterns

• Tumbling: Listeria

• Darting: Campylobacter

• Rapid Darting / Shooting Star: Vibrio

• Gliding: Capnocytophaga

• Twitching: Kingella

Methods to Demonstrate Motility

1. Flagellar Stains: Fisher and Conn, Loeffler's, Leifson, Casares-Gil, Van Ermengem, Gray's.

2. Hanging Drop Preparation: Suspension drop placed on a coverslip with vaseline edges, inverted over a concave glass slide.

3. Semi-Solid Media (e.g., SIM):

   • Listeria: Inverted Christmas tree / umbrella pattern.

   • Erysipelothrix rhusiopathiae: Test tube brush pattern.

Pili (Fimbriae)

Non-flagellar, hair-like protein structures approximately $2\text{ }\mu\text{m}$ in length, extending from the cytoplasmic membrane to the exterior.

• Common / Somatic Pili: Mediate adherence to tissue surfaces (e.g., Neisseria gonorrhoeae).

• Sex Pili: Facilitate genetic transfer during bacterial conjugation (e.g., Escherichia coli).

REQUIREMENTS FOR BACTERIAL GROWTH

Nutritional Requirements

• Autotrophs / Lithotrophs: Utilize inorganic carbon sources (e.g., $\text{CO}_2$).

• Heterotrophs / Organotrophs: Require organic compounds as carbon sources (e.g., glucose).

• Phototrophs: Utilize light energy.

• Chemotrophs: Derive energy from chemical compounds.

Oxygen Requirements

• Obligate / Strict Aerobe: Requires oxygen; produces catalase, peroxidase, and superoxide dismutase.

  • Examples: Pseudomonas aeruginosa, Mycobacterium, Brucella, Francisella.

• Obligate / Strict Anaerobe: Cannot grow in the presence of free oxygen.

  • Examples: Clostridium perfringens, Clostridium botulinum, Veillonella, Actinomyces.

• Facultative Anaerobe: Capable of growth under both aerobic and anaerobic conditions; contains superoxide dismutase.

  • Examples: Family Enterobacteriaceae, Staphylococcus aureus.

• Aerotolerant Anaerobe: Does not require oxygen for growth, but survives in its presence.

  • Examples: Lactobacillus, Propionibacterium.

• Microaerophilic: Requires restricted oxygen levels ($2\%\text{--}20\%\text{ }\text{O}_2$).

  • Examples: Campylobacter jejuni, Helicobacter pylori.

• Capnophilic: Requires elevated carbon dioxide levels ($5\%\text{--}10\%\text{ }\text{CO}_2$).

  • Examples: Haemophilus influenzae, Neisseria gonorrhoeae.

Temperature Requirements

• Psychrophiles / Cryophiles: Optimal growth between $0\,^\circ\text{C}$ and $20\,^\circ\text{C}$.

  • Examples: Listeria monocytogenes, Yersinia enterocolitica.

• Mesophiles: Optimal growth between $20\,^\circ\text{C}$ and $45\,^\circ\text{C}$. Includes most clinical human pathogens.

• Thermophiles / Hyperthermophiles: Optimal growth between $50\,^\circ\text{C}$ and $60\,^\circ\text{C}$.

  • Examples: Geobacillus stearothermophilus, Thermus aquaticus, Sulfolobus, Pyrococcus, Pyrodictium.

• Extreme Mesophiles: Adapted to extreme survival conditions (e.g., Bacillus infernus).

pH Requirements

• Acidophiles: Require acidic growth media.

  • Example: Lactobacillus acidophilus (grows on tomato juice agar at pH $3.0$).

• Neutrophiles: Grow optimally at neutral pH ranges ($7.2\text{--}7.4$).

• Alkaliphiles: Require alkaline growth media.

  • Example: Vibrio species.

Salt Concentration Tolerances

• $6.5\%$ Sodium Chloride: Enterococcus species.

• $7.5\%$ Sodium Chloride: Staphylococcus aureus.

• $8\text{--}10\%$ Sodium Chloride: Vibrio species (Exceptions: Vibrio mimicus and Vibrio cholerae).

Stages of Bacterial Growth

1. Lag Phase: Absence of cell division; cells increase in size and metabolic activity; growth rate is $0\%$.

2. Log (Exponential) Phase: Constant rate of rapid binary fission; maximum metabolic activity; period of highest antibiotic susceptibility.

3. Stationary Phase: Rate of cell death equals rate of cell division; driven by nutrient depletion and accumulation of toxic metabolites; characterized by toxin synthesis, metachromatic granule accumulation, and endospore formation.

4. Death Phase: Rate of dying cells exceeds rate of viable cell division; exponential decrease in viable population.

BACTERIAL METABOLISM

Glucose is the primary carbohydrate substrate oxidized or fermented into pyruvate through three main pathways.

Pathways Converting Glucose to Pyruvic Acid

• Embden-Meyerhof-Parnas (EMP) Glycolytic Pathway: Major anaerobic glycolytic pathway ($90\%$ of glucose breakdown); utilized by all members of Enterobacteriaceae.

• Pentose-Phosphate (Phosphogluconate) Pathway: Aerobic pathway ($10\%$ of glucose breakdown); utilized by Lactobacilli and Brucella abortus.

Pathways Converting Pyruvic Acid to Energy and By-Products

• Respiration (Aerobic Process): Krebs cycle / Tricarboxylic Acid cycle; carried out by obligate aerobes and facultative anaerobes to convert pyruvate to carbon dioxide and water.

• Fermentation (Anaerobic Process): Carried out by obligate anaerobes and facultative anaerobes, yielding specific organic end-products.

Fermentation End-Products and Associated Organisms

BACTERIAL GENETICS

• Mutation: Alteration in the original nucleotide sequence of a gene within the organism's genome.

• Genetic Exchange Mechanisms:

  • Conjugation: Requires cell-to-cell contact mediated by a donor sex pilus.

  • Transduction: Transfer of bacterial DNA from donor to recipient cell mediated by a bacteriophage (bacterial virus).

  • Transformation: Direct uptake of free, naked extracellular DNA by a competent recipient cell following donor lysis.

  • Transposition: Excision and integration of transposable genetic elements ("jumping genes") from one genomic site to another.

• Recombination (Homologous Recombination): Physical exchange of a homologous DNA segment from a donor cell into the recipient cell's genomic architecture.

BACTERIAL VIRULENCE FACTORS

Terminology Definitions

• Colonization: Persistence and multiplication of microbiota at a mucosal site without host tissue harm.

• Infection: Growth and multiplication of microorganisms that induce host tissue damage.

• Pathogenicity: The inherent ability of an organism to cause disease.

• Virulence: The degree or intensity of pathogenicity displayed by an organism.

Microbial Virulence Factors

Virulence factors are frequently encoded on Pathogenicity Islands (PAIs), discrete genetic units containing virulence-associated genes and mobility elements.

1. Adherence Factors: Pili, adherence proteins, adhesins, and biofilms. (Attachment is the initial prerequisite step of pathogenesis).

2. Anti-Phagocytic Factors: Capsular polysaccharides, M proteins, mycolic acids, and Protein A.

3. Enzymes:

   • Coagulase: Converts soluble fibrinogen to an insoluble fibrin clot.

   • Fibrinolysin / Kinase: Dissolves fibrin clots.

   • Hyaluronidase (Spreading Factor / Duran-Reynals Factor): Hydrolyzes hyaluronic acid within connective tissues.

4. Toxins: Biochemically active substances released by bacteria that disrupt host cell functions.

Endotoxin Detection: Limulus Amebocyte Lysate (LAL) Test

• Reagent Source: Blood lysate derived from the horseshoe crab (Limulus polyphemus).

• Principle: Endotoxin triggers amebocyte white blood cell lysis, generating a firm gel clot.

• Application: Detects trace Gram-negative bacterial endotoxins in parenteral fluids, body fluids, and surgical instruments.

Comparison of Exotoxins and Endotoxins

Additional Specific Virulence Factors

• IgA Protease: Cleaves secretory IgA antibodies; produced by Neisseria gonorrhoeae, Haemophilus influenzae, and Streptococcus pneumoniae.

• Leukocidins: Selectively destroy host neutrophils and macrophages.

• Intracellular Survival: Facilitates evasion of humoral immunity within host cells; displayed by Mycobacterium, Legionella, Brucella, Listeria, and Histoplasma.

• Yersinia Outer Membrane Proteins (YOPS): Inhibits neutrophil/macrophage phagocytosis and blunts cytokine cascades.

Clinical Manifestations of Disease

• Signs: Objective, measurable indications of illness observed by healthcare providers (e.g., fever, rash, localized swelling).

• Symptoms: Subjective manifestations reported directly by the patient (e.g., headache, general malaise, nausea, pain).

LABORATORY SAFETY AND INFECTION CONTROL

• Standard Precautions: Fundamental principle asserting that all human blood, body fluids, and clinical specimens must be handled as infectious and potentially hazardous.

• Exposure Control Plan: Institutional administrative protocol defining employee biohazardous task risks and containment measures. Enforced jointly by the Microbiology Supervisor and Laboratory Director.

• Post-Exposure Investigation: Overseen by the Safety Officer and Microbiology Supervisor following accidental laboratory exposures.

• Handwashing: The single most critical practice required to prevent institutional pathogen transmission.

Transmission-Based Precautions

Airborne Precautions

• Prevents inhalation of small-particle droplet nuclei (<5\text{ }\beta\text{m}) capable of traveling over $3\text{ feet}$.

• Requires private patient placement in monitored negative air pressure isolation rooms.

• Healthcare personnel must wear N95 respirators or HEPA-filtered devices. Patient wears a standard surgical mask during transport.

• Examples: Measles, Varicella, Tuberculosis, Smallpox.

Droplet Precautions

• Prevents transmission via large-particle droplets (>5\text{ }\beta\text{m}) that do not travel beyond $3\text{ feet}$.

• Requires private room or cohorting with patients harboring identical pathogens.

• Healthcare personnel must wear surgical masks when operating within $3\text{ feet}$ of the patient.

• Examples: Bordetella pertussis, invasive Haemophilus influenzae type b, invasive Neisseria meningitidis, Diphtheria, Mycoplasma pneumoniae, Adenovirus, Influenza virus, Mumps, Parvovirus B19, Rubella, Streptococcal pharyngitis.

Contact Precautions

• Prevents direct or indirect contact transmission.

• Requires gloves, gown, face shield/eye protection prior to patient contact, alongside strict hand hygiene.

• Examples: Clostridium difficile, Escherichia coli O157:H7, Shigella, Hepatitis A virus, Rotavirus, Respiratory Syncytial Virus (RSV), Parainfluenza virus, Viral hemorrhagic fevers.

METHODS OF MICROBIAL CONTROL

Sterilization

The absolute destruction and elimination of all forms of microbial life, including vegetative cells, fungal spores, viruses, and bacterial endospores.

Physical Methods of Sterilization

Moist Heat

Kills microorganisms via protein denaturation, structural protein coagulation, and nucleic acid degradation.

1. Autoclave:

   • Mechanism: Steam under pressure.

   • Standard Culture Media Parameters: $121\,^\circ\text{C}$ at $15\text{ pounds per square inch (psi)}$ for $15\text{ minutes}$.

   • Medical Waste Parameters: $132\,^\circ\text{C}$ at $15\text{ psi}$ for $30\text{--}60\text{ minutes}$.

   • Efficacy: Kills all bacteria, viruses, and spores; does not deactivate prions.

   • Biological Indicator: Geobacillus stearothermophilus endospores.

2. Fractional Sterilization (Tyndallization):

   • Mechanism: Flowing steam in an Arnold Sterilizer at $100\,^\circ\text{C}$ for $30\text{ minutes}$ on $3\text{ consecutive days}$.

   • Cycle Function: Day 1 kills vegetative cells; Day 2 kills germinated spores; Day 3 eliminates residual spores.

   • Application: Heat-labile media containing milk or serum.

3. Inspissation:

   • Mechanism: Thickening of protein-rich media through controlled evaporation at $75\text{--}80\,^\circ\text{C}$ for $2\text{ hours}$ on $3\text{ consecutive days}$.

   • Application: Coagulation of high-protein, egg-based culture media (e.g., Lowenstein-Jensen).

Dry Heat

Destroys microorganisms via protein oxidation and denaturation in the absence of moisture.

1. Direct Flaming: Used for sterilizing wire inoculating loops and needles.

2. Dry Heat Oven: Operates at $160\text{--}180\,^\circ\text{C}$ for $1\text{--}2\text{ hours}$. Used for heat-stable glasswares, metallic instruments, non-aqueous oils, and powders.

   • Biological Indicator: Bacillus subtilis var. niger (incubated at $35\text{--}37\,^\circ\text{C}$).

3. Cremation: Used for systemic disease control and cadaver disposal.

4. Incineration: Most common physical treatment for biohazardous medical wastes. Operates at $870\text{--}980\,^\circ\text{C}$.

Filtration

1. Liquid Filtration: Solutions are pulled through a cellulose nitrate membrane ($0.22\text{ }\beta\text{m}$ pore size) to ensure complete bacterial sterility. Primary choice for heat-sensitive antibiotic solutions, toxic chemicals, radioisotopes, carbohydrate solutions, and vaccines.

   • Asbestos Filters (Seitz): $98\%$ particle filtration efficacy.

2. Air Filtration: High-Efficiency Particulate Air (HEPA) filters designed to trap particles and organisms larger than $0.3\text{ }\beta\text{m}$ from isolation suites, biological safety cabinets, and operating rooms.

Ionizing Radiation (Cold Sterilization)

• Employs short-wavelength, high-energy gamma rays to sterilize heat-sensitive pre-packaged plastic items (e.g., surgical gloves, syringes, catheters).

• Biological Indicator: Bacillus pumilus.

Chemical Methods of Sterilization

• Ethylene Oxide (EtO): Most common gas chemical sterilant for cold sterilization of heat-labile materials.

  • Biological Indicator: Bacillus subtilis var. globigii.

• Formaldehyde Vapor and Vapor-Phase Hydrogen Peroxide ($\text{H}_2\text{O}_2$): Employed for space decontamination and HEPA filter sterilization within Biological Safety Cabinets.

• $2\%$ Glutaraldehyde: Cold sporicidal liquid sterilant used for medical devices (e.g., endoscopes, bronchoscopes).

• Peracetic Acid: Highly effective sporicidal agent in the presence of organic debris.

Disinfection

The process of destroying or removing pathogenic microorganisms from inanimate objects, but not necessarily eliminating all bacterial spores.

Physical Methods of Disinfection

Pasteurization

Used in food processing to eliminate pathogenic vegetative bacteria (Salmonella, Streptococcus, Brucella, Listeria, Mycobacterium bovis) while preserving product quality. Does not kill bacterial endospores. Checked using the phosphatase test (successful pasteurization yields a negative phosphatase result).

• Grade A Milk Standard: Maximum $75,000\text{ CFU/mL}$ pre-pasteurization; maximum $15,000\text{ CFU/mL}$ post-pasteurization.

• Ultra-High Temperature (UHT): $140\,^\circ\text{C}$ for $2\text{ seconds}$.

• Flash Method (HTST): $72\,^\circ\text{C}$ for $15\text{ seconds}$.

• Batch Method (LTLT): $63\,^\circ\text{C}$ for $30\text{ minutes}$.

Boiling

Boiling at $100\,^\circ\text{C}$ for $15\text{--}30\text{ minutes}$ destroys all non-spore-forming vegetative bacterial pathogens.

Non-Ionizing Radiation

Ultraviolet (UV) light possesses low energy and long wavelengths, requiring direct surface exposure to induce thymine dimerization (ineffective for deep penetration).

Chemical Disinfectants and Antiseptics

• Antiseptics: Chemical germicides formulated for safe application onto living skin or mucosal tissue.

  • Alcohols: Non-sporicidal. $70\%$ Ethanol is more effective than $95\%$ Ethanol because water is required for protein denaturation.

  • Iodophors: Combination of iodine and a solubilizing detergent. Most effective skin antiseptic. $70\%$ Ethanol followed by an iodophor is standard for blood culture site preparation.

  • Chlorhexidine: Used for surgical site prep; alternative to iodine tincture for blood culture draws.

  • Hexachlorophene: Topical antiseptic formulation.

  • $10\%$ Hydrogen Peroxide: Cleansing and debridement agent for open wounds.

• Disinfectants: Applied strictly to non-living surface materials.

  • Halogens: Sodium Hypochlorite (household bleach) recommended at a $1:10$ dilution. Inactivates Hepatitis B Virus (HBV) within $10\text{ minutes}$ and HIV within $2\text{ minutes}$.

  • Heavy Metals: Mercury (Merthiolate), Copper (algaecide), Silver ($1\%$ Silver Nitrate [$\text{AgNO}_3$] used historically in Crede's prophylaxis for N. gonorrhoeae ophthalmia neonatorum), Manganese ($\text{KMnO}_4$).

  • Aldehydes: $1\text{--}8\%$ Formaldehyde or Glutaraldehyde; inactivates proteins and nucleic acids.

  • Quaternary Ammonium Compounds (QUATS): Benzalkonium chloride (Zephiran). Inactivated by organic matter; Pseudomonas aeruginosa displays intrinsic resistance.

  • Phenolics: Phenol is tuberculocidal and serves as the standard baseline reference disinfectant.

Phenol Coefficient (PC)

Measures the bactericidal power of a test chemical agent relative to pure phenol using Staphylococcus aureus (Gram-positive) and Salmonella typhi (Gram-negative) as standard test organisms:

• \text{PC} > 1: Disinfectant is more effective than phenol.

• \text{PC} < 1: Disinfectant is less effective than phenol.

• $\text{PC} = 1$: Disinfectant possesses efficiency equal to phenol.

Biological Indicators Summary Table

BIOLOGICAL SAFETY LEVELS AND BIOTERRORISM

Biosafety Levels (BSL)

• Biosafety Level 1 (BSL-1):

  • Risk: Well-characterized agents not known to consistently cause disease in immunocompetent adult humans.

  • Examples: Bacillus subtilis, Mycobacterium gordonae, Naegleria gruberi.

  • Precautions: Standard Personal Protective Equipment (PPE), basic sterile technical practices.

• Biosafety Level 2 (BSL-2):

  • Risk: Moderate individual risk agents present in clinical blood/body fluids via ingestion, percutaneous, or mucous membrane exposure.

  • Examples: Bacillus anthracis, Yersinia pestis, Staphylococcus aureus, enteric pathogens (Salmonella, Shigella), HIV, HBV.

  • Precautions: Restricted access, contact precautions, biohazard warning signs, immunizations.

• Biosafety Level 3 (BSL-3):

  • Risk: Indigenous or exotic agents with potential for aerosol transmission causing severe or lethal disease.

  • Examples: Mycobacterium tuberculosis, Rickettsia spp., Francisella tularensis, Brucella spp., Coxiella burnetii, Coccidioides immitis, St. Louis Encephalitis virus.

  • Precautions: Controlled negative airflow, mandatory HEPA filtration, dedicated containment equipment.

• Biosafety Level 4 (BSL-4):

  • Risk: High individual life-threatening risk from exotic aerosol-transmitted agents lacking vaccines or therapies.

  • Examples: Ebola virus, Marburg virus, Lassa virus, Machupo virus, Smallpox (Variola virus), MERS-CoV.

  • Precautions: Full positive-pressure suit containment, total airlock exit decontamination showers, maximum facility isolation.

Bioterrorism Agent Categories

• Category A (Highest Priority): Easily disseminated or transmitted person-to-person; high mortality potential; major public health impact.

  • Agents: Anthrax (Bacillus anthracis), Botulism (Clostridium botulinum), Plague (Yersinia pestis), Smallpox (Variola major), Tularemia (Francisella tularensis), Filoviruses (Ebola, Marburg), Arenaviruses (Lassa, Machupo).

• Category B (Second Highest Priority): Moderately easy to disseminate; moderate morbidity rates; low mortality rates.

  • Agents: Brucellosis (Brucella spp.), Epsilon toxin (Clostridium perfringens), Salmonella spp., E. coli O157:H7, Shigella, Glanders (Burkholderia mallei), Melioidosis (Burkholderia pseudomallei), Psittacosis (Chlamydia psittaci), Q fever (Coxiella burnetii), Ricin toxin (Ricinus communis), Staphylococcal enterotoxin B, Typhus fever (Rickettsia prowazekii), Viral encephalitis, Vibrio cholerae, Cryptosporidium parvum.

• Category C (Third Highest Priority): Emerging pathogens engineered for mass dissemination due to availability and ease of production.

  • Agents: Nipah virus, Hantaviruses.

BIOLOGICAL SAFETY CABINETS (BSCs)

• Class I BSC: Unsterilized room air flows inward across the work surface; exhaust air passes through a HEPA filter before discharge outdoors or back into the room. Protects personnel and environment, but does not protect the sample/specimen from contamination. Handles BSL-1 agents and low-risk chemicals.

• Class II BSC (Laminar Flow BSC): Protects personnel, environment, and sample product. Handles BSL-2 and BSL-3 bacterial/fungal pathogens.

  • Class IIA1: Self-contained; $70\%$ of air recirculated within the work area through HEPA filters; $30\%$ exhausted back into the room or outdoors via canopy.

  • Class IIA2: Maintains $100\text{ linear feet per minute (lfm)}$ intake air velocity under negative pressure plenums. Recirculates $70\%$ air, exhausts $30\%$ via canopy ducting.

  • Class IIB1: Recirculates $30\%$ of air; exhausts $70\%$ through dedicated hard ducts to the outside via HEPA filters. Selected for minute amounts of volatile toxic chemicals and radionuclides.

  • Class IIB2: No recirculation ($0\%$); $100\%$ of intake air is exhausted directly outdoors through dedicated hard-ducted HEPA filtration systems. Permits work with volatile toxic chemicals and radionuclides.

• Class III BSC: Completely gas-tight, hermetically sealed enclosure fitted with attached heavy-duty rubber gloves. Intake and exhaust air are double-HEPA filtered or incinerated. Provides maximum containment for BSL-4 exotic agents.

Biological Safety Cabinet Operational Specifications

SPECIMEN COLLECTION, HANDLING, AND TRANSPORT

• Collect specimens during the acute phase of illness (within $2\text{--}3\text{ days}$ for viral agents), prior to starting antimicrobial therapy.

• Use sterile, leak-proof collection containers (Exceptions: stool collected in clean containers; skin/hair/nail scrapings in screw-top clean containers).

Collection Media Guidelines

• Pre-Moistened Swab in Transport Medium: Upper respiratory tract (nasopharynx, throat, larynx), genitourinary tract (urethra, cervix, vaginal canal), rectum, outer ear, superficial wounds/abscesses.

• Clean, Leak-Proof Container: Stool (transfer to transport medium if transport time exceeds $1\text{ hour}$).

• Bedside Plating: Corneal scrapings.

• Clean Screw-Top Tube: Hair, nails, skin scrapings.

Transport Time Requirements

All routine specimens should reach the laboratory within $2\text{ hours}$ at room temperature, except:

• Immediate Transport: Prostatic fluid in glass tubes (within $2\text{ hours}$ if swabbed), suprapubic bladder aspirates, bone biopsies.

• Transport within $15\text{ minutes}$: Sterile body fluids (CSF, amniotic, peritoneal, bile, synovial, pericardial, pleural, aqueous/vitreous ocular fluids).

• Transport within $1\text{ hour}$: Unpreserved stool, gastric biopsy.

• Transport within $24\text{ hours}$: Preserved stool, rectal swabs, preserved urine, skin/hair/nail scrapings.

Specimen Preservatives and Transport Media

• Boric Acid: Maintains appropriate microbial colony counts in urine.

• Buffered Formalin ($10\%$): Preserves trophozoite and cyst morphology for Ova & Parasite (O&P) stool exams.

• Stuart's and Amie's Media: Preserves microbial viability without supporting multiplication.

• $0.025\%$ Sodium Polyanethol Sulfonate (SPS): Anticoagulant for blood/body fluids; neutralizes complement, phagocytosis, and aminoglycoside antibiotics.

• Heparin: Anticoagulant for viral cultures and isolation of Mycobacterium from blood.

• JEMBEC & Transgrow Systems: Transport systems for Neisseria species containing an internal $\text{CO}_2$ generator.

• Cary-Blair Medium: Stool pathogen transport medium (especially Vibrio and Campylobacter).

• Todd-Hewitt / Lim Broth: Selective enrichment for Streptococcus agalactiae from vaginal/rectal swabs.

• Leibovitz-Emory Medium: Viral transport medium.

• Buffered Glycerol Saline: Common stool transport medium (toxic to Vibrio and Campylobacter).

• Charcoal Addition: Added to media to absorb toxic fatty acids that kill fastidious organisms (Neisseria gonorrhoeae, Bordetella pertussis).

Swab Composition Recommendations

• Dacron and Rayon Swabs: Recommended for general bacterial and viral cultures.

• Cotton Swabs: Recommended for viral cultures; toxic to Neisseria (unless treated with charcoal).

• Calcium Alginate Swabs: Toxic to viruses; acceptable for Neisseria and Bordetella.

• Contraindications: Swabs are strictly unacceptable for anaerobic and fungal cultures.

Storage Temperatures

• $4\,^\circ\text{C}$ (Refrigeration): Urine, stool, viral specimens, sputa, foreign devices (catheter tips), swabs (except genital), stool for C. difficile toxin assay (up to 3 days).

• $-20\,^\circ\text{C}$: Serological serum samples (up to 1 week).

• $-70\,^\circ\text{C}$: Viral specimens (long-term transport/storage), tissue samples, stool for C. difficile toxin assay if delayed $>3 ext{ days}$.

• $37\,^\circ\text{C}$ (Incubator): CSF and blood culture bottles.

• $22\text{--}25\,^\circ\text{C}$ (Room Temperature): Anaerobic cultures, sterile body fluids, genital specimens (N. gonorrhoeae), hair, nails, skin scrapings.

Anticoagulants

• $0.025\%$ SPS: Inhibits complement, phagocytosis, and aminoglycosides. High concentrations inhibit Neisseria gonorrhoeae, Gardnerella vaginalis, Streptobacillus moniliformis, and Peptostreptococcus anaerobius (can be neutralized by adding $1\%$ gelatin).

• Heparin: Preferred for viral cultures and Mycobacterium isolation from blood. May inhibit Gram-positive bacteria and yeasts.

• EDTA, Heparin, and Citrate: Acceptable for general isolation, but EDTA and Heparin are unacceptable for PCR testing due to enzyme inhibition.

CLINICAL SPECIMEN PROCESSING

Blood and Bone Marrow Aspirates

• Collect prior to the peak of fever spike using sterile venipuncture.

• Site Preparation: Cleanse with $70\%$ alcohol followed by an iodophor (iodine/betadine).

• Timing & Sets: Draw 2 to 3 sets from separate venipuncture sites within a $24\text{-hour}$ period.

• Volume Guidelines:

  • Adults: $10\text{--}20\text{ mL}$ per blood culture bottle ($2\text{--}4$ sets = total $40\text{ mL}$ in emergency settings).

  • Infants / Small Children: $1\text{--}5\text{ mL}$ per set.

  • Blood-to-Broth Ratio: Maintain a $1:5$ to $1:10$ ratio to dilute serum antibacterial factors.

• Special Additives:

  • Thiol Broth / ARD (Antimicrobial Removal Device): Used for patients on antimicrobial therapy.

  • Penicillinase: Inactivates penicillin.

  • Magnesium Sulfate ($\text{MgSO}_4$): Inactivates tetracyclines.

  • PABA (Para-aminobenzoic acid): Inactivates sulfonamides.

• Turnaround Time (TAT) & Growth Indicators: Check daily for turbidity, hemolysis, gas production, or clots for $7\text{ days}$.

  • Extended Incubation: $21\text{ days}$ for Brucella and Subacute Bacterial Endocarditis (SBE); $8\text{ weeks}$ for Leptospira.

• Workup for Positive Bottles: Perform Gram stain and subculture. If Gram stain is negative but turbidity exists, perform Acridine Orange fluorochrome staining.

Cerebrospinal Fluid (CSF)

• Processed immediately (<15\text{ minutes} transport time).

• Tube Distribution: Tube 1 (Chemistry/Serology), Tube 2 (Microbiology - stored at room temperature $37\,^\circ\text{C}$), Tube 3 (Hematology cell count), Tube 4 (Special studies/molecular).

• Primary Plating Media: Blood Agar Plate (BAP), Chocolate Agar Plate (CAP), and Thioglycollate (THIO) broth (mandatory for CSF collected via shunts).

• Common Isolates: Neisseria meningitidis, Streptococcus pneumoniae, Streptococcus agalactiae, Escherichia coli, Staphylococcus aureus, Listeria monocytogenes.

Respiratory Tract Specimens

Throat Swab

• Normal Flora: Viridans group streptococci (alpha-hemolytic streptococci).

• Primary Pathogen: Streptococcus pyogenes (Group A Streptococcus).

• Transport: Swab in Stuart's or Amies transport medium; keep at room temperature for <2\text{ hours}.

• Special Media Additions: Required for suspected Corynebacterium diphtheriae, Neisseria gonorrhoeae, or Haemophilus influenzae epiglottitis.

Sputum

• Collect first-morning deep-cough specimens on 3 consecutive days.

• Evaluate quality using Bartlett's Classification System under low-power field (LPF):

• Interpretation: A net score of $0$ or less indicates heavy saliva contamination (reject specimen).

• Selective Media: Gentamicin BAP (isolates S. pneumoniae); Bacitracin BAP (isolates H. influenzae).

Pulmonary Specimen Types

• Spontaneously Produced Sputum: Specimen of choice for routine lower respiratory tract cultures.

• Aerosol-Induced Sputum: Hypertonic saline inhalation used for non-productive patients (children $\ge 5\text{ years}$).

• Gastric Lavage: Specimen of choice for infants/children (<3\text{ years}) and non-ambulatory patients who swallow sputum overnight. Neutralize with sodium bicarbonate within $1\text{ hour}$.

• Bronchial Lavages / Washings / Brushings: Specimen of choice for non-tuberculous mycobacteria and opportunistic fungal pathogens.

• Endotracheal Aspirates (ETA): Mechanical ventilator samples; reject if $>10$ squamous epithelial cells per LPF.

Urine

• Collection Methods: Midstream Clean-Catch (specimen of choice), Catheterized (discard initial $15\text{ mL}$), Suprapubic Aspirate (only specimen acceptable for anaerobic urine culture), First-Morning (preferred for molecular nucleic acid testing).

• Preservation: Boric acid maintains accurate colony counts for up to $24\text{ hours}$ without refrigeration.

• Major Pathogens: Escherichia coli (primary cause of UTI); Staphylococcus saprophyticus (causes UTI in young, sexually active females).

• Quantitative Colony Count Formula:

  • Using a $1\text{ }\beta\text{L}$ ($0.001\text{ mL}$) loop: $\text{Colonies counted} \times 1000 = \text{CFU/mL}$ of urine.

  • Using a $10\text{ }\beta\text{L}$ ($0.01\text{ mL}$) loop: $\text{Colonies counted} \times 100 = \text{CFU/mL}$ of urine.

  • Diagnostic Significance: $\ge 100,000\text{ CFU/mL}$ ($10^5\text{ CFU/mL}$) indicates significant urinary tract infection.

Gastrointestinal Tract Specimens

• Stool: Clean container; transfer to Cary-Blair transport medium if delay exceeds $1\text{ hour}$. Do not perform routine Gram stain for stool pathogens. Standard workup screens for Salmonella, Shigella, and Campylobacter (specify if requesting Vibrio, Aeromonas, Plesiomonas, Yersinia, or E. coli O157:H7).

• Rectal Swabs: Insert swab $2.5\text{ cm}$ past the anal sphincter; visible feces must coat the swab. Transport in Cary-Blair medium.

• Gastric Biopsy: Rapid urease testing or culture for Helicobacter pylori.

Genital Tract Specimens

• Collection: Dacron or rayon swabs with plastic shafts. Charcoal transport media (Amies/Stuart's with charcoal, JEMBEC, or Gono-Pak) absorbs toxic lipids.

• Preferred Method: First-voided urine is the specimen of choice for Nucleic Acid Amplification Tests (NAAT) for Chlamydia trachomatis and Neisseria gonorrhoeae.

Wounds and Abscesses

• Superficial Abscess: Cleanse site with saline/alcohol; swab leading active edge of lesion.

• Deep Abscess: Aspirate purulent material from active wall using a needle and syringe; transport in anaerobic transport containers.

Critical Values in Microbiology

1. Positive blood culture bottles or bone marrow aspirates.

2. Positive CSF Gram stain, direct wet mount, or culture.

3. Positive Acid-Fast Smear.

4. Detection of Streptococcus pyogenes in surgical wounds.

5. Gram stain findings suggestive of gas gangrene (Clostridium perfringens).

6. Positive blood smear for Plasmodium (Malaria).

7. Positive Cryptococcal antigen test.

8. Detection of Select Bioterrorism Agents.

Criteria for Specimen Rejection

1. Unlabeled or mislabeled specimen (mismatch between requisition and container).

2. Specimen transported at incorrect temperatures.

3. Incorrect transport media (e.g., anaerobic culture submitted in aerobic media).

4. Quantity Not Sufficient (QNS).

5. Leaking specimen containers.

6. Transport time exceeds allowable limits without preservation.

7. Specimen received in chemical fixatives (e.g., formalin for bacterial culture).

8. Anaerobic culture requested on sites with normal anaerobic biota (vagina, mouth, throat).

9. Dried-out swabs.

10. Specimen processing yields questionable clinical value (e.g., Foley catheter tips).

BACTERIAL IDENTIFICATION AND STAINING

Microscopic Examination

• Unstained Living State: Wet mount (NSS mixture) or Hanging Drop preparation (evaluates true motility).

• Stained Fixed State: Heat fixation or $95\%$ methanol fixation prior to dye application.

Staining Techniques

• Simple / Direct Stain: Uses a single dye; colors all bacteria uniformly.

• Differential Stain: Uses primary and counterstains to separate bacterial groups based on cell wall composition (e.g., Gram stain, Acid-Fast stain).

• Indirect / Relief / Negative Stain: Stains the background dark while leaving structures or cells unstained (e.g., India Ink, Nigrosin, Borris method).

• Special Stains: Selective stains designed to highlight specific structural components.

Summary of Specialized Stains

• Capsule Stains: Muir's, Anthony's, Tyler's, Hiss's, Welch's, Gins's.

• Endospore Stains: Dorner's, Schaeffer-Fulton, Wirtz & Conklin, Heat and Acetic Acid method.

• Flagellar Stains: Gray's, Fisher & Conn, Loeffler's, Leifson, Casares-Gil, Van Ermengem.

• Polar Bodies Stain: Wayson's stain.

• Spirochete Stains: Levaditi, Fontana, Warthin-Starry.

• Rickettsia Stains: Gimenez, Macchiavello, Giemsa.

• Metachromatic Granule Stains: Neisser's, Albert's, Ljubinsky's, Lindergan's, Ponder's, Burke's, LAMB (Loeffler Alkaline Methylene Blue).

• Fluorescent / Other Stains:

  • Acridine Orange: Fluorochrome dye binding nucleic acids (fluoresces bright orange); confirms negative Gram stains.

  • Auramine-Rhodamine: Fluorochrome stain for mycobacteria (bright yellow/orange against dark background).

  • Calcofluor White: Fluorochrome binding chitin in fungal cell walls (apple-green/blue-white fluorescence).

  • Malachite Green: Endospore stain (spores stain green, vegetative cells stain pink/red with safranin).

  • Lactophenol Cotton Blue (LPCB): Stains fungal cell wall structures.

  • Diene's Stain: Stains Mycoplasma colonies.

Gram Staining

Developed by Hans Christian Gram in 1884. Binds teichoic acid within Gram-positive cell walls.

• Unsuitable Organisms: Intracellular bacteria (Chlamydia, Rickettsia), organisms lacking cell walls (Mycoplasma, Ureaplasma), thin spirochetes below light microscopy resolution (Treponema).

• General Rules:

  • All cocci are Gram-positive except Neisseria, Veillonella, and Moraxella.

  • All bacilli are Gram-negative except Mycobacterium, Corynebacterium, Clostridium, Bacillus, Erysipelothrix, Lactobacillus, Listeria, Nocardia, and Actinomyces.

  • All spiral organisms report as Gram-negative.

  • Yeast cells stain Gram-positive.

Gram Stain Reagents and Reactions

• Counterstain Modifications: Extend safranin to $2\text{ minutes}$ or replace with $0.2\%$ basic fuchsin for Bordetella pertussis; use $0.1\%$ basic fuchsin for Legionella.

• Quality Control: Staphylococcus aureus (Gram-positive control); Escherichia coli (Gram-negative control).

Errors in Gram Staining

• Gram-Positive Appears Gram-Negative: Over-decolorization, using old/dying cultures, acidic iodine mordant, prior penicillin treatment (cell wall degradation), omitting iodine.

• Gram-Negative Appears Gram-Positive: Under-decolorization, excessively thick smear preparation.

Gram Stain Modifications

• Hucker's Modification: Uses Crystal Violet with ammonium oxalate.

• Burke's Modification: Uses sodium bicarbonate and ether-acetone decolorizer.

Quantification Reporting Scales

Organism Quantification

• $4+$ (Many): $\ge 25$ organisms per oil immersion field (OIF, $1000\times$).

• $3+$ (Moderate): $6\text{--}10$ organisms per OIF.

• $2+$ (Few): $3\text{--}5$ organisms per OIF.

• $1+$ (Rare): <10 total organisms observed across entire smear.

Host Cell Quantification

• $4+$ (Many): $\ge 25$ cells per low-power field (LPF, $100\times$).

• $3+$ (Moderate): $10\text{--}25$ cells per LPF.

• $2+$ (Few): $2\text{--}10$ cells per LPF.

• $1+$ (Rare): <2 cells per LPF.

Acid-Fast Staining

Stains mycolic acids (hydroxy-methoxy-acids) present in cell walls. Acid-fast organisms include Mycobacterium, Nocardia (partially acid-fast), Cryptosporidium, Cystoisospora, and Cyclospora. (Mycobacterium appears as non-staining "Gram ghosts" on routine Gram stains).

Acid-Fast Methods Comparison

Additional Acid-Fast Modifications

• Pappenheim's Method: Differentiates Mycobacterium smegmatis (blue, negative) from Mycobacterium tuberculosis (red, positive).

• Baumgarten's Method: Differentiates Mycobacterium leprae (red, positive) from Mycobacterium tuberculosis (blue, negative).

STUDY OF CULTURAL CHARACTERISTICS

Culture Media Types

• Agar Properties: Derived from red algae. Solidifies at $40\text{--}50\,^\circ\text{C}$; melts at $80\text{--}90\,^\circ\text{C}$. Pouring temperature for plates is $55\text{--}60\,^\circ\text{C}$ ($25\text{ mL}$ per plate).

Classification by Physical State

• Liquid Media: No solidifying agent (e.g., Nutrient Broth, Brain Heart Infusion, Alkaline Peptone Water, Thioglycollate Broth).

• Semi-Solid Media: Contains $0.5\text{--}1.0\%$ agar (e.g., SIM medium, Gelatin medium).

• Solid Media (Liquefiable): Contains $2.0\text{--}3.0\%$ agar (e.g., BAP, MacConkey, Salmonella-Shigella Agar).

• Solid Media (Non-Liquefiable): Non-agar based (e.g., Rice medium for fungi).

• Biphasic Media: Contains both liquid and solid phases in a single bottle (e.g., Castaneda bottle, HBT agar).

Classification by Composition

• Synthetic / Chemically Defined: Exact chemical structure of all components is known (e.g., BG-11 for cyanobacteria).

• Complex / Non-Synthetic: Contains undefined biological extracts, peptones, or digests (e.g., Nutrient Broth, Trypticase Soy Broth, MacConkey Agar).

• Tissue Media: Living cell lines required for obligate intracellular pathogens:

  • McCoy cells: Chlamydia trachomatis.

  • HeLa cells: Cervical carcinoma.

  • A549 cells: Lung carcinoma.

  • Vero cells: African green monkey kidney.

  • Hep-2 cells: Laryngeal carcinoma.

Classification by Purpose

• Simple / Supportive Media: Supports non-fastidious bacterial growth (e.g., Nutrient Agar, Trypticase Soy Agar).

• Enriched Media: Contains basal media supplemented with blood, serum, or vitamins for fastidious bacteria (e.g., Blood Agar Plate, Chocolate Agar Plate).

• Enrichment Broths: Liquid media containing specific inhibitors to suppress unwanted flora while favoring target pathogens (e.g., Selenite F Broth, Tetrathionate Broth, GN Broth, Alkaline Peptone Water, BCYE, Lim Broth).

• Selective Media: Solid media containing specific inhibitory agents (dyes, bile salts, systemic antibiotics) to suppress background flora (e.g., TCBS, PEA, SSA, CTBA).

• Differential Media: Contains specific substrates and indicators that yield distinct colonial morphologies based on metabolic reactions (e.g., MacConkey Agar, Blood Agar).

Hemolytic Patterns on Blood Agar Plates (BAP)

• Alpha ($\alpha$) Hemolysis: Partial lysis of red blood cells surrounding colonies; yields a greenish/brownish halo due to hemoglobin reduction to methemoglobin.

• Beta ($\beta$) Hemolysis: Complete lysis of red blood cells surrounding colonies; yields a clear, transparent zone.

• Gamma ($\gamma$) Hemolysis: Absence of red blood cell lysis; no media color change.

• Alpha-Prime ($\alpha'$) Hemolysis: Small inner zone of intact red blood cells immediately surrounding the colony, encompassed by an outer zone of complete $\beta$ hemolysis (characteristic of Viridans streptococci).

• Double Hemolysis: Inner zone of clear $\beta$ hemolysis surrounded by an outer zone of partial $\alpha$ hemolysis (characteristic of Clostridium perfringens).

Primary Culture Media Index

• Alkaline Peptone Water (APW): Alkaline enrichment broth (pH $8.4$) for Vibrio species.

• Ashdown Medium: Selective isolation medium for Burkholderia pseudomallei.

• Bacteroides Bile Esculin (BBE) Agar: Selective and differential for Bacteroides fragilis group (contains $20\%$ bile and gentamicin; turns black).

• Buffered Charcoal-Yeast Extract (BCYE) Agar: Enriched selective medium for Legionella species.

• Bile Esculin Agar (BEA): Differential isolation of Group D Streptococci and Enterococcus (esculin hydrolysis yields black ferric complex).

• Bile Esculin Azide Agar with Vancomycin: Selective isolation of Vancomycin-Resistant Enterococci (VRE).

• Bismuth Sulfite Agar (BSA): Selective medium for isolating Salmonella typhi (yields metallic black colonies with halos).

• Blood Cystine Glucose Agar: Enriched medium for cultivating Francisella tularensis.

• Bordet-Gengou (Potato-Blood-Glycerol) Agar: Isolation medium for Bordetella pertussis (yielding "mercury droplet" colonies).

• Brilliant Green Agar (BGA): Highly selective medium for non-typhoidal Salmonella species.

• Campy-Blood Agar: Selective isolation medium for Campylobacter jejuni (incubated at $42\,^\circ\text{C}$ under microaerophilic conditions).

• Cefsulodin-Irgasan-Novobiocin (CIN) Agar: Selective medium for Yersinia enterocolitica (yields "bull's-eye" pink colonies).

• Cetrimide Agar: Selective isolation medium for Pseudomonas aeruginosa (stimulates pyocyanin/pyoverdine pigmentation).

• Charcoal-Cephalexin-Blood Agar (Regan-Lowe): Enrichment and transport medium for Bordetella pertussis.

• Chocolate Agar Plate (CAP): Enriched medium providing Hemin (X factor) and NAD (V factor) for fastidious Haemophilus and pathogenic Neisseria.

• Columbia CNA Agar: Selective medium containing Colistin and Nalidixic Acid to isolate Gram-positive cocci.

• Cycloserine-Cefoxitin-Fructose Agar (CCFA): Selective and differential medium for Clostridium difficile (yellow colonies with a chartreuse fluorescence).

• Cystine-Tellurite Blood Agar (CTBA): Isolation medium for Corynebacterium diphtheriae (black colonies with brown halos).

• Ellinghausen-McCullough-Johnson-Harris (EMJH) / Fletcher's Media: Semi-solid media for growing Leptospira interrogans.

• Eosin Methylene Blue (EMB) Agar: Differential medium for Gram-negative enteric bacilli (E. coli yields a green metallic sheen).

• Human Blood Bilayer Tween (HBT) Medium: Selective medium for Gardnerella vaginalis.

• Lowenstein-Jensen (LJ) Medium: Egg-based medium containing malachite green for isolating Mycobacterium tuberculosis.

• MacConkey (MAC) Agar: Selective/differential medium containing crystal violet and bile salts to isolate Gram-negative lactose fermenters (pink) and non-fermenters (colorless).

• MacConkey Sorbitol Agar (SMAC): D-sorbitol substituted for lactose to screen for E. coli O157:H7 (sorbitol-negative, colorless colonies).

• Mannitol Salt Agar (MSA): Contains $7.5\%\text{ NaCl}$ to isolate Staphylococcus aureus (mannitol-positive, yellow colonies).

• New York City (NYC) Agar: Selective isolation medium for Neisseria gonorrhoeae and genital Mycoplasma.

• Phenylethyl Alcohol (PEA) Agar: Inhibits Gram-negative organisms to selectively isolate Gram-positive cocci and obligate anaerobes.

• Thayer-Martin (TM) / Modified Thayer-Martin (MTM) Agar: Selective CAP containing Vancomycin (kills Gram-positive), Colistin (kills Gram-negative), Nystatin (kills fungi), and Trimethoprim (kills Proteus swarming) to isolate Neisseria gonorrhoeae and Neisseria meningitidis.

• Thiosulfate-Citrate-Bile Salts-Sucrose (TCBS) Agar: Selective and differential for Vibrio species (sucrose-fermenting V. cholerae yields yellow colonies; non-fermenting V. parahaemolyticus yields green colonies).

• Tinsdale Agar: Selective medium for Corynebacterium diphtheriae (yields dark black colonies with dark brown halos).

• Triple Sugar Iron (TSI) Agar: Slant/butt medium containing $0.1\%$ glucose, $1.0\%$ lactose, $1.0\%$ sucrose, phenol red, and sodium thiosulfate/ferrous sulfate to assess carbohydrate fermentation and $\text{H}_2\text{S}$ production.

ANTIMICROBIAL SUSCEPTIBILITY TESTING (AST)

Terminology Definitions

• Minimal Inhibitory Concentration (MIC): The lowest antimicrobial concentration that visibly inhibits in vitro bacterial growth.

• Minimal Bactericidal Concentration (MBC): The lowest antimicrobial concentration that kills $99.9\%$ of the initial bacterial inoculum.

Standardized AST Methods

Broth Dilution Method

• Inoculum Concentration: $5 \times 10^5\text{ CFU/mL}$.

• Medium: Mueller-Hinton Broth (MHB).

• Microdilution: Uses microwell trays with volume capacities of $0.05\text{--}0.1\text{ mL}$.

• Macrodilution: Uses standard test tubes; functions as the reference method for determining MIC and MBC.

• Trailing Effect: Heavy growth at low antibiotic concentrations followed by persistent light growth across higher concentrations.

Agar Dilution Method

• Inoculum Concentration: $1 \times 10^4\text{ CFU/mL}$.

• Reference Application: Reference method for testing obligate anaerobes (Brucella agar with laked blood and Vitamin K) and Neisseria gonorrhoeae.

Disk Diffusion Method (Kirby-Bauer Test)

• Standard Medium: Mueller-Hinton Agar (MHA).

  • Agar Depth: Exactly $4\text{ mm}$.

  • Agar pH: $7.2\text{--}7.4$

  • Filter Disk Size: $6\text{ mm}$.

• Inoculum Standardization: Adjusted to a $0.5\text{ McFarland Standard}$, equivalent to an organism density of $1.5 \times 10^8\text{ CFU/mL}$ (prepared using $99.5\text{ mL}$ of $1\%\text{ }\text{H}_2\text{SO}_4$ and $0.5\text{ mL}$ of $1.175\%$ barium chloride [$\text{BaCl}_2$]).

• Inoculation and Incubation: Within $15\text{ minutes}$ of standardizing inoculum, streak MHA plate in three directions. Apply antibiotic disks within $15\text{ minutes}$. Incubate at $35\text{--}37\,^\circ\text{C}$ in ambient air (no $\text{CO}_2$) for $16\text{--}18\text{ hours}$.

• Disk Placement Limits: Maximum $12\text{ disks}$ on a $150\text{ mm}$ plate; maximum $5\text{--}6\text{ disks}$ on a $100\text{ mm}$ plate. Maintain a minimum $24\text{ mm}$ distance from center-to-center and $15\text{ mm}$ from disk to plate margin.

• Interpretation: Zone diameters measured in millimeters using calipers or rulers. Categorized as Susceptible (S), Susceptible-Dose Dependent (SDD), Intermediate (I), Resistant (R), or Nonsusceptible (NS).

Sources of AST Error

• Inoculum too light: Yields falsely large inhibition zones (false susceptibility).

• Inoculum too heavy: Yields falsely small inhibition zones (false resistance).

• Excessive agar depth (>4\text{ mm}): Yields falsely small zones.

• Inadequate agar depth (<4\text{ mm}): Yields falsely large zones.

Specialized Susceptibility Tests

• D-Test: Detects inducible Clindamycin resistance among Staphylococcus aureus strains resistant to Erythromycin. Clindamycin ($2\text{ }\beta\text{g}$) and Erythromycin ($15\text{ }\beta\text{g}$) disks are placed $15\text{--}20\text{ mm}$ apart. Induction of resistance results in flattening or blunting of the clindamycin zone adjacent to the erythromycin disk, generating a characteristic "D" pattern.

• E-Test (Epsilometer Test): Quantitative dilution technique using an agar plastic strip imprinted with a predefined exponential gradient of a single antibiotic; the MIC is read where the symmetrical ellipse edge intersects the strip.

• Chromogenic Cephalosporin (Nitrocefin) Test: Rapid test for direct $\beta$-lactamase enzyme production (yields a red color change upon $\beta$-lactam ring cleavage).

CLASSIFICATION OF ANTIMICROBIAL AGENTS

• Bacteriostatic: Inhibits bacterial multiplication without direct killing.

• Bactericidal: Directly kills target bacteria.

• Narrow-Spectrum: Targets a restricted subset of bacterial species.

• Broad-Spectrum: Active against diverse Gram-positive and Gram-negative species.

Antimicrobial Mechanisms of Action

Inhibition of Cell Wall Synthesis

• Penicillins: Natural Penicillins (Penicillin G, Penicillin V), Synthetic Penicillins (Methicillin, Oxacillin, Ampicillin, Carbenicillin, Piperacillin).

• Cephalosporins: 1st Gen (Cephalexin), 2nd Gen (Cefoxitin), 3rd Gen (Ceftriaxone), 4th Gen (Cefepime), 5th Gen (Ceftaroline).

• Carbapenems: Imipenem, Meropenem.

• Monobactams: Aztreonam.

• Glycopeptides: Vancomycin, Teicoplanin.

Inhibition of Protein Synthesis

• Binds to 30S Ribosomal Subunit:

  • Tetracyclines: Tetracycline, Doxycycline, Minocycline, Demeclocycline.

  • Aminoglycosides: Gentamicin, Tobramycin, Amikacin, Kanamycin, Neomycin.

• Binds to 50S Ribosomal Subunit:

  • Macrolides: Erythromycin, Clarithromycin, Azithromycin.

  • Lincosamides: Lincomycin, Clindamycin.

  • Chloramphenicol

Inhibition of Folic Acid Synthesis

• Sulfonamides, Trimethoprim, Nitrofurantoin.

Inhibition of Nucleic Acid Synthesis

• Fluoroquinolones (Inhibit DNA Gyrase/Topoisomerase IV): Ciprofloxacin, Ofloxacin, Levofloxacin.

• Rifampin: Inhibits DNA-dependent RNA polymerase $\beta$ subunit.

• Metronidazole: Induces DNA strand breakage.

Inhibition of Cell Membrane Function

• Polymyxins (Polymyxin B, Colistin/Polymyxin E), Daptomycin, Bacitracin, Nystatin.

LABORATORY QUALITY CONTROL

• Daily Maintenance: Monitor temperatures of incubators, water baths, refrigerators, and freezers. Check reagent performance using positive and negative control strains.

• Equipment Quality Control Schedule:

  • Each Use: Autoclave temperature/pressure, GasPak jar anaerobic indicators.

  • Daily: Incubator temperatures, anaerobic chamber environment.

  • Weekly: Autoclave spore testing (Geobacillus stearothermophilus), Gram stain control slides (S. aureus and E. coli).

  • Quarterly: Microscope phase/optical alignments.

  • Annually: Analytical balance calibration, Biological Safety Cabinet air velocity certification.

FAMILY MICROCOCCACEAE

Includes the Gram-positive cocci genera Staphylococcus, Micrococcus, Planococcus, and Stomatococcus.

Micrococcus vs. Staphylococcus Differentiation

Genus Staphylococcus

Staphylococcus aureus

• General Characteristics: Facultative anaerobe; normal flora of anterior nares and nasopharynx. Forms smooth, circular, opaque, creamy golden-yellow colonies on BAP with $\beta$ hemolysis. Produces jet-black colonies on Tellurite/Vogel-Johnson agar.

Virulence Factors

• Lipase: Hydrolyzes skin lipids, facilitating skin and soft tissue infections (furuncles, carbuncles, boils).

• Hyaluronidase (Duran-Reynals Factor): Spreading factor that hydrolyzes hyaluronic acid in connective tissue.

• $\beta$-Lactamase (Penicillinase): Inactivates penicillin.

• Coagulase: Converts fibrinogen to fibrin, forming protective clots around bacteria.

• Staphylokinase (Fibrinolysin): Dissolves fibrin clots to facilitate spread.

• Thermonuclease (DNase): Degrades host DNA to reduce pus viscosity.

• Protein A: Surface protein that binds the Fc region of IgG, preventing opsonization and phagocytosis.

• Toxins:

  • Alpha ($\alpha$) Toxin: Disrupts vascular smooth muscle; cytolytic to RBCs and WBCs.

  • Beta ($\beta$) Toxin: Heat-labile sphingomyelinase C.

  • Panton-Valentine Leukocidin (PVL): Selectively lethal to white blood cells; associated with severe community-acquired MRSA.

  • Enterotoxins (A, B, C, D, E, F): Heat-stable toxins. Enterotoxins A, B, and D cause food poisoning. Enterotoxin F causes Toxic Shock Syndrome Toxin-1 (TSST-1).

  • Exfoliative Toxin (Epidermolytic Toxin): Causes Staphylococcal Scalded Skin Syndrome (SSSS) / Ritter's Disease.

Coagulase-Negative Staphylococci (CoNS)

Staphylococcus epidermidis

• Normal skin flora; white non-hemolytic colonies on BAP.

• Virulence Factor: Slime layer (exopolysaccharide) mediating adherence to prosthetic heart valves, intravenous lines, and catheters.

• Antibiotic Profile: Novobiocin susceptible.

Staphylococcus saprophyticus

• Major cause of community-acquired urinary tract infections in young, sexually active females.

• Diagnostic Profile: Novobiocin resistant (<16\text{ mm} zone around a $5\text{ }\beta\text{g}$ disk).

Staphylococcus lugdunensis

• Slide-coagulase positive, tube-coagulase negative; PYR positive.

• Carries mecA gene; causes aggressive endocarditis and bacteremia.

Primary Diagnostic Tests for Staphylococci

1. Catalase Test: $3\%\text{ }\text{H}_2\text{O}_2$ applied to colonies yields immediate effervescence/bubbling ($\text{Staphylococcus} = +$; $\text{Streptococcus} = -$). Note: Do not take colonies from Blood Agar Plates due to pseudoperoxidase activity of RBCs.

2. Modified Oxidase Test: Tetramethyl-p-phenylenediamine dihydrochloride in DMSO ($\text{Micrococcus} = +$ [dark blue]; $\text{Staphylococcus} = -$).

3. Coagulase Tests:

   • Slide Coagulase: Detects cell-bound coagulase (clumping factor) using EDTA rabbit plasma ($S. aureus = +$).

   • Tube Coagulase: Detects free extracellular coagulase by incubating plasma at $35\,^\circ\text{C}$ for $1\text{--}4\text{ hours}$ to observe clot formation ($S. aureus = +$).

4. PYR Test: Detects L-pyrrolidonyl-arylamidase ($S. lugdunensis = +$ [red]; $S. aureus = -$).

5. Mannitol Salt Agar (MSA): $S. aureus$ ferments mannitol, turning the phenol red indicator yellow.

6. Cefoxitin Disk Screen for MRSA: Cefoxitin ($30\text{ }\beta\text{g}$) disk screening detects mecA-mediated oxacillin/methicillin resistance ($\le 21\text{ mm}$ zone indicates MRSA). Molecular PCR detection of mecA remains the definitive gold standard.

FAMILY STREPTOCOCCACEAE

Gram-positive, catalase-negative cocci occurring in pairs or chains. Facultative anaerobes that ferment carbohydrates into lactic acid without gas production.

Classification Systems

Hemolytic Classification (Smith and Brown)

• Alpha ($\alpha$): Incomplete/greenish hemolysis (S. pneumoniae, Viridans group).

• Beta ($\beta$): Complete clear hemolysis (S. pyogenes, S. agalactiae).

• Gamma ($\gamma$): Non-hemolytic (Enterococcus, Group D Non-Enterococcus).

Serological Classification (Lancefield)

Based on cell wall C-carbohydrate antigen specificity (Groups A, B, C, D, F, G). S. pneumoniae and Viridans group lack Lancefield antigens.

Group A Streptococcus (Streptococcus pyogenes)

Virulence Factors

• Streptolysin O: Oxygen-labile, highly immunogenic hemolysin; causes subsurface $\beta$ hemolysis. Elicits Antistreptolysin O (ASO) antibodies.

• Streptolysin S: Oxygen-stable, non-immunogenic hemolysin; causes surface $\beta$ hemolysis.

• Pyrogenic / Erythrogenic Toxins (A, B, C): Mediates the rash of Scarlet Fever and Streptococcal Toxic Shock Syndrome.

• M Protein: Major virulence factor; anti-phagocytic and mediates mucosal attachment.

• Streptokinase / Hyaluronidase / Streptodornase (DNase B).

Clinical Manifestations

• Acute Pharyngitis ("Strep Throat"), Impetigo, Cellulitis, Erysipelas, Scarlet Fever (strawberry tongue, punctate rash).

• Necrotizing Fasciitis: Rapidly progressive "flesh-eating" soft tissue destruction.

• Post-Streptococcal Sequelae:

  • Rheumatic Fever: Cross-reactive antibodies target heart valves following untreated streptococcal pharyngitis.

  • Acute Glomerulonephritis: Immune-complex deposition in kidney glomeruli following skin or throat infections.

Diagnostic Tests

• Dick's Test: Skin test evaluating susceptibility to Scarlet Fever.

• Schultz-Charlton Test: Neutralization blanching test for Scarlet Fever rash.

• Bacitracin Susceptibility (Taxo A): Susceptible ($\ge 10\text{ mm}$ zone).

• PYR Test: Positive (red color).

Group B Streptococcus (Streptococcus agalactiae)

• Normal biota of female genitourinary and lower gastrointestinal tracts.

• Leading cause of neonatal sepsis and meningitis.

• Screening recommended for pregnant women at $35\text{--}37\text{ weeks}$ gestation using Lim Broth or Granada medium.

Diagnostic Tests

• CAMP Test: Produces an arrowhead-shaped zone of enhanced $\beta$ hemolysis at the junction of S. agalactiae and a $\beta$-lysin producing Staphylococcus aureus streak.

• Hippurate Hydrolysis: Positive (deaminates sodium hippurate to form a deep blue color with ninhydrin).

• Bacitracin / SXT Profile: Resistant to both Bacitracin and SXT.

Group D Streptococci and Enterococcus

Enterococcus (E. faecalis, E. faecium)

• Normal bowel biota; resistant to penicillin, cephalosporins, and vancomycin (VRE).

• Diagnostic Profile: Grows in $40\%$ Bile; hydrolyzes Bile Esculin (black); grows in $6.5\%\text{ NaCl}$; PYR positive.

• E. faecalis vs. E. faecium: E. faecalis is Pyruvate positive; E. faecium is Pyruvate negative.

Non-Enterococcus Group D (Streptococcus bovis, S. equinus)

• Diagnostic Profile: Grows in $40\%$ Bile; hydrolyzes Bile Esculin (black); fails to grow in $6.5\%\text{ NaCl}$; PYR negative; Penicillin susceptible.

• Clinical Correlation: Isolation of Streptococcus bovis in blood cultures strongly correlates with underlying colon cancer.

Streptococcus pneumoniae

• Gram-positive, lancet-shaped / bullet-shaped diplococci.

• Major cause of adult bacterial meningitis, otitis media, and lobar pneumonia (yielding rusty-colored sputum).

• Colony Morphology: Alpha-hemolytic, mucoid, dome-shaped colonies that collapse inward with age, producing a "doughnut" or "nail-head" appearance due to autolysin production.

Diagnostic Tests

1. Optochin (P Disk) Susceptibility: Susceptible ($\ge 14\text{ mm}$ zone of inhibition around a $5\text{ }\beta\text{g}$ disk).

2. Bile Solubility Test: Soluble ($10\%$ sodium desoxycholate dissolves colonies, leaving a clear solution).

3. Quellung Reaction (Capsular Swelling): Positive (capsular swelling under specific antiserum).

Viridans Group Streptococci

• Alpha-hemolytic, normal flora of oral cavity; primary cause of Subacute Bacterial Endocarditis (SBE) following dental procedures.

• Produces glucans and dextrans facilitating attachment to damaged heart valves.

• Diagnostic Profile: Optochin resistant (<14\text{ mm}); Bile insoluble.

Nutritionally Variant Streptococci (NVS)

• Genera Abiotrophia and Granulicatella (S. adjacens, S. defectivus).

• Require Pyridoxal (Vitamin $B_6$) for growth.

• Exhibit satellitism around Staphylococcus aureus colonies on blood agar.

Diagnostic Identification Table for Streptococci

GRAM-NEGATIVE COCCI

Genera Neisseria and Moraxella

Gram-negative, kidney bean-shaped diplococci; non-motile, capnophilic ($5\text{--}10\%\text{ }\text{CO}_2$), cold-sensitive.

• General Profile: Oxidase positive, Catalase positive (Exception: Neisseria elongata is catalase negative).

• Oxidase Screening Test: Tetramethyl-p-phenylenediamine dihydrochloride yields a deep purple color.

Neisseria gonorrhoeae (Gonococcus)

• Fastidious; requires enriched Chocolate Agar or selective media (Thayer-Martin, Martin-Lewis, NYC) at $35\text{--}37\,^\circ\text{C}$.

• Virulence Factors: Pili (fimbriae), IgA protease, Endotoxin (LOS), Porin (Por I), Opa (Protein II).

• Clinical Pathologies: Acute Gonorrhea, Pelvic Inflammatory Disease (PID), Fitz-Hugh-Curtis Syndrome (perihepatitis), Ophthalmia Neonatorum.

• Superoxol Test: Positive ($30\%\text{ }\text{H}_2\text{O}_2$ yields immediate vigorous bubbling).

• Carbohydrate Utilization: Ferments Glucose ONLY.

Neisseria meningitidis (Meningococcus)

• Encapsulated diplococcus causing epidemic bacterial meningitis and meningococcemia.

• Pathologies: Waterhouse-Friderichsen Syndrome (massive bilateral adrenal hemorrhage and DIC).

• Carbohydrate Utilization: Ferments Glucose AND Maltose.

• ONPG Test: Negative (distinguishes it from Neisseria lactamica, which is ONPG positive).

Moraxella catarrhalis

• Normal URT flora; causes otitis media and respiratory infections.

• Colony Characteristics: Opaque, pink-gray colonies displaying a "hockey puck" consistency (can be pushed intact across agar).

• Diagnostic Profile: Asaccharolytic (does not ferment carbohydrates); DNase positive; Butyrate Esterase positive (blue color change).

Carbohydrate Utilization Patterns (CTA Media)

GRAM-POSITIVE BACILLI

Aerobic Spore-Forming Bacilli: Genus Bacillus

Bacillus anthracis

• Bioterrorism Agent (Category A). Non-motile, non-hemolytic Gram-positive rod in long chains ("bamboo rod" appearance).

• Capsule Composition: Poly-D-glutamic acid (protein capsule highlighted by Mc-Fadyean's stain).

• Exotoxins: Composed of Protective Antigen (PA), Edema Factor (EF), and Lethal Factor (LF).

  • $\text{PA} + \text{EF} = \text{Edema Toxin}$

  • $\text{PA} + \text{LF} = \text{Lethal Toxin (Cell Death)}$

• Clinical Forms:

  1. Cutaneous Anthrax: Papule develops into a black necrotic eschar ("malignant pustule").

  2. Pulmonary Anthrax (Woolsorter's Disease): Inhalation of spores; rapidly fatal hemorrhagic mediastinitis.

  3. Gastrointestinal Anthrax: Ingestion of contaminated meat.

• Colony Morphology: Large, flat, non-hemolytic grey colonies with swirling projections ("Medusa-head" appearance); holds shape when lifted ("beaten egg-white" consistency).

• Diagnostic Tests:

  • Penicillin Susceptibility: Susceptible; displays string-of-pearls reaction on agar containing penicillin.

  • Gamma Phage Lysis: Positive.

  • Gelatin Hydrolysis: Negative.

Bacillus cereus

• Motile, $\beta$-hemolytic, penicillin-resistant.

• Causes two distinct types of food poisoning:

  • Emetic Form: Short incubation ($1\text{--}6\text{ hours}$); heat-stable enterotoxin associated with fried rice.

  • Diarrheal Form: Long incubation ($8\text{--}16\text{ hours}$); heat-labile enterotoxin associated with meats and vegetables.

B. anthracis vs. B. cereus Differentiation

Non-Spore-Forming Gram-Positive Bacilli

Listeria monocytogenes

• Gram-positive coccobacillus; foodborne pathogen associated with unpasteurized soft cheeses, cold cuts, and coleslaw.

• Causes perinatal infection (Granulomatosis infantiseptica), stillbirth, and neonatal/elderly meningitis.

• Virulence Factors: Listeriolysin O (oxygen-labile hemolysin), Actin A (facilitates direct cell-to-cell spread).

• Diagnostic Features:

  • Motility: Tumbling motility in hanging drop at room temperature ($25\,^\circ\text{C}$); umbrella-shaped / inverted Christmas tree motility in semi-solid agar at $25\,^\circ\text{C}$ (non-motile at $37\,^\circ\text{C}$).

  • Cold Enrichment: Enhanced recovery at $4\,^\circ\text{C}$.

  • CAMP Test: Positive, producing a rectangular-shaped zone of hemolysis.

  • Anton Test: Induces purulent conjunctivitis in rabbit eyes.

  • Catalase Test: Positive.

Corynebacterium diphtheriae (Klebs-Loeffler Bacillus)

• Pleomorphic Gram-positive rod displaying club-shaped ends arranged in palisades or "Chinese letter" formations.

• Contains Babes-Ernst metachromatic granules (stained with Loeffler's Methylene Blue or Albert's stain).

• Diphtheria Toxin: Lysogenic $\beta$-phage carrying the tox gene produces a potent exotoxin that inhibits host cell protein synthesis by inactivating Elongation Factor-2 (EF-2).

• Pathology: Respiratory Diphtheria features a tough, adherent grey-white pseudomembrane over the pharynx/tonsils and severe neck edema ("Bull's neck").

• Media and Culture:

  • Loeffler's Serum Slant / Pai Media: Enhances granule formation.

  • Cystine-Tellurite Blood Agar / Tinsdale Medium: Produces dark grey/black colonies surrounded by a dark brown halo (due to cystinase activity).

• Toxigenicity Testing:

  • Elek Test: In vitro immunodiffusion assay demonstrating precipitin lines of toxin-antitoxin reaction.

  • Schick Test: In vivo skin test assessing immunity to diphtheria toxin.

Other Corynebacterium Species

• C. jeikeium (Group JK): Multidrug-resistant pathogen associated with catheter endocarditis.

• C. minutissimum: Causes Erythrasma (skin infection displaying a coral-red fluorescence under Wood's lamp due to porphyrin production).

• C. urealyticum: Rapid urease-positive pathogen causing urinary tract infections and encrusted cystitis.

Erysipelothrix rhusiopathiae

• Gram-positive, catalase-negative rod; occupational hazard for fish, poultry, and meat handlers.

• Causes Erysipeloid (localized reddish skin swelling on hands/fingers).

• Diagnostic Profile: Only Gram-positive rod that produces Hydrogen Sulfide ($\text{H}_2\text{S}$) in TSI; produces a test-tube brush pattern in gelatin stab cultures.

Gardnerella vaginalis

• Gram-variable rod associated with Bacterial Vaginosis (BV).

• Clinical Presentation: Homogeneous, thin discharge with a pH >4.5.

• Whiff / KOH Test: Addition of $10\%\text{ KOH}$ to vaginal secretions produces a strong fishy amine odor.

• Microscopy: Clue cells (vaginal epithelial cells heavily coated with bacteria, obscuring border margins).

• Media: Human Blood Bilayer Tween (HBT) Agar; sensitive to SPS.

Branching / Fungus-Like Gram-Positive Actinomycetes

FAMILY ENTEROBACTERIACEAE

Gram-negative fermentative enteric bacilli. All members ferment glucose, reduce nitrate to nitrite (Exceptions: Photorhabdus, Xenorhabdus), and are cytochrome oxidase negative (Exception: Plesiomonas shigelloides). All are catalase positive (Exception: Shigella dysenteriae type 1) and motile via peritrichous flagella (Exceptions: Shigella, Klebsiella, and Yersinia pestis at $37\,^\circ\text{C}$).

Antigenic Architecture

• O Antigen (Somatic): Heat-stable lipopolysaccharide component of the outer cell wall.

• K Antigen (Capsular): Heat-labile polysaccharide envelope (e.g., K1 in E. coli, Vi in Salmonella typhi).

• H Antigen (Flagellar): Heat-labile protein components located on flagella.

Primary Diagnostic Biochemical Reactions

Triple Sugar Iron (TSI) Interpretations

• $A/A$ (Yellow Slant / Yellow Butt): Ferments glucose plus lactose and/or sucrose.

• $K/A$ (Red Slant / Yellow Butt): Ferments glucose only.

• $K/K$ (Red Slant / Red Butt): Non-fermenter.

• $\text{H}_2\text{S}$ Production: Black precipitation in butt.

• Gas Production: Cracks, bubbles, or displacement of agar medium.

Organism Profiles on TSI

• $A/A$ Gas (+), $\text{H}_2\text{S}$ (-): Escherichia, Klebsiella, Enterobacter.

• $A/A$ Gas (+), $\text{H}_2\text{S}$ (+): Citrobacter freundii.

• $K/A$ Gas (+), $\text{H}_2\text{S}$ (+): Salmonella, Proteus, Edwardsiella.

• $K/A$ Gas (-), $\text{H}_2\text{S}$ (-): Shigella, Providencia, Yersinia.

• $K/K$ Gas (-), $\text{H}_2\text{S}$ (-): Pseudomonas (Non-enterobacteriaceae).

IMViC Reaction Patterns

1. Indole: Tryptophanase converts tryptophan to indole. Ehrlich's or Kovac's reagent yields a red ring ($E. coli = +$).

2. Methyl Red (MR): Measures mixed acid fermentation pathways (\text{pH} < 4.5 yields red color).

3. Voges-Proskauer (VP): Detects acetoin production using $\alpha$-naphthol ($\text{Barritt's A}$) and $40\%\text{ KOH}$ ($\text{Barritt's B}$) yielding a pink-red color.

4. Citrate Utilization: Ability to use sodium citrate as sole carbon source on Simmons Citrate Agar ($\text{bromothymol blue}$ turns blue).

• $E. coli$: $++--$

• Klebsiella / Enterobacter: $--++$

Phenylalanine Deaminase (PAD) Test

Converts phenylalanine to phenylpyruvic acid. Addition of $10\%\text{ ferric chloride}$ ($\text{FeCl}_3$) yields an intense green color.

• PAD Positive Group: Proteus, Providencia, Morganella.

Genera and Species Profiles

Escherichia coli

• Indole positive, lactose fermenter (pink on MAC, green metallic sheen on EMB).

• Pathotypic Variants:

  • ETEC (Enterotoxigenic): Traveler's diarrhea ("Turista"); produces heat-labile (LT) and heat-stable (ST) enterotoxins.

  • EIEC (Enteroinvasive): Dysentery-like illness with bloody stool; positive Sereny test.

  • EPEC (Enteropathogenic): Infantile diarrhea outbreaks in nurseries.

  • EHEC (Enterohemorrhagic - E. coli O157:H7): Produces Shiga-like toxins (Verotoxins I & II); causes Hemorrhagic Colitis and Hemolytic Uremic Syndrome (HUS). Sorbitol-negative on MacConkey Sorbitol Agar (SMAC) and MUG-test negative.

  • UPEC (Uropathogenic): Leading cause of urinary tract infections.

  • MNEC (Meningitis/Sepsis): Expresses K1 capsular antigen.

Klebsiella Species

• Non-motile, heavily encapsulated (yielding mucoid colonies and a positive string test using $10\%\text{ KOH}$).

• Klebsiella pneumoniae: Causes severe pneumonia with "currant-jelly" sputum. Reaction: Indole negative, VP positive, Urease positive.

• Klebsiella oxytoca: Identical to K. pneumoniae, but Indole positive.

Enterobacter Species

• Motile, lactose fermenting. Ornithine Decarboxylase (ODC) positive.

• Enterobacter sakazakii (Cronobacter sakazakii): Produces yellow pigmentation; associated with neonatal meningitis from contaminated powdered milk formula.

Proteus, Providencia, and Morganella Group

• All are PAD positive.

• Proteus spp.: Displays swarming motility on blood agar; emits a distinct "gunpowder" or burnt sugar odor. Causes UTIs associated with renal struvite/staghorn stones due to potent urease activity.

  • Proteus mirabilis: Indole negative, $\text{H}_2\text{S}$ positive, Ampicillin/Cephalosporin susceptible.

  • Proteus vulgaris: Indole positive, $\text{H}_2\text{S}$ positive, Chloramphenicol resistant.

• Providencia spp.: Non-swarming; Indole positive, Citrate positive.

• Morganella morganii: Indole positive, Urease positive, Citrate negative, ODC positive.

Yersinia Species

• Yersinia pestis: Category A bioterrorism agent; cause of Plague ("Black Death") transmitted by the rat flea (Xenopsylla cheopis). Exhibits bipolar safety-pin staining with Wayson's stain. Produces "stalactite" growth in broth and "hammered copper" colonies on BAP. Non-motile at all temperatures.

• Yersinia enterocolitica: Cause of enterocolitis and pseudoappendicitis. Transmitted via contaminated milk/pork. Capable of cold enrichment ($4\,^\circ\text{C}$). Produces "bull's-eye" colonies on CIN agar. Motile at $25\,^\circ\text{C}$, non-motile at $37\,^\circ\text{C}$.

Salmonella Species

• Gram-negative rods, $\text{H}_2\text{S}$ positive, lactose non-fermenters, Indole negative, LDC positive.

• Salmonella typhi: Causes Typhoid Fever (producing "pea-soup" stools). Diagnostic timeline: Blood cultures positive in week 1; Stool/Urine positive in weeks 2--3; Bone marrow aspirate is the overall gold standard. Gallbladder serves as the primary chronic carrier reservoir. Widal test detects anti-O and anti-H antibodies.

Shigella Species

• Biochemically inert, non-motile, lactose non-fermenters, $\text{H}_2\text{S}$ negative. Causes Bacillary Dysentery.

• Serogroup Subdivisions:

  • Group A: Shigella dysenteriae (Catalase negative; produces Shiga toxin).

  • Group B: Shigella flexneri.

  • Group C: Shigella boydii.

  • Group D: Shigella sonnei (ONPG positive, ODC positive; late lactose fermenter).

Serratia Species

• Produces extracellular enzymes: DNase, Lipase, and Gelatinase positive.

• Serratia marcescens: Produces a characteristic red/pink pigment (prodigiosin) at room temperature.

NON-ENTERIC GRAM-NEGATIVE INTESTINAL PATHOGENS

Family Vibrionaceae

Gram-negative, curved/comma-shaped rods. Oxidase positive, Catalase positive, Indole positive. Display rapid darting / shooting-star motility. Positive String test (cell lysis in $0.5\%$ sodium desoxycholate yields a viscous thread). Halophilic (requires $\text{NaCl}$; Exceptions: V. cholerae and V. mimicus).

Vibrio cholerae

• Non-halophilic. Causative agent of Cholera, characterized by severe dehydration and "rice-water" stools.

• Cholera Toxin (Choleragen): Hyperstimulates adenylate cyclase, increasing intracellular $\text{cAMP}$ to cause massive mucosal water and electrolyte secretion.

• Growth on TCBS Agar: Sucrose fermenter yielding yellow colonies.

• Biotype Differentiation (Serogroup O1):

  • Classical Biotype: Polymyxin B ($50\text{ U}$) susceptible, VP negative, non-hemolytic, chicken RBC agglutination negative.

  • El Tor Biotype: Polymyxin B resistant, VP positive, $\beta$-hemolytic, chicken RBC agglutination positive.

Other Vibrio Species

• Vibrio parahaemolyticus: Halophilic; primary cause of seafood-borne gastroenteritis ("summer diarrhea"). Sucrose non-fermenter on TCBS (green colonies). Kanagawa Positive strains demonstrate $\beta$ hemolysis on Wagatsuma high-salt agar.

• Vibrio vulnificus: Halophilic; lactose-fermenting Vibrio. Causes septicemia and necrotizing wound infections following marine exposure in patients with liver disease.

• Vibrio alginolyticus: Highly halophilic ($8\text{--}10\%\text{ NaCl}$); causes ear and wound infections.

Aeromonas and Plesiomonas

• Aeromonas hydrophila: Freshwater organism; oxidase positive, fermentative. Causes diarrhea and "red-leg disease" in frogs. Resistant to vibriostatic compound 0/129.

• Plesiomonas shigelloides: Member of Enterobacteriaceae that is oxidase positive. Sensitive to 0/129. Positive for Lysine, Ornithine, and Arginine (Tri-decarboxylase positive).

Helicobacter pylori

• Microaerophilic, curved Gram-negative rod inhabiting human gastric mucosa. Causes gastritis, peptic ulcers, and gastric carcinoma.

• Diagnostic Profile: Potent urease activity. Diagnosed via Urea Breath Test, stool antigen assays, or endoscopic gastric biopsy stained with Warthin-Starry silver stain.

Campylobacter Species

• Microaerophilic ($5\%\text{ }\text{O}_2$, $10\%\text{ }\text{CO}_2$), curved/spiral rods exhibiting "seagull-wing" morphology and darting motility.

• Campylobacter jejuni: Leading bacterial cause of gastroenteritis; associated with post-infectious Guillain-Barré syndrome. Grows optimally at $42\,^\circ\text{C}$ on Campy-BAP or Skirrow's media. Hippurate Hydrolysis positive; Nalidixic Acid susceptible, Cephalothin resistant.

• Campylobacter fetus: Causes bacteremia and animal abortion; grows at $37\,^\circ\text{C}$, fails to grow at $42\,^\circ\text{C}$. Nalidixic Acid resistant, Cephalothin susceptible.

GRAM-NEGATIVE NON-FERMENTATIVE BACILLI

Obligate aerobes that fail to ferment carbohydrates. Yield $\text{K/K}$ (alkaline) reactions on TSI. Characterized using Hugh-Leifson Oxidation-Fermentation (OF) media.

Pseudomonas aeruginosa

• Motile, oxidase positive, catalase positive Gram-negative rod. Grows at $42\,^\circ\text{C}$.

• Pigment Production: Pyocyanin (blue-green), Pyoverdine/Fluorescein (yellow-green, fluorescent), Pyorubin (red), Pyomelanin (brown/black).

• Odor: Emits a distinct fruity grape-like or corn taco odor (due to 2-aminoacetophenone).

• Clinical Pathologies: Major cause of cystic fibrosis pneumonia, burn wound sepsis, swimmer's ear (otitis externa), hot-tub folliculitis, and ecthyma gangrenosum.

Burkholderia Species

• Burkholderia cepacia: Causes respiratory infections in cystic fibrosis patients; emits an earthy/dirt-like odor. Grows on PC/OFPBL agar.

• Burkholderia mallei: Non-motile agent of Glanders in horses.

• Burkholderia pseudomallei: Agent of Melioidosis ("Vietnamese Time Bomb"). Produces wrinkled, dry colonies on Ashdown agar and displays safety-pin bipolar staining.

Stenotrophomonas maltophilia

• Oxidase negative, DNase positive, Lysine Decarboxylase positive. Strong oxidizer of maltose.

Acinetobacter baumannii

• Gram-negative coccobacillus ("Iraqibacter"); non-motile, oxidase negative, catalase positive. Forms purple-tinted colonies on MacConkey agar.

FASTIDIOUS GRAM-NEGATIVE BACILLI

Genus Haemophilus

Requires growth factors: X Factor (Hemin) and/or V Factor (NAD/Coenzyme I). Demonstrates satellitism around Staphylococcus aureus on blood agar.

Haemophilus influenzae (Pfeiffer's Bacillus)

• Requires BOTH X and V factors. Non-hemolytic on horse blood.

• Serotype b: Encapsulated with Polyribosylribitol Phosphate (PRP); primary cause of pediatric meningitis, epiglottitis, and pneumonia.

• H. influenzae biogroup aegyptius causes Brazilian Purpuric Fever.

Haemophilus aegyptius (Koch-Weeks Bacillus)

• Requires X and V factors; causes acute contagious purulent conjunctivitis ("pink eye").

Haemophilus ducreyi

• Requires X factor ONLY (V factor independent). Causative agent of Chancroid (painful genital ulcers with suppurative buboes).

• Microscopy: Gram-negative rods arranged in parallel chains described as a "school of red fish" or "railroad track" pattern.

Growth Factor Requirement Table

Note: Porphyrin/ALA test determines the ability to synthesize hemin independently. Porphyrin-positive organisms do not require exogenous X factor.

HACEK GROUP ORGANISMS

Dysgonic Gram-negative bacilli that constitute normal oral biota; primary causes of HACEK-driven subacute infective endocarditis:

1. Aggregatibacter aphrophilus (formerly H. aphrophilus): Requires elevated $\text{CO}_2$ ($5\%$).

2. Aggregatibacter actinomycetemcomitans: Forms star-shaped / "dots and dashes" center colonies. Catalase positive.

3. Cardiobacterium hominis: Forms characteristic rosette arrangements on Gram stain.

4. Eikenella corrodens: Pits/corrodes agar surfaces; emits a distinct bleach-like odor. Associated with human bite wounds ("clenched-fist injuries"). Assaccharolytic.

5. Kingella kingae: Demonstrates twitching motility; pits agar; causes pediatric septic arthritis and endocarditis.

MISCELLANEOUS GRAM-NEGATIVE BACILLI

• Brucella Species: Category B bioterrorism agent. Obligate intracellular pathogen causing Undulant Fever / Malta Fever. Transmitted via unpasteurized dairy or direct animal contact. Cultured in biphasic Castaneda bottles for up to $21\text{ days}$.

  • B. abortus (Cattle): Requires $\text{CO}_2$; $\text{H}_2\text{S}$ positive; inhibited by Thionine.

  • B. melitensis (Goats/Sheep): No $\text{CO}_2$ required; $\text{H}_2\text{S}$ negative; grows in Thionine and Fuchsin.

  • B. suis (Pigs): No $\text{CO}_2$ required; $\text{H}_2\text{S}$ positive; inhibited by Basic Fuchsin.

  • B. canis (Dogs): No $\text{CO}_2$ required; $\text{H}_2\text{S}$ negative.

• Francisella tularensis: Category A bioterrorism agent. Fastidious intracellular rod causing Tularemia / Rabbit Fever. Requires cysteine/cystine enrichment (Blood Cysteine Glucose Agar). Diagnosed skin test: Forshay test.

• Legionella pneumophila: Causative agent of Legionnaires' Disease and Pontiac Fever. Transmitted via aerosolized water systems/cooling towers. Requires L-cysteine and iron on BCYE agar (yields grey-blue "cut-glass" colonies). Stained with Dieterle silver stain. Detected via urine antigen assays.

• Bordetella pertussis: Encapsulated cause of Whooping Cough (Catarrhal, Paroxysmal with repetitive coughing fits, and Convalescent stages). Isolated on Bordet-Gengou or Regan-Lowe media (producing pearl-like / mercury-droplet colonies).

• Pasteurella multocida: Gram-negative rod with safety-pin bipolar staining, isolated following cat or dog bites. Emits a musty/mushroom odor. Oxidase positive, Catalase positive, Indole positive.

• Streptobacillus moniliformis: Causes Rat-Bite Fever and Haverhill Fever. Produces "fluff-ball" / "breadcrumb" growth in broth and "fried-egg" colonies on agar.

• Klebsiella granulomatis (Calymmatobacterium): Agent of Granuloma Inguinale (Donovanosis). Demonstrates Donovan bodies (cluster of pleomorphic rods within mononuclear cell vacuoles).

SPIROCHETES

Helical Gram-negative spiral bacteria propelled by periplasmic axial filaments. Stained using silver impregnation methods (Levaditi, Fontana, Warthin-Starry) and visualized under dark-field microscopy.

Treponema pallidum subsp. pallidum

Causative agent of Venereal Syphilis. Obligate human intracellular parasite.

Clinical Stages

1. Primary Syphilis: Hard, painless chancre at site of inoculation. Diagnostic via dark-field microscopy of lesion aspirate.

2. Secondary Syphilis: Generalized maculopapular rash and condyloma lata (moist mucosal warts).

3. Latent Syphilis: Asymptomatic period.

4. Tertiary Syphilis: Gummas (granulomatous lesions), Neurosyphilis (Tabes dorsalis), and cardiovascular aortitis.

5. Congenital Syphilis: Transplacental infection yielding Hutchinson's Triad (Hutchinson's notched teeth, Interstitial keratitis, and 8th nerve deafness).

Serological Testing

• Nontreponemal Tests (Screening): Detects anti-cardiolipin regain antibodies (RPR, VDRL).

• Treponemal Tests (Confirmatory): Detects specific antibodies against T. pallidum (FTA-ABS, TP-PA, MHA-TP).