bacteriology things i need to memorize

What are the 3 bacterial coats?

Plasma membrane, cell wall, and capsule

What are the external appendages?

Flagella and pili/fimbriae

What is the plasma membrane composed of and what are its functions?

Composition: A bilayer of phospholipoprotein, similar to eukaryotic cells but lacking sterols.

Functions:

• Selective transport of molecules in and out of the cell.

• Excretion of extracellular enzymes (ex: hydrolytic enzymes for digesting large molecules, enzymes that degrade antibiotics like penicillinase).

• Respiration: Contains respiratory enzymes, functioning analogously to mitochondria in eukaryotes.

• Cell wall biosynthesis: Houses enzymes essential for cell wall synthesis.

• Reproduction: helps in cell division.

• Chemotactic system: Receptors for attractants and repellants are located here, initiating cellular responses.

Where is the cell wall located? What is the structure and function of it? What is a peptidoglycan?

Location: surrounds the cytoplasmic membrane.

Structure: 10-25 nm thick, strong, rigid with some elasticity. Its strength is due to peptidoglycan.

Peptidoglycan: A unique bacterial polymer composed of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). Tetrapeptide side chains are attached to NAM. This structure is unique to bacteria.

• Components of the tetrapeptide side chain include L-alanine, D-glutamine, L-lysine, and D-alanine.

Functions:

• Provides shape and rigidity to the bacterium.

• Prevents osmotic lysis (bursting due to water influx).

• Plays a role in cell division.

• Determines Gram staining characteristics.

What is the capsule composed of? What ode it look like and how is it formed? What are its functions?

Composition: Primarily polysaccharide, occasionally polypeptide.

Appearance: Often appears as an unstained halo with Gram stain.

Formation: Formed only inside the host (in vivo).

Functions:

• Attachment to surfaces.

• Protection against phagocytosis by immune cells (a virulence factor).

• Protection against desiccation (drying out).

What does flagella look like? What is it composed of and its function? What is its arrangement and structure?

Description: Long, whip-like appendages used for motility. Can only be seen with an electron microscope.

Composition: Made of protein called flagellin. Flagellin is highly antigenic and forms the "H" antigen of bacteria.

Function: Organ of motility.

Arrangement: Can be monotrichous (single polar), lophotrichous (multiple polar), or peritrichous (distributed over the entire cell).

Structure: Consists of three parts: filament, hook, and basal body (motor). The basal body anchors the flagellum and rotates to propel the bacterium.

What does the pili/fimbriae look like? What is it composed of and what are its functions?

Description: Hair-like appendages, thinner and shorter than flagella.

Composition: Made of protein called pillin.

Functions:

• Adherence: Attach the organism to host surfaces, acting as a virulence factor.

• Conjugation: A special, longer pilus called the sex pilus (F pilus) is involved in the transfer of DNA between bacteria.

What is a cytoplasm and nucleoid?

Cytoplasm: The gel-like substance filling the cell, enclosed by the plasma membrane.

Nucleoid:

• Composition: Contains the bacterial chromosome, which is typically a single, circular DNA molecule.

• Function: Houses the genetic material of the bacterium.

What are plasmids and ribosomes?

Plasmids:

• Composition: Extrachromosomal, circular DNA molecules.

• Function: Carry non-essential genes that can provide advantages such as antibiotic resistance or toxin production.

Ribosomes:

• Composition: Made of protein and RNA.

• Function: Site of protein synthesis.

• Characteristics: Bacterial ribosomes are 70S (composed of 50S and 30S subunits), which is distinct from eukaryotic ribosomes (80S). This difference makes bacterial ribosomes a selective target for antibiotics.

What are inclusion granules and mesosomes?

Inclusion Granules:

• Function: Serve as nutrient or energy reserve granules (e.g., for phosphate storage).

Mesosomes:

• Description: Inward invaginations of the cytoplasmic membrane into the cytoplasm.

• Functions: Historically thought to be involved in cell division, sporulation, and bacterial respiration (analogous to mitochondria), though their exact role is debated.

What is the procedure for gram staining?

Procedure:

1. Application of Crystal Violet (primary stain, purple dye).

2. Application of Iodine (mordant, fixes the dye).

3. Alcohol wash (decolorization).

4. Application of Safranin (counterstain, pale red dye).

What does gram-positive bacterial look like? What is its cell wall structure?

Gram Stain Appearance: Retain the crystal violet stain and appear purple.

Cell Wall Structure:

• Peptidoglycan: Thick layer (about 50% of cell wall thickness, ~40 sheets).

• Teichoic Acids: Polymers of ribitol or glycerol phosphate. They are major surface antigens and are linked to peptidoglycan and the plasma membrane (lipoteichoic acids).

• Outer Membrane: Absent.

What does gram-negative bacterial look like? What is its cell wall structure?

Gram Stain Appearance: Lose the crystal violet stain during alcohol wash and are counterstained by safranin, appearing pale red.

Cell Wall Structure:

• Peptidoglycan: Thin layer (about 5-10% of cell wall thickness, ~1-2 sheets).

• Outer Membrane: Present. This is a complex layer external to the peptidoglycan.

  • Lipopolysaccharide (LPS): A major component of the outer membrane.

    • Lipid A: The toxic component, known as endotoxin.

    • Polysaccharides: Including the O antigen (surface antigen).

  • Porins: Proteins embedded in the outer membrane that regulate the passage of small molecules.

• Periplasmic Space: The region between the plasma membrane and the outer membrane, containing the thin peptidoglycan layer.

What is the origin of endotoxin/lipid A? Where is it released? What is its toxicity? What are its effects?

Origin: A component of the LPS in the cell walls of Gram-negative bacteria.

Release: set free when the bacterial cell wall lyses.

Toxicity: Associated with the Lipid A component.

Effects:

• Activates macrophages, causing fever.

• Activates the complement cascade, leading to inflammation (e.g., via TNF-α and IL-1).

• Activates blood factors, potentially causing disseminated intravascular coagulation (DIC) and hemorrhage.

• Can lead to septic shock, organ failure, and death.

What are the differences between exotoxins and endotoxins?

Exotoxins: Secreted by living bacteria (mainly Gram-positive), are proteins, and have specific actions.

Endotoxins: Integral part of Gram-negative cell walls (LPS), are lipopolysaccharides (Lipid A), and have generalized effects (fever, shock).

What is planktonic growth? What is biofilm growth?

Planktonic Growth: Free-floating, individual bacterial cells suspended in a liquid medium.

Biofilm Growth:

• Description: Organized communities of bacteria encased in a self-produced matrix of extracellular polymeric substances (EPS).

• Formation: Bacteria adhere to a surface and begin to multiply, producing the EPS matrix.

• Role in Disease: Biofilms are often associated with chronic infections because the bacteria within them are more resistant to antibiotics and host immune defenses.

What are endospores? How are they formed? What is germination in terms of endospores?

Definition: A resting form of certain bacteria (e.g., Bacillus and Clostridiumgenera) that allows survival under adverse environmental conditions. Endospores do not grow or reproduce.

Formation (Sporulation): Occurs when environmental conditions become unfavorable. The cell undergoes asymmetric division, and a protective cortex and outer coat are formed around a core containing the bacterial DNA.

Germination: The process by which an endospore returns to its vegetative (actively growing) state when favorable conditions are restored. This can happen rapidly (within 15 minutes).

What are endospores resistant to? What are the mechanisms of resistance?

Resistance: Endospores are highly resistant to:

• Heat (including autoclaving).

• Disinfectants.

• Drying.

• Radiation.

Mechanisms of Resistance:

• High content of calcium + dipicolinic acid.

• Impermeability of the cortex and outer coat.

• Low water content.

• Very low metabolic activity.

What is the problem with clostridium difficiles endospores?

Its endospores are highly resistant to cleaning agents and can survive in hospitals, leading to nosocomial infections (hospital-acquired infections), particularly antibiotic-associated diarrhea. The endospore can germinate into vegetative cells in the gut when antibiotics disrupt the normal flora.

What is the difference between direct and indirect methods of diagnosing bacterial infections?

Direct Methods: These methods involve the direct examination and testing of the clinical specimen.

Indirect Methods: These methods detect the host's response to the infection, rather than the pathogen itself.

What are the 4 steps in the direct methods?

Specimen collection

Microscopic examination

Cultivation

Biochemical identification

What are the main things to look out for when collecting a specimen?

Timing: Collect specimens before starting antibiotic treatment.

Appropriateness: Choose the specimen that best represents the site of infection.

Sterility: Use sterile containers and avoid contaminating the specimen.

Transport: Transport the specimen to the laboratory as early as possible and under appropriate conditions.

Microscopic examination: why do we stain or not stain the specimen? What information do we get from this?

Unstained Preparations (Wet Mount): Used to observe bacterial motility.

Stained Preparations: Used to visualize bacterial morphology and staining characteristics.

Information Gained:

• Bacterial Morphology: Size, shape (cocci, bacilli, spiral), and special arrangements.

• Staining Affinity: Gram staining (Gram-positive/-negative), Ziehl-Neelsen staining (acid-fast bacilli), India Ink stain (Cryptococcus), Silver stain (Pneumocystis jirovecii).

• Special Features: Spore formation, capsule formation.

Microscopic examination: What are the key stains used in microbiological diagnosis?

Gram Stain: Differentiates bacteria into Gram-positive (purple) and Gram-negative (pink/red) based on cell wall composition.

Ziehl-Neelsen Stain: Used for acid-fast bacilli (e.g., Mycobacteria) due to their high lipid content in the cell wall.

Cultivation: what are the types of culture media?

Fluid Media: Growth is observed as turbidity.

Solid Media: Growth appears as colonies (macroscopic products of bacterial division). Solid media are generally preferred for isolation, identification, and susceptibility testing.

Cultivation: what is simple media?

Simple Media: Support the growth of a wide range of non-fastidious bacteria (e.g., Nutrient agar, Nutrient broth).

Cultivation: what is enriched media?

Enriched Media: Contain extra nutrients to support the growth of fastidious organisms:

• Blood Agar: used for observing hemolytic reactions.

  • Beta (β) Hemolysis: Complete lysis of red blood cells, resulting in a clear zone. (Streptococcus pyogenes)

  • Alpha (α) Hemolysis: Partial lysis of red blood cells, resulting in a greenish discoloration. (Streptococcus pneumoniae)

  • Gamma (γ) Hemolysis (Non-hemolytic): No lysis of red blood cells, no change in the agar. (Streptococcus bovis)

• Chocolate Agar: Contains lysed red blood cells (hemoglobin and hemin), supporting the growth of fastidious organisms like Neisseria meningitidis.

Cultivation: what is selective media?

Selective Media: Contain inhibitors that prevent the growth of unwanted organisms, allowing specific pathogens to grow (Lowenstein Jensen (L.J.) medium for M. tuberculosis).

Cultivation: what is selective and differential media?

Selective and Differential Media: Combine selective properties with indicators to differentiate between different types of bacteria based on their metabolic capabilities.

• MacConkey's Agar: Selective for Gram-negative bacilli.

  • Contains bile salts (selective) and lactose with a pH indicator (differential).

  • Lactose Fermenters: Produce pink colonies (e.g., E. coli).

  • Lactose Non-Fermenters: Produce colorless or yellow colonies (e.g., Salmonella).

What tests are used to see a biochemical reaction to identify the specimen?

Catalase Test: Differentiates catalase-positive organisms (Staphylococci) from catalase-negative organisms (Streptococci). A positive test shows rapid bubble formation due to oxygen production from the breakdown of hydrogen peroxide (H2​O2​).

Coagulase Test: Differentiates Staphylococcus aureus (coagulase-positive) from Coagulase-Negative Staphylococci (CONS) (S. epidermidis). Coagulase converts plasma fibrinogen to fibrin, forming a visible clot.

Analytical Profile Index (API): A system of biochemical tests in a standardized format for rapid bacterial identification. Results are read and entered into a database for interpretation.

What are the indirect methods used?

Serological Methods: Detect the presence or rising titers of antibodies (IgM, IgG) in the patient's serum. A rising titer of IgG antibodies over time is often indicative of a current or recent infection.

Other Methods: Include skin tests (for tuberculosis) and specific tests like QuantiFERON-TB.

Antibiotic susceptibility test: what is the disc diffusion method (Kirby-Bauer)?

Principle: A lawn of bacteria is spread on an agar plate, and paper discs impregnated with different antibiotics are placed on the surface.

Interpretation: After incubation, zones of inhibition (clear areas where bacterial growth is inhibited) are measured. Larger zones indicate greater susceptibility.

Categories: Susceptible, Intermediate, Resistant.

Antibiotic susceptibility test: what is the tube dilution method? What is the MIC?

Purpose: To determine the Minimal Inhibitory Concentration (MIC).

MIC: The lowest concentration of an antibiotic that inhibits the visible growth of a microorganism after overnight incubation.

Procedure: Serial dilutions of an antibiotic are prepared in broth or agar. Each dilution is inoculated with the test organism. The MIC is the highest dilution (lowest concentration) showing no visible growth.

Antibiotic susceptibility test: what is an E-test?

Purpose: For determination of MIC.

Principle: A plastic strip containing a gradient of an antibiotic is placed on an inoculated agar plate.

Interpretation: After incubation, an elliptical zone of inhibition is observed. The MIC is read at the point where the ellipse intersects the strip's concentration scale.

When is a non-culture technique used?

Non-culture techniques are particularly valuable when:

Rapid diagnosis is required.

The microorganism is difficult, impossible, or dangerous to culture on artificial media.

The pathogen is a slow-growing organism.

What are the 2 non-culture techniques?

Identification of a Specific Microbial Antigen: This involves detecting a structural component or a product of the microorganism by reacting it with a specific antibody.

Identification of a Specific Gene Sequence (Nucleic Acid): This is achieved using various molecular methods like Polymerase Chain Reaction (PCR) or DNA probes.

What is seen in Ag-Ab reactions?

An antibody will only bind to the specific antigen that elicited its production. This specificity allows for the identification of one reactant if the other is known.

Direct non-culture method (serology): what reactions are accompanied with a visual reaction?

Agglutination Reactions: Involve the clumping of particulate antigens (bacteria) by antibodies. This is a visible reaction.

Direct non-culture method (serology): what reactions are detected by labeled reagents? (IF)

Immunofluorescence (IF): Uses fluorescent dyes attached to Ab to visualize Ag-Ab complexes.

Direct IF: A known, fluorescently labeled Ab is used to detect an unknown Ag directly in a specimen (rabies virus in brain tissue).

• Specimen is fixed to a slide.

• Known fluorescein-labeled Ab is added.

• Fluorescence is observed under a fluorescence microscope if the Ag is present.

Indirect IF: A known Ag is fixed to a slide, and the patient's serum (containing unknown Ab) is added. Then, a fluorescently labeled anti-human globulin is added. Fluorescence indicates the presence of Ab in the patient's serum.

• Detects Ab in serum, thus indirectly diagnosing disease.(Syphilis diagnosis using Treponema pallidum antigen)

Direct non-culture method (serology): what reactions are detected by labeled reagents? (ELISA)

Enzyme-Linked Immunosorbent Assay (ELISA): Uses enzymes conjugated to Ab or Ag, which produce a color change when a substrate is added.

• Sensitivity and Specificity: ELISA is known for its high sensitivity and specificity.

• Quantitative Potential: The intensity of the color change, measured by a spectrophotometer, is proportional to the amount of Ag or Ab, allowing for quantitative measurement.

• Direct ELISA (Ag Detection): An enzyme-labeled Ab is used to detect Ag in a sample.

• Indirect ELISA (Ab Detection): Known Ag is put on a plate, then add the patient's serum. An enzyme-labeled anti-human immunoglobulin is then added.

What is the principle and technique of identifying specific gene sequences (molecular methods)?

Principle: Detects the presence of specific DNA or RNA sequences unique to the pathogen.

Techniques: PCR (Polymerase Chain Reaction), DNA probes.

Indirect (serologic) methods: what is it used for? What are skin tests?

Principle: Known microbial antigens are used to detect antibodies against the microorganism in the patient's serum.

Skin Tests: Based on cell-mediated hypersensitivity. These tests indicate past exposure and an intact immune response (Tuberculin test).

Indirect (serologic) methods: what is the Ab detections based on? What is rising titre?

Current/Active Infection Diagnosis: Typically based on:

• Detection of Specific IgM Antibodies: IgM is the first antibody produced during an infection and indicates a recent or active infection.

• Rising Titre of Specific IgG Antibodies: A significant increase (e.g., a 4-fold or greater increase) in IgG antibody levels between two serum samples taken at different times indicates an active immune response to an ongoing infection.

• A Single High Titre of IgG Antibodies: In certain diseases, a very high IgG titer in a single sample can be indicative of an active infection.

What infection is part of the streptococcus genus and is found as resident flora in the nose and throat?

Streptococci

What is the morphology f streptococci?

Gram-positive cocci, typically arranged in chains or pairs.

What is the result in the catalase test for streptococci?

Catalase-negative. This is a key differentiator from Staphylococcusspecies, which are catalase-positive.

What are the 2 ways to classify streptococci?

1. Lancefield Groups: Based on specific carbohydrate antigens present in the cell wall.

   • Specific antibodies against these carbohydrate group antigens are used for identification.

2. Hemolytic Reactions on Sheep Blood Agar:

   • Complete (beta) hemolysis: Red blood cells are completely lysed around the colonies (S. pyogenes). Caused by hemolysins that destroy RBCs.

   • Partial (alpha) hemolysis: Incomplete lysis of red blood cells, resulting in a greenish discoloration around the colonies due to the formation of methemoglobin or biliverdin (S. pneumoniae).

   • Non-hemolytic: No lysis of red blood cells (S. bovis).

What are the microscopic and biochemical characteristics of streptococcus pyogenes?

Gram-positive cocci, arranged in chains.

Catalase-negative.

Requires enriched media with blood for growth.

Exhibits beta-hemolysis on sheep blood agar.

Virulence factors off S. pyogenes: what are the factors mediating adherence (colonization)?

M protein: A surface protein crucial for colonization of the skin and evasion of phagocytosis. It is highly immunogenic and defines approximately 80 M serotypes of S. pyogenes.

Fibronectin-binding protein (Protein F): Aids in adherence to host tissues.

Lipoteichoic acids (LTAs): Also involved in adherence to host cells.

Virulence factors of S. pyogenes: what are the antiphagocytic factors that mediate invasion and evasion?

M protein: Hinders phagocytosis.

Hyaluronic acid capsule: Chemically similar to host connective tissue, making it non-immunogenic and allowing the bacterium to evade recognition by the host immune system.

Virulence factors of S. pyogenes: what are the enzymes that mediate invasion and evasion?

C5a peptidase/protease: Degrades C5a, a chemoattractant for phagocytes, thus reducing neutrophil recruitment to the site of infection.

Streptokinase (fibrinolysin): Breaks down fibrin clots, facilitating the spread of bacteria. It has therapeutic uses in dissolving blood clots (myocardial infarction).

Streptolysins: Pore-forming toxins that lyse host cell membranes.

• Streptolysin O: Highly immunogenic; detection of antibodies against it (ASOT) is used diagnostically.

• Streptolysin S: Non-immunogenic.

Hyaluronidase: Degrades hyaluronic acid in connective tissue, promoting bacterial spread.

Nucleases: Degrade DNA and RNA, helping to break down cellular debris and facilitate spread.

Virulence factors of S. pyogenes: what are the exotoxins that mediate invasion and evasion?

Streptococcal Pyrogenic Exotoxins (SPEs - A, B, C): Responsible for scarlet fever, sepsis, toxic shock syndrome, and necrotizing fasciitis. SPE-A is associated with scarlet fever, while SPE-B is a protease involved in necrotizing fasciitis. SPE-A and SPE-C are associated with toxic shock syndrome.

What are the main infections that are caused by S. pyogenes?

Pharyngitis (Sore Throat/Tonsillitis):

• Most common streptococcal infection.

• Transmitted via droplets.

Scarlet Fever:

• when pharyngitis is caused by an S. pyogenes strain producing an erythrogenic toxin (SPE-A).

• scarlet red rash and a "strawberry tongue."

Skin and Soft Tissue Infections:

• Impetigo (Pyoderma): infection of superficial skin layers, leading to blisters and crusted sores.

• Cellulitis: Infection of the deeper layers of the skin.

Invasive Streptococcal Infections: severe, life-threatening infections.

• Puerperal Fever: Infection of the endometrium and surrounding structures following childbirth or abortion, which can lead to sepsis and toxic shock syndrome.

• Acute Endocarditis: Infection of the heart valves, rapid onset and high fatality.

• Necrotizing Fasciitis: rapidly progressing infection causing extensive tissue destruction, referred to as "flesh-eating bacteria." Associated with SPE-B (protease).

• Streptococcal Toxic Shock Syndrome (STSS): A severe complication of invasive infections, often starting with skin or soft tissue infection. Characterized by shock, renal failure, and acute respiratory distress syndrome (ARDS). Associated with SPE-A and SPE-C.

What are the laboratory diagnosis used for identifying S. pyogenes infections?

Specimens: throat swabs pus, high vaginal swab, blood

Direct detection: gram stained smear and rapid Ag detection test

Cultivation: blood agar and blood culture technique

Identification: colony morphology, gram stained smear, catalase test, and lancefield grouping.

Lab diagnosis of S. pyogenes: what do we used for direct detection?

Gram-stained smear: To visualize Gram-positive cocci in chains.

Rapid Antigen Detection Tests (RADT): Detect streptococcal group A antigen in throat swabs, providing results in minutes.

Lab diagnosis of S. pyogenes: what do we used for cultivation?

Blood Agar: Specimens (except blood) are plated on blood agar and incubated at 37°C. Colonies of S. pyogenes are typically surrounded by beta-hemolysis.

Blood Culture Technique: For suspected bacteremia. A large volume of blood (5-10 ml) is added to a broth medium and incubated at 37°C. Subcultures are performed every 48 hours for up to 10 days to increase the chance of isolating low numbers of pathogens.

Lab diagnosis of S. pyogenes: what do we used for identification?

Colony Morphology: Beta-hemolytic colonies on blood agar.

Gram Stain: Gram-positive cocci in chains.

Catalase Test: Negative.

Lancefield Grouping: Serological tests (using specific antibodies) to confirm Group A antigen.

Post strep sequelae: acute rheumatic fever (ARF) what is this pathogenesis, diagnosis, and prevention of recurrence?

Follows pharyngeal infections caused by specific "rheumatogenic" M types (1, 3, 5, 6, 18).

Pathogenesis: Antibodies produced against S. pyogenes M protein cross-react with epitopes on host tissues, particularly in the heart, joints, and brain, leading to inflammation and damage.

Diagnosis: Based on the Modified Jones Criteria, requiring evidence of recent S. pyogenes infection (elevated ASO titer, positive throat culture) plus either:

• Two major criteria.

• One major criterion and two minor criteria.

• Major Criteria: Carditis (valvulitis, pericarditis, myocarditis), Migratory polyarthritis, Erythema annulare, Subcutaneous nodules, Sydenham's chorea.

• Minor Criteria: Fever, Arthralgias/arthritis, Elevated ESR, Positive CRP, Leukocytosis, Prior history of Rheumatic Fever.

Prevention of Recurrence: Life-long antibiotic prophylaxis (penicillin) is recommended to prevent recurrent rheumatic fever and subsequent valvular damage.

Post strep sequelae: Acute glomerulonephritis (AGN) what is its pathogenesis, diagnosis, and recurrence?

Can follow pharyngeal or skin infections caused by "nephritogenic" strains (M types 4, 25 for throat infections; M types 2, 60 for skin infections).

Pathogenesis: Deposition of antigen-antibody complexes on the basement membrane of kidney glomeruli, leading to inflammation and damage.

Diagnosis: Clinical presentation and evidence of recent streptococcal infection.

Recurrence: Uncommon. Antibiotic prophylaxis is generally not necessary after an initial attack.

What is the treatment used for S. pyogenes?

Antibiotics: Penicillin is the drug of choice for S. pyogenes infections, typically administered for 10 days.

Penicillin Allergy: Erythromycin can be used for patients allergic to penicillin.

Chemoprophylaxis: Long-acting penicillin is used for chemoprophylaxis against recurrent S. pyogenes infections, particularly to prevent repeated attacks of rheumatic fever and subsequent cardiac damage.

Importance of Treatment: Prompt identification and treatment of S. pyogenesinfections are crucial to prevent the development of serious post-streptococcal sequelae like ARF and AGN.

What is the difference between commensals and pathogens?

Commensals: Microorganisms that live in or on a living host without causing harm. They may even provide benefits to the host. The a lot of these microorganisms in the body is called microbiota or normal flora.

Pathogens: Bacteria that always cause disease, regardless of the host's condition. They have an inherent ability to breach host defenses.

What are opportunistic pathogens?

These are commensals or other bacteria that do not typically cause disease under normal conditions.

They can cause disease if:

• The host is immunocompromised.

• They gain access to a site other than their normal habitat (e.g., through injury or a breach in sterile barriers).

Examples include Staphylococcus aureus causing wound infections or Streptococcus pneumoniae causing pneumonia when inhaled into the lungs.

What is symbiosis? What are the 3 types of a symbiotic relationships?

Symbiosis describes the relationship between two different organisms living in close physical association. The types of symbiotic relationships are:

Mutualism (++): Both organisms benefit.

Commensalism (+ 0): One organism benefits, and the other is unaffected.

Parasitism (+ -): One organism (the parasite) benefits at the expense of the other (the host).

What is the infectious process from infected person to someone else?

Reservoir: The natural habitat of the infectious agent (humans, animals, soil).

Portal of Exit: The means by which the infectious agent leaves the reservoir (respiratory secretions, feces).

Mode of Transmission: The method by which the infectious agent is transferred from the reservoir to a susceptible host (direct contact, indirect contact, airborne).

Portal of Entry: The means by which the infectious agent enters the susceptible host (respiratory tract, gastrointestinal tract, genitourinary tract, skin).

Susceptible Host: An individual who is at risk of infection (elderly, infants, immunocompromised individuals).

What are the 5 stages of infection?

1. Source of Infection: The reservoir from which the pathogen originates.

2. Mode of Transmission: How the pathogen travels from the source to the host.

3. Portal of Entry: How the pathogen enters the host.

4. Multiplication of Organisms: The pathogen must multiply within the host to establish an infection.

5. Portal of Exit: The means by which the pathogen leaves the host to potentially infect others.

What are the outcomes of infection? What is the difference between subclinical infections and diseases?

Microbial Factors (Virulence): The ability of the pathogen to cause damage.

Host Resistance Factors (Immunity): host's ability to fight off the infection.

Environmental Factors: External influences that can affect the interaction.

Most infections are subclinical (or silent/abortive), meaning they occur without noticeable symptoms.

Disease occurs when there is actual damage to host tissues due to microbial invasion or toxin production.

What is pathogenicity? What is virulence? What is virulence factors?

Pathogenicity is the qualitative ability of an organism to produce disease.

Virulence is the quantitative measure of pathogenicity, indicating the degree of pathogenicity or the severity of the disease caused. Virulence is determined and can be carried on plasmids or chromosomes.

Virulence factors are structures or products that enable a microorganism to cause disease.

What are adhesive virulence factors?

Proteins that enable bacteria to attach to receptors on host surfaces, crucial for colonization and establishing infection.

• Fimbriae (in Escherichia coli): Help bacteria attach to the urinary tract lining.

• Glycocalyx (in Viridans Streptococci): Can mediate attachment to heart valves.

What are invasive virulence factors?

Mechanisms that help bacteria invade host tissues and spread.

Enzymes:

• Collagenase and Hyaluronidase: Break down connective tissue.

• IgA Protease: Degrades IgA antibodies, which are important in mucosal immunity.

• Leukocidin: Kills white blood cells (phagocytes).

• Deoxyribonuclease (DNase): Breaks down DNA, potentially from host cell debris or other bacteria.

• Lecithinase: Degrades cell membranes.

Antiphagocytic Factors: Mechanisms that help bacteria evade phagocytosis by immune cells.

• Capsule: A polysaccharide layer that prevents phagocytes from attaching to the bacterium.

• Cell Wall Proteins: (Protein A of Staphylococcus aureus, M protein of Streptococcus pyogenes) prevent attachment to phagocytes.

• Coagulase: (in Staphylococcus aureus) converts fibrinogen to fibrin, forming a clot around

What is toxic production in virulence factors?

Bacterial products that damage host cells or disrupt host functions.

There are two main types: Exotoxins and Endotoxins.

What are endotoxins? (Origin, structure, release, effects)

Origin: Primarily from the cell wall of Gram-negative bacteria.

Structure: Lipopolysaccharide (LPS), consisting of Lipid A (toxic component) and polysaccharide (O antigen, which is immunogenic).

Release: Released when the bacterial cell lyses.

Effects:

• Activate macrophages to produce pyrogens (fever-inducing substances).

• Activate the complement cascade, leading to inflammation.

• Induce the release of cytokines like Tumor Necrosis Factor-alpha (TNF-α) and Interleukin-1 (IL-1).

• Can lead to septic shock, characterized by hypotension, disseminated intravascular coagulation (DIC), multiple organ failure, and potentially death.

• Increase vascular permeability.

• Can cause fever, hypoglycemia, and activation of clotting factors.

What are exotoxins? (Origin, structure, release, toxicity, immunogenicity, types based on target)

Origin: Secreted by living bacteria (Gram-positive and -negative).

Structure: Proteins.

Release: Actively secreted by the bacteria.

Toxicity: Highly potent, often specific in their action.

Immunogenicity: Can stimulate the production of antitoxins.

Types (based on target):

• Neurotoxins: Affect nerve cells (botulinum toxin, tetanus toxin).

• Cardiotoxins: Affect heart cells.

• Hepatotoxins: Affect liver cells.

• Leukotoxins: Affect leukocytes (WBC).

• Enterotoxins: Affect the lining of the gastrointestinal tract, causing diarrhea and vomiting (cholera toxin, toxins from Staphylococcus aureus).

• Cytotoxins: Affect a wide variety of cells, often by damaging cell membranes.

What is biofilm? How is it formed?

A biofilm is an aggregate of bacteria (or other microbes) attached to a surface (biotic or abiotic) and encased in an exopolysaccharide matrix.

Formation Steps:

1. Attachment: Planktonic bacteria adhere to a surface.

2. Growth/Proliferation: Bacteria multiply and form microcolonies.

3. Maturation: The biofilm develops a complex structure, and bacteria differentiate.

4. Dispersion: Individual bacteria or clusters detach from the biofilm to colonize new surfaces.

What is the significance of a biofilm?

Biofilms provide protection for bacteria from antibiotics and host immune defenses (phagocytosis, complement).

Bacteria within biofilms exhibit altered gene expression and are often more resistant to antimicrobial agents.

Biofilms can form on natural surfaces (like teeth as dental plaque) and medical devices (catheters, implants, artificial joints), leading to persistent infections.

What chronic infection is caused by mycobacterium tuberculosis/tubercle bacilli?

Tuberculosis (TB)

What is TBs cell wall composed of? What are the key components? What rate the properties caused by its high lipid concentration?

Mycobacterium tuberculosis possesses a unique cell wall structure rich in lipids.

Key components include:

• Mycolic acids: Long-chain fatty acids (FAs) with 60 to 90 carbons.

• Cord factor: A cytotoxin.

• Mycobacterial sulfolipids: Inhibit phagolysosome fusion.

This high lipid concentration confers several properties:

• Impermeability to common stains and dyes.

• Resistance to many antibiotics.

• Resistance to osmotic lysis.

• Resistance to killing by both acidic and alkaline compounds.

• Ability to survive inside macrophages, inducing a cell-mediated immune response.

What staining method is used for TB?

Ziehl-Neelsen (Z-N) or Kinyoun method:

• Uses carbol-fuchsin dye, heat, and acid-alcohol for decolorization, followed by methylene blue.

• Acid-fast bacilli (Mycobacteria) appear RED against a blue background.

Auramine-rhodamine stain:

• Results in bacilli fluorescing orange-yellow against a black background.

What is the morphology of TB?

Tubercle bacilli are acid-alcohol-fast bacilli.

They are slender, may be curved, and are arranged singly or in small groups.

What are the predisposing factors that lead to TB?

Overcrowding.

Areas of poor standard of living.

Low body resistance, often exacerbated by debilitating diseases like Diabetes Mellitus (DM).

Handling infected materials:

• Infected meat (butchers).

• Infected wounds (healthcare personnel)

What is the mode of infection for human tubercle bacilli?

Expectorated in sputum of patients with pulmonary TB.

Contaminate dust.

Transmission routes:

• Inhalation of contaminated dust: Leads to pulmonary tuberculosis.

• Swallowing of contaminated dust: Leads to TB of the tonsils or intestine.

• Inoculation through the skin: Leads to cutaneous TB.

What is the mode of infection for bovine tubercle bacilli?

Exist in the milk of tuberculous cows.

Transmitted to humans by swallowing infected milk.

Leads to tuberculosis of the tonsils or intestine.

What is the tissue reaction to TB bacilli?

The primary tissue reaction to Mycobacterium tuberculosis is the formation of a granuloma, also known as a tubercle.

This is a hallmark of chronic granulomatous inflammation, specifically a Type IV hypersensitivity reaction.

What are the 7 steps of granuloma formation in TB?

1. Inhalation and Macrophage Engulfment: Bacilli reach the terminal alveoli and are engulfed by alveolar macrophages.

2. Intracellular Survival and Replication: The bacilli resist destruction within phagosome by preventing lysosome fusion. They replicate inside the macrophage.

3. Antigen Presentation: Macrophages act as antigen-presenting cells (APCs), presenting the bacillus to helper T-lymphocytes (CD4+ Th cells).

4. T-cell Activation and Cytokine Release:

   • APCs present TB antigens in association with MHC II to naive CD4+ Th cells.

   • These differentiate into Th1 cells.

   • Macrophages and dendritic cells produce IL-12, which drives Th1 differentiation.

   • Th1 cells secrete Interferon-gamma (IFN-γγ) and Tumor Necrosis Factor (TNF).

5. Macrophage Activation: IFN-γγ activates macrophages, leading to their transformation into epithelioid cells and giant cells.

6. Adhesion Molecule Expression: TNF acts on endothelium to increase the expression of adhesion molecules, facilitating the migration of monocytes and lymphocytes to the infection site.

7. Granuloma Maturation: Cytokines attract more macrophages and lymphocytes, leading to the formation of a granuloma

What are the components of TB granuloma?

Central Caseous Necrosis:

• Grossly appears as a yellowish-white, cheesy material resembling casein.

• Microscopically, nuclei disappear, cells fuse, forming a pink, structureless material.

Epithelioid Cells:

• Derived from activated macrophages.

• Characterized by abundant pale pink cytoplasm, indefinite cell borders, and oval, vesicular nuclei.

Langhans Giant Cells:

• Large multinucleated cells formed by the fusion of epithelioid cells or macrophages.

• Nuclei are arranged at the periphery of the cell in a horse-shoe pattern.

Peripheral Lymphocytes: T-lymphocytes surround the central core.

Fibrosis: A fibrous capsule may form around the granuloma, especially in healed lesions.

Endarteritis: Inflammation of the small blood vessels within the granuloma wall.

What is primary pulmonary tuberculosis? What is the ghon complex?

Occurs during the first exposure to tubercle bacilli. It involves a triad known as the Ghon complex:

1. Ghon Focus: A primary tubercle, typically found in the middle third of the lung, often subpleural. It is a yellowish caseous lesion, 1-2 cm in diameter.

2. Lymphangitis: Inflammation of the draining lymphatic vessels.

3. Lymphadenitis: Caseous involvement of the hilar lymph nodes.

What are the different fates of primary pulmonary tuberculosis?

Healing: Most common outcome with mild infection and good immunity. Lesions become fibrosed and encapsulated.

Spread of Infection:

• Direct Spread: To the lung parenchyma or pleura.

• Blood Spread:

  • Few bacilli: Destroyed by the immune system.

  • Moderate bacilli: Settle in one organ (e.g., brain, kidney), leading to isolated organ TB.

  • Large number of bacilli: Disseminated to many organs (lungs, kidneys, liver, spleen), resulting in miliary TB.

Encapsulation and Reactivation: The primary focus can remain dormant and reactivate later with lowered body resistance.

What is military TB? How is it seen on an X-ray?

Characterized by the widespread dissemination of bacilli, producing numerous minute lesions (2-3 mm) resembling millet seeds in distant organs.

Chest X-ray shows well-defined and diffuse micronodular opacities in both lung fields.

What is the source of infection for secondary pulmonary TB? What are the types of lesions?

Source of Infection:

• Inhalation of tubercle bacilli.

• Reactivation of a healed primary focus.

Types of Lesions (depend on immunity and bacillary load):

• Healed Apical Lesion (Fibrotic TB): Occurs with good resistance. Characterized by fibrous scarring.

• Chronic Fibro-caseous TB: Some degree of resistance leads to a slow-progressing lesion with attempts at healing. Cavities may form, often in the apical or posterior segments of the upper lobes (due to better oxygenation and reduced lymphatic drainage). Blood vessels traversing the cavity wall are thickened (endarteritis) and appear as cords or ridges.

• Acute Caseous Tuberculous Pneumonia (Acute Pulmonary TB): Occurs with very low resistance. Caseation necrosis predominates, involving an entire lobe or lung with rapid spread and minimal fibrosis.

What are the complications of secondary pulmonary TB?

Hemoptysis: Due to erosion of blood vessels within tuberculous cavities.

Spread of Infection:

• Via blood, causing isolated organ TB or miliary TB.

• Direct spread to the pleura.

Intestinal Tuberculosis: From swallowing infected sputum.

Lung Fibrosis and Right-Sided Heart Failure: Due to chronic lung damage.

How is TB diagnosed?

Detection of Organism:

• Microscopy of sputum or broncho-alveolar lavage using Ziehl-Neelsen stain to identify acid-fast bacilli (red against a blue background).

• Culture of the organism.

Chest X-ray: Used to identify characteristic lesions, differentiate between primary, secondary, and miliary TB.