MICRO 2042 Practical #1 Study Guide

Culture Media

• Mannitol Salt Agar (MSA):

  • Full Name: Mannitol Salt Agar.

  • Inhibitor / Selective Agent: $7.5\%$ sodium chloride (salt).

  • Indicator: Phenol red.

  • Interpretation of Yellow MSA: Mannitol fermentation is positive; acid production lowers the pH and changes the phenol red indicator to yellow.

  • Target Organisms Selected: Salt-tolerant organisms, especially Staphylococcus species.

• MacConkey Agar:

  • Selectivity: Selects for Gram-negative bacteria; inhibits the growth of most Gram-positive bacteria.

  • Indicator: Neutral red.

  • Interpretation of Pink / Red Growth: Lactose fermentation positive.

  • Interpretation of Clear / Colorless Growth: The organism does not ferment lactose (lactose fermentation negative).

• Columbia CNA Agar (CNA):

  • Selectivity: Selects for Gram-positive bacteria.

  • Inhibitors: Colistin and nalidixic acid.

  • Indicator: None.

  • Growth Meaning: Growth indicates that the organism tolerated the antibiotic selection.

• Blood Agar:

  • Primary Usage: Used to observe hemolysis patterns; it functions as an enriched and differential medium.

  • Indicator Component: Blood cells (red blood cells).

• Mueller-Hinton Agar:

  • Primary Usage: Standard susceptibility medium used for antimicrobial disk diffusion testing.

  • Indicator: Not applicable (N/A).

  • Positive Reaction / Observation: Visible zones of inhibition surrounding antibiotic disks.

• Culture Media Summary Table:

  • Mannitol Salt Agar (MSA): Selects salt-tolerant organisms / inhibits salt-sensitive organisms; Indicator is phenol red; Positive reaction is yellow agar ($\text{mannitol fermentation positive}$).

  • MacConkey Agar: Selects Gram-negative bacteria / inhibits Gram-positive bacteria; Indicator is neutral red; Positive reaction is pink/red growth ($\text{lactose fermentation positive}$).

  • Columbia CNA Agar: Selects Gram-positive bacteria / inhibits Gram-negative bacteria; No indicator; Positive reaction is growth indicating selection tolerance.

  • Blood Agar: Enriched and differential medium; Indicator is blood cells; Positive reactions: Alpha ($\alpha$) = greenish discoloration, Beta ($\beta$) = clear zone, Gamma ($\gamma$) = no change.

  • Mueller-Hinton Agar: Standard susceptibility medium; Indicator N/A; Positive reaction is clear zones of inhibition around antibiotic disks.

Hemolysis

• Alpha ($\alpha$) Hemolysis:

  • Definition: Partial red blood cell destruction.

  • Visual Presentation: Produces a greenish discoloration surrounding the bacterial colony.

• Beta ($\beta$) Hemolysis:

  • Definition: Complete red blood cell destruction.

  • Visual Presentation: Produces a completely clear and transparent zone around the bacterial colony.

• Gamma ($\gamma$) Hemolysis:

  • Definition: No visible hemolysis.

  • Visual Presentation: No visible change in the surrounding blood agar medium.

• Hemolysis Identification Rules:

  • Completely clear zone around colony: Beta ($\beta$) hemolysis.

  • Greenish halo around colony: Alpha ($\alpha$) hemolysis.

  • No visible change in medium: Gamma ($\gamma$) hemolysis.

• Hemolysis Memory Cues:

  • $\text{GREEN} = \text{ALPHA}$

  • $\text{CLEAR} = \text{BETA}$

  • $\text{NO CHANGE} = \text{GAMMA}$

Oxygen Requirements & Thioglycollate Medium

• Microbial Classification of Clostridium sporogenes:

  • Classified as an obligate anaerobe.

  • Growth Behavior: Clostridium sporogenes grows inside an anaerobic box but fails to grow under normal atmospheric conditions because oxygen is harmful or strongly inhibitory to its growth.

• Oxygen Gradient in Thioglycollate Medium Tubes:

  • Highest Oxygen Concentration: Located near the top surface of the tube.

  • Lowest Oxygen Concentration: Located near the bottom of the tube.

• Oxygen Requirement Growth Patterns in Thioglycollate:

  • Obligate Aerobe: Growth occurs only at the top of the tube where oxygen concentration is highest.

  • Obligate Anaerobe: Growth occurs only at the bottom of the tube where oxygen concentration is lowest.

  • Facultative Anaerobe: Growth is present throughout the entire tube, but is distinctly heavier near the top surface.

  • Aerotolerant Anaerobe: Relatively even growth distributed throughout the entire tube.

  • Microaerophile: Growth forms a thin band located just below the top surface, where oxygen concentration is lower than atmospheric concentration.

• Thioglycollate Pattern Summary Table:

  • Obligate Aerobe: Top of tube.

  • Obligate Anaerobe: Bottom of tube.

  • Facultative Anaerobe: Growth throughout tube; heavier at top.

  • Aerotolerant Anaerobe: Growth evenly throughout tube.

  • Microaerophile: Thin band just below the top surface.

Microscopy & Staining

• Magnification Principles:

  • Total Magnification Formula: $\text{Total Magnification} = \text{Ocular Magnification} \times \text{Objective Magnification}$

  • Calculation Example: $10\times \text{ocular} \times 100\times \text{objective} = 1000\times \text{total magnification}$

• Microscope Components & Optical Properties:

  • Condenser: Focuses light rays through the specimen.

  • Diaphragm: Controls the precise amount of light passing through the specimen.

  • Resolution: The ability to distinguish two close objects as separate and distinct entities.

  • Immersion Oil (used with $100\times$ Objective): Reduces light refraction and light loss, thereby improving resolution.

• Gram Staining Procedure & Reagents:

  • Correct Reagent Order: Crystal violet $\rightarrow$ iodine $\rightarrow$ alcohol $\rightarrow$ safranin.

  • Primary Stain: Crystal violet.

  • Mordant: Iodine.

  • Decolorizer: Alcohol.

  • Counterstain: Safranin.

  • Gram-Positive Cell Appearance: Purple.

  • Gram-Negative Cell Appearance: Pink / red.

• Negative Staining Technique:

  • Reagent: Nigrosin.

  • Purpose: To observe true cell size, shape, and cellular arrangement without the morphological distortion caused by heat-fixing.

  • Mechanism: Both the nigrosin stain and the bacterial cell surface possess negative charges; electrical repulsion prevents the stain from entering the cells, leaving cells clear against a dark background.

• Smear Preparation Considerations:

  • Excessive Smear Thickness: If a smear is prepared too thick, individual cells and their Gram reaction colors become difficult to discern clearly.

Antibiotic Susceptibility Testing

• Kirby-Bauer Disk Diffusion Methodology:

  • Test Method: Kirby-Bauer disk diffusion method using antimicrobial-impregnated paper disks.

  • Growth Medium: Mueller-Hinton agar.

  • Bacterial Lawn Preparation: A sterile swab is used to spread the organism evenly across the entire plate surface in multiple directions.

  • Zone of Inhibition: The clear area surrounding an antibiotic disk where bacterial growth has been inhibited.

• Susceptibility Terminology & Definitions:

  • Susceptible: The organism is inhibited by the antibiotic under tested conditions; a clinically appropriate dose of the antibiotic may be effective for treatment.

  • Resistant: The organism is not adequately inhibited by the antibiotic under tested conditions.

  • Bactericidal: Antimicrobial action that directly kills bacteria.

  • Bacteriostatic: Antimicrobial action that inhibits bacterial growth and reproduction without killing them.

• Major Modes of Antibiotic Action:

  • Inhibition of cell wall synthesis.

  • Inhibition of protein synthesis.

  • Inhibition of nucleic acid synthesis.

  • Inhibition of cell membrane function.

  • Inhibition of metabolic pathways.

• E-Test & Minimum Inhibitory Concentration (MIC):

  • E-Test Function: Determines the Minimum Inhibitory Concentration (MIC) of an antibiotic using a specialized plastic gradient strip.

  • MIC Definition: The lowest concentration of an antimicrobial agent that completely prevents visible bacterial growth.

• Standardized Antibiotic Disk Diffusion Interpretation Standards Table:

  • E-15 (Erythromycin): Resistant if $\le 13\,\text{mm}$; Intermediate if $14\text{--}22\,\text{mm}$; Susceptible if $> 23\,\text{mm}$.

  • SXT (Trimethoprim / Sulfamethoxazole): Resistant if $\le 10\,\text{mm}$; Intermediate if $11\text{--}15\,\text{mm}$; Susceptible if $> 16\,\text{mm}$.

  • P-10 (Penicillin): Resistant if $\le 14\,\text{mm}$; Intermediate if $15\text{--}29\,\text{mm}$; Susceptible if $> 29\,\text{mm}$.

  • S-10 (Streptomycin): Resistant if $\le 11\,\text{mm}$; Intermediate if $12\text{--}14\,\text{mm}$; Susceptible if $> 15\,\text{mm}$.

  • C-5 (Ciprofloxacin): Resistant if $\le 15\,\text{mm}$; Intermediate if $16\text{--}20\,\text{mm}$; Susceptible if $> 21\,\text{mm}$.

  • PB-300 (Polymyxin B): Resistant if $\le 8\,\text{mm}$; Intermediate if $9\text{--}10\,\text{mm}$; Susceptible if $> 11\,\text{mm}$.

Media Preparation & Bacterial Nutrition

• Media Fundamentals:

  • Agar: A solidifying agent derived from algae used to make solid culture media.

  • Agar Concentration in Liquid Broth: $0\%$

• Classification of Culture Media:

  • Selective Media: Media formulation that inhibits the growth of specific organisms while favoring the growth of others.

  • Differential Media: Media formulation that distinguishes between different groups of organisms based on visible biochemical or growth reactions.

  • Complex / Rich Medium: Medium in which the exact chemical composition is not completely defined (e.g., contains extracts or digests).

  • Synthetic / Defined Medium: Medium in which every single chemical component and its precise concentration are completely known.

• Nutritional & Metabolic Categorizations:

  • Phototroph: An organism that obtains its energy from light.

  • Chemotroph: An organism that obtains its energy from chemical compounds.

  • Autotroph: An organism that obtains its carbon from carbon dioxide ($\text{CO}_2$) or inorganic carbon sources.

  • Heterotroph: An organism that obtains its carbon from organic compounds.

Acid-Fast Staining

• Acid-Fast Staining Protocol & Reagents:

  • Primary Stain: Carbolfuchsin.

  • Decolorizer: Acid-alcohol.

  • Counterstain: Methylene blue.

  • Acid-Fast Cell Color: Red / pink.

  • Non-Acid-Fast Cell Color: Blue.

• Structural Basis of Acid-Fastness:

  • Difficulty in Staining Mycobacterium: Mycobacterium species possess complex cell walls containing high concentrations of waxy mycolic acids and lipids, making them resistant to conventional water-based staining methods.

Temperature & Microbial Control

• Thermal Growth Classifications:

  • Psychrophile: Cold-loving microorganism.

  • Mesophile: Moderate-temperature organism; includes many human-associated microorganisms and pathogens.

  • Thermophile: High-temperature loving organism.

  • Hyperthermophile: Extremely high-temperature loving organism.

• Thermal Death Measurements:

  • Thermal Death Time (TDT): The duration of time required to kill all microorganisms in a specified sample at a designated temperature and under set conditions.

  • Thermal Death Point (TDP): The lowest temperature required to kill all microorganisms in a liquid sample within a specified period of time.

  • D-Value (Decimal Reduction Time): The time required at a specific temperature to reduce a microbial population by $90\%$ (a one-log decrease).

• Autoclaving Parameters:

  • Standard Autoclave Operating Conditions: Approximately $121\,^{\circ}\text{C}$ at $15\,\text{psi}$ of steam pressure for roughly $15\text{--}20\,\text{min}$, dependent upon load volume and course protocol.

  • Three Central Autoclaving Factors: Pressure, temperature, and time.

UV Radiation

• UV Germicidal Characteristics:

  • Most Germicidal Spectrum: UV-C wavelength range, with peak germicidal efficiency occurring around $260\,\text{nm}$.

  • Mechanism of DNA Damage: Induces the formation of pyrimidine/thymine dimers in DNA strands.

  • Enzymatic Repair Mechanism: Photolyase enzyme can repair UV-induced thymine dimers via photoreactivation.

• Practical Limitations of UV Sterilization:

  • Penetration Power: UV light exhibits poor penetrating ability; shaded, covered, or opaque surfaces prevent effective treatment.

  • Experimental Controls: Covering a portion of an agar plate with an opaque shield during UV exposure creates an untreated control group for baseline comparison.

pH & Osmosis

• pH Growth Classifications:

  • Acidophile: Prefers acidic environmental conditions.

  • Neutrophile: Prefers near-neutral pH conditions.

  • Alkaliphile / Basophile: Prefers alkaline / basic conditions.

• Osmotic Concepts & Cell Behaviors:

  • Osmosis: The net movement of water across a selectively permeable membrane toward the region containing a higher effective solute concentration.

  • Hypertonic Environment: Solute concentration outside the cell is higher than inside; net water moves out of the cell. Walled cells undergo plasmolysis.

  • Hypotonic Environment: Solute concentration outside the cell is lower than inside; net water moves into the cell. Cells lacking strong walls may swell or undergo lysis.

  • Isotonic Environment: Solute concentrations are equal inside and outside the cell; no net movement of water occurs.

• Halotolerance & Extremophiles:

  • Halophile: Requires high sodium chloride (salt) concentrations for growth.

  • Halotolerant: Tolerates high salt concentrations but does not require high salt for growth.

  • Classification of Halobacterium salinarum (H. salinarum): Classified as a halophile.

Antiseptic, Disinfectant, & Sterilization Terminology

• Antiseptic: An antimicrobial chemical formula intended for safe application to living tissue.

• Disinfectant: An antimicrobial chemical formula intended for application to nonliving (inanimate) surfaces.

• Sterilization: The total removal or complete destruction of all forms of microbial life, including bacterial endospores.

• Sanitization: The reduction of microbial populations on surfaces to levels deemed safe by public health standards.

Catalase Test

• Test Protocol & Reaction:

  • Reagent: Hydrogen peroxide ($\text{H}_2\text{O}_2$).

  • Positive Result: Immediate vigor of bubble formation due to oxygen gas release.

  • Chemical Reaction Equation: $2\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_2$

• Microbial Group Results:

  • Staphylococcus species: Usually catalase-positive.

  • Streptococcus and Enterococcus species: Usually catalase-negative.

Fungi & Lichens

• Mycology Basics:

  • Definition: The scientific study of fungi.

  • Domain: Eukarya.

  • Dimorphic Fungus: A fungus capable of existing in two distinct structural forms (such as yeast and mold phases) depending on environmental parameters.

• Lichen Biology:

  • Definition: A long-term mutualistic association between a fungus and a photosynthetic organism (such as a green alga or cyanobacterium).

  • Fungal Partner Contribution: Provides structural integrity, protection against environmental stress, and assists in water and mineral absorption.

  • Photosynthetic Partner Contribution: Provides organic carbon nutrients produced via photosynthesis.

• Fungal Cultivation Media:

  • Standard Medium: Sabouraud dextrose agar (SDA), used according to specific course protocols.

Serratia & Water Testing

• Microbial Characteristics of Serratia marcescens:

  • Pigment Production: Classically produces a distinct red pigment called prodigiosin.

  • Temperature Effect: Prodigiosin synthesis is generally more pronounced at room temperature / cooler incubation conditions than at $37\,^{\circ}\text{C}$.

• Water Quality & Differential Testing:

  • Eosin Methylene Blue (EMB) Agar: Selective for Gram-negative bacteria and differential for lactose fermentation.

  • Coliform Bacteria: Gram-negative, non-spore-forming rods that ferment lactose with gas production, often facultative anaerobes, serving as indicator organisms for microbial water quality.

  • Indicator of Fecal Contamination: Escherichia coli (E. coli) serves as the primary bioindicator organism for fecal contamination.

  • Enzymatic Detection Methods (e.g., Colisure): Detects the presence of $\beta\text{-galactosidase}$ and $\beta\text{-glucuronidase}$ enzymes; enzymatic hydrolysis resulting in fluorescence indicates E. coli presence.

Micropipettes & Scientific Nomenclature

• Volumetric Conversions:

  • $1\,\text{mL} = 1000\,\mu\text{L}$

  • $100\,\mu\text{L} = 0.1\,\text{mL}$

  • $1\,\mu\text{L} = 0.001\,\text{mL}$

• Binomial Nomenclature Formatting Standard:

  • Example Organism: Bacillus megaterium

  • Typed Formatting Rules: The genus name is capitalized (Bacillus), the species epithet is written in lowercase (megaterium), and both words are italicized when printed (Bacillus megaterium).

  • Handwritten Formatting Rules: Write the genus capitalized and species lowercase, and underline each word separately (e.g., Bacillus megaterium).

Rapid Memorization Summary

• Mannitol Salt Agar (MSA): Salt selection + Staphylococcus; Indicator is phenol red; Yellow result indicates mannitol fermentation.

• MacConkey Agar: Selects Gram-negative; Indicator is neutral red; Pink result indicates lactose fermenter; Clear result indicates non-fermenter.

• Columbia CNA Agar: Selects Gram-positive; Inhibitors are colistin and nalidixic acid.

• Blood Agar: Alpha ($\alpha$) = green; Beta ($\beta$) = clear; Gamma ($\gamma$) = no change.

• Clostridium sporogenes: OBLIGATE ANAEROBE $\rightarrow$ Grows at the bottom of thioglycollate tubes.

• Thioglycollate Growth Patterns: Aerobe at top; Anaerobe at bottom; Facultative throughout but heavy at top; Aerotolerant evenly throughout; Microaerophile thin band below surface.

• Gram Stain Sequence: Crystal violet $\rightarrow$ iodine $\rightarrow$ alcohol $\rightarrow$ safranin.

• Gram Cell Colors: Gram-positive = purple; Gram-negative = pink / red.

• Negative Stain Protocol: Nigrosin reagent; Dark background; Bacterial cells remain unstained.

• Kirby-Bauer Testing: Mueller-Hinton agar + antibiotic disks + measured zone of inhibition.

• Bactericidal vs Bacteriostatic: Bactericidal kills bacteria; Bacteriostatic stops / inhibits bacterial growth.

• Catalase Reaction: $2\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_2$; Bubbling indicates a positive result.

• Acid-Fast Stain Sequence: Carbolfuchsin $\rightarrow$ acid-alcohol $\rightarrow$ methylene blue.

• Autoclave Standard: $121\,^{\circ}\text{C}$, $15\,\text{psi}$, for $\sim 15\text{--}20\,\text{min}$.

• UV Disinfection: UV-C spectrum; $\sim 260\,\text{nm}$; Creates thymine dimers.