Microbiology Lab Exam Study Guide: Skin, Throat, Food Quality, and Lab Techniques

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Skin & Throat Microbiology

Species Naming and Microbiota

Correctly writing species names is essential in microbiology. The genus is capitalized and the species is lowercase, both italicized (e.g., Staphylococcus aureus).

Normal flora (microbiota): Microorganisms that naturally inhabit the skin and throat without causing disease. Example: Staphylococcus epidermidis (skin), Streptococcus salivarius (throat).
Pathogens: Microbes capable of causing disease. Example: Staphylococcus aureus (skin), Streptococcus pyogenes (throat).
Correct spelling: Always italicize and capitalize genus, lowercase species.

Media Types: Differential vs. Selective

Microbiological media are used to grow and differentiate microorganisms based on their physiological properties.

Differential media: Distinguish between organisms based on observable reactions (e.g., color change).
Selective media: Inhibit growth of some organisms while allowing others to grow.
Both: Some media are both selective and differential.

Medium	Selective?	Differential?	Key Reaction/Compound
Blood Agar	No	Yes	Hemolysis (alpha, beta, gamma)
Tinsdale Agar	Yes	Yes	Tellurite for Corynebacterium
Mannitol Salt Agar	Yes	Yes	High NaCl, mannitol fermentation
Baird-Parker Agar	Yes	Yes	Tellurite reduction, lecithinase activity
DNase Agar	No	Yes	DNA hydrolysis

Example: On Baird-Parker Agar, S. aureus forms black colonies with a white precipitation zone due to tellurite reduction and lecithinase activity.
Extra credit: Know compounds like NaCl (selective in MSA), tellurite (selective in BP), mannitol (differential in MSA), DNA (differential in DNase Agar).

Catalase and Oxidase Tests

Catalase Test

Used to differentiate between groups of bacteria based on their ability to decompose hydrogen peroxide.

Reaction: (bubbles indicate positive test)
Use: Differentiates Staphylococcus (catalase positive) from Streptococcus (catalase negative).

Oxidase Test

Detects presence of cytochrome c oxidase.

Reaction: Oxidase reagent + cytochrome c oxidase → color change (purple/blue)
Use: Differentiates Pseudomonas (oxidase positive) from Enterobacteriaceae (oxidase negative).

Epidemiology

Types of Epidemics

Epidemics are classified based on their spread and source.

Common source epidemic: Many cases arise from a single source (e.g., contaminated water).
Propagated epidemic: Spread person-to-person, cases build over time.
Lab experiment: Typically models a propagated epidemic.

Graphical Representation

Common source: Sharp rise and fall in cases.
Propagated: Gradual rise, multiple peaks.

Food Quality Control (Microbial Contamination)

Foodborne Illness Mechanisms

Infection: Consumption of food containing live pathogens.
Intoxication: Consumption of food containing microbial toxins.

LPS (Lipopolysaccharide)

Definition: Component of Gram-negative bacterial cell wall; also known as endotoxin.
Contamination: Food can be contaminated by LPS from dead Gram-negative bacteria.
Heat resistance: LPS is not destroyed by heating.

Foodborne Pathogens

Chicken: Salmonella enterica, Campylobacter jejuni
Beef: Escherichia coli (e.g., O157:H7)

Coliforms and Fecal Coliforms

Coliforms: Gram-negative, lactose-fermenting, rod-shaped bacteria (e.g., Escherichia coli).
Fecal coliforms: Subset of coliforms, specifically from fecal origin.
Importance: Indicators of sanitary quality and potential contamination.

Media for Cell Counts and Detection

Total viable cell counts: Plate Count Agar (PCA) or Standard Methods Agar (SMA).
Detection of coliforms and E. coli: VRB-MUG agar (violet red bile agar with MUG substrate).
Stomacher machine: Homogenizes food samples for uniform microbial distribution.

Campylobacter Characteristics

Microscopy: Spiral-shaped, Gram-negative rods.
Oxygen requirements: Microaerophilic (requires reduced oxygen).
Generation of conditions: Use of gas packs or jars to create microaerophilic environment.

Media Types for Food Microbiology

Medium	Type	Purpose
Campy agar	Selective	Isolation of Campylobacter
VRB-MUG	Selective & Differential	Detection of coliforms and E. coli
SMA	Non-selective	Total viable cell counts

Dilution Problems

Serial dilution is used to estimate microbial concentration in samples.

Example: Starting with 25g of food, dilute in 225 mL buffer for a 1:10 dilution, then serially dilute further.
Formula:

Lab Techniques and Interpretations

Cell Plating Techniques

Streaking for isolation: Used to obtain pure colonies.
Spread plating: Used for quantification.
Pour plating: Used for quantification and isolation.

Gram Stain

Purpose: Differentiates bacteria into Gram-positive (purple) and Gram-negative (pink).
Steps: Crystal violet, iodine, alcohol decolorization, safranin counterstain.

Aseptic Techniques

Purpose: Prevent contamination during inoculation.
Methods: Flame sterilization, use of sterile tools, proper handling.

Other Equipment

Triple Scale Hydrometer: Measures density (specific gravity) of liquids.
pH Meter: Measures acidity/alkalinity.
Micropipettor: Accurate liquid measurement.
Microscope: Observing cell morphology.
Scale: Weighing samples.

Colony and Microbe Description

Colony description: Size (mm), color, margin (edge), elevation, shape.
Microbe description under microscope: Cell shape, arrangement, Gram reaction, size, presence of structures (e.g., flagella).

Interpretation of Plates

Counting types: Observe colony morphology to estimate number of different species.
Example: Plate with three distinct colony types indicates three microbial species.

Solution Calculations

Formula for dilution:
Application: Used to prepare solutions of desired concentration.

Additional info: Academic context was added for media compounds, dilution formulas, and detailed descriptions of lab techniques to ensure completeness and clarity.