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). The skin and throat harbor both normal flora (microbiota) and potential pathogens.

• Normal Flora Example (Skin): Staphylococcus epidermidis

• Pathogen Example (Skin): Staphylococcus aureus

• Normal Flora Example (Throat): Streptococcus mitis

• Pathogen Example (Throat): Streptococcus pyogenes

Differential vs. Selective Media

Microbiological media are used to grow and differentiate microorganisms. Selective media favor the growth of certain microbes, while differential media distinguish between organisms based on biochemical reactions.

• Blood Agar: Differential (hemolysis patterns), not selective.

• Tinsdale Agar: Both selective and differential (for Corynebacterium diphtheriae).

• Mannitol Salt Agar (MSA): Both selective (high salt for staphylococci) and differential (mannitol fermentation).

• Baird-Parker Agar: Both selective (for staphylococci) and differential (tellurite reduction, lecithinase activity).

• DNase Agar: Differential (detects DNase activity).

Example: On Baird-Parker Agar, S. aureus forms black colonies with a white precipitation zone due to tellurite reduction and lecithinase activity.

Additional info: Selective compounds include NaCl (MSA), potassium tellurite (Baird-Parker), and cystine (Tinsdale). Differential reactions often involve color changes or precipitation.

Catalase and Oxidase Tests

These tests identify microbial metabolic capabilities.

• Catalase Test: Detects the enzyme catalase, which breaks down hydrogen peroxide.

• Reaction:

• Oxidase Test: Detects cytochrome c oxidase, involved in electron transport.

• Reaction: Oxidation of tetramethyl-p-phenylenediamine (TMPD) by cytochrome c oxidase.

• Usefulness: Catalase differentiates staphylococci (catalase-positive) from streptococci (catalase-negative). Oxidase differentiates enteric bacteria (oxidase-negative) from non-enterics (oxidase-positive).

Epidemiology

Types of Epidemics

Epidemics are classified based on their spread and source.

• Common Source Epidemic: Rapid rise in cases from a single source (e.g., contaminated food).

• Propagated Epidemic: Gradual rise, person-to-person transmission.

Example: The lab experiment demonstrated a common source epidemic.

Graph: Common source shows a sharp peak; propagated shows a gradual increase and decrease.

Food Quality Control (Microbial Contamination)

Foodborne Illness Mechanisms

Foodborne illnesses arise from:

• Infection: Ingestion of live pathogens (e.g., Salmonella).

• Intoxication: Ingestion of microbial toxins (e.g., Staphylococcus aureus enterotoxin).

LPS and Food Contamination

LPS (Lipopolysaccharide): A component of Gram-negative bacterial cell walls, also known as endotoxin.

• Contamination: Food can be contaminated by LPS if Gram-negative bacteria are present.

• Heat Resistance: LPS is heat-stable and cannot be destroyed by cooking.

Examples of Foodborne Pathogens:

• Chicken: Salmonella enterica, Campylobacter jejuni

• Beef: Escherichia coli (especially 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, indicating contamination.

• Importance: Used as indicators of sanitary quality and potential pathogen presence in food and water.

Media for Cell Counts and Detection

• Total Viable Cell Counts: Plate Count Agar (PCA) or Standard Methods Agar (SMA) is used.

• Detection of Coliforms and E. coli: VRB-MUG (Violet Red Bile with MUG) medium; E. coli produces fluorescence under UV due to MUG hydrolysis.

Food Sample Preparation

Stomacher Machine: Homogenizes food samples to ensure even distribution of microbes for testing.

Campylobacter Characteristics

• Microscopy: Gram-negative, spiral-shaped ("gull-wing" appearance).

• Oxygen Requirements: Microaerophilic (requires reduced oxygen).

• Generation of Conditions: Use of gas packs or jars to create microaerophilic environment.

Media Types and Interpretation

• Campy Agar: Selective for Campylobacter.

• VRB-MUG: Selective and differential for coliforms and E. coli.

• SMA: Non-selective, used for total viable counts.

Plate Interpretation: Identify colony types, count colonies, and interpret differential reactions (e.g., color change, fluorescence).

Dilution Problems

Serial dilutions are used to estimate microbial counts in samples.

• Example: Starting with 25g of food, dilute in 225mL buffer for a 1:10 dilution, then perform further serial dilutions.

• Calculation Formula:

Laboratory Techniques

Common Techniques

• Streaking for Isolation: Separates individual colonies.

• Gram Stain: Differentiates Gram-positive (purple) and Gram-negative (pink) bacteria.

• Aseptic Technique: Prevents contamination during inoculation.

• Triple Scale Hydrometer: Measures density (often for fermentation studies).

• pH Meter: Measures acidity/alkalinity.

• Scale: Weighs samples.

• Micropipettor: Accurately dispenses small volumes.

• Microscope: Observes cell morphology.

• Dilutions: Reduces concentration for counting.

Cell Plating Techniques

• Spread Plate: Spreads sample evenly for colony counting.

• Pour Plate: Mixes sample with agar for colony counting.

• Streak Plate: Isolates pure colonies.

Interpretation of Plates and Microscopy

• Petri Plate: Count and identify different colony types (based on size, color, margin, elevation, shape).

• Colony Description: Size (mm), color, margin (edge), elevation (flat, raised), shape (round, irregular).

• Microscopy Observations: Cell shape, arrangement, Gram reaction, motility, presence of spores, inclusion bodies.

Table: Media Types and Their Properties

Additional info: Media selection is crucial for isolating and identifying specific microbes in clinical and food samples.