micro bio lab exam 1

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).

• Species Name Format: Always italicize; Genus capitalized, species lowercase.

Differential vs. Selective Media

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

• Selective Media: Inhibit growth of some organisms while allowing others. Example: Mannitol Salt Agar (MSA) selects for staphylococci due to high salt concentration.

• Differential Media: Allow differentiation of organisms based on biochemical reactions. Example: Blood Agar differentiates based on hemolysis.

• Both Selective and Differential: Media can possess both properties. Example: MSA is both selective (salt) and differential (mannitol fermentation).

• Media Used in Lab:

  • Blood Agar: Differential for hemolysis (alpha, beta, gamma).

  • Tinsdale Agar: Selective for Corynebacterium diphtheriae, differential for tellurite reduction.

  • Mannitol Salt Agar (MSA): Selective for staphylococci, differential for mannitol fermentation (yellow color indicates fermentation).

  • Baird-Parker Agar: Selective for staphylococci, differential for tellurite reduction and lecithinase activity (black colonies with white precipitation).

  • DNase Agar: Differential for DNase activity (clear zone around colonies).

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

Additional info: Selective agents include salt (MSA), tellurite (Baird-Parker), and antibiotics. Differential agents include mannitol, blood, DNase substrate.

Catalase and Oxidase Tests

Biochemical Tests for Microbial Identification

These tests help differentiate major groups of bacteria based on enzyme activity.

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

• Reaction: $2H_2O_2 \rightarrow 2H_2O + O_2$ (bubbles indicate positive test)

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

• Reaction: Oxidase reagent is oxidized, turning purple/blue.

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

Epidemiology

Types of Epidemics and Graphs

Epidemiology studies the spread and control of diseases.

• Types of Epidemics:

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

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

• Lab Example: The experiment modeled a common source epidemic.

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

Additional info: Epidemic curves are used to distinguish outbreak types.

Food Quality Control (Microbial Contamination)

Foodborne Illness Mechanisms

Microbes cause foodborne illnesses via two main mechanisms:

• Infection: Consumption of food containing live pathogens (e.g., Salmonella).

• Intoxication: Consumption of food containing microbial toxins (e.g., Staphylococcus aureus enterotoxin).

LPS and Food Contamination

• LPS (Lipopolysaccharide): 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 heat-stable; cannot be destroyed by cooking.

• Examples: Most common pathogens in chicken: Salmonella enterica, Campylobacter jejuni; in beef: Escherichia coli (O157:H7).

Coliforms and Fecal Coliforms

• Coliforms: Gram-negative, lactose-fermenting, rod-shaped bacteria (e.g., Escherichia coli).

• Fecal Coliforms: Subset of coliforms, originate from feces; indicate fecal contamination.

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

Media and Detection Methods

• Total Viable Cell Counts: Plate Count Agar (PCA) or Standard Methods Agar (SMA) used to enumerate live bacteria.

• Detection of Coliforms and E. coli: VRB-MUG agar (violet red bile agar with MUG) detects coliforms (red colonies) and E. coli (fluorescent under UV due to MUG hydrolysis).

• Stomacher Machine: Homogenizes food samples for uniform microbial distribution.

Campylobacter Characteristics

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

• Oxygen Requirements: Microaerophilic (requires reduced oxygen).

• Generation of Conditions: Incubation in microaerophilic jars or gas packs.

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: Ability to identify colony types and interpret results is essential.

Dilution Problems

• Sample Calculation: If starting with 25g of food, serial dilutions are performed to obtain countable plates.

• Formula: $\text{CFU/g} = \frac{\text{Number of colonies} \times \text{Dilution factor}}{\text{Volume plated (mL)} \times \text{Sample weight (g)}}$

Additional info: Dilution calculations are fundamental for quantifying microbial contamination.

Lab Techniques and Interpretations

Microbiological Techniques

Mastery of basic lab techniques is crucial for accurate microbial analysis.

• Streaking for Isolation: Technique to obtain pure colonies from mixed cultures.

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

• Aseptic Technique: Prevents contamination during inoculation.

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

• pH Meter: Measures acidity/alkalinity of samples.

• Scale: Weighs samples for accurate measurements.

• Micropipettor: Transfers precise volumes of liquids.

• Microscope: Observes microbial morphology.

• Dilutions: Reduces concentration for quantification.

• Cell Plating Techniques: Spread plate, pour plate, streak plate.

Interpretation of Plates and Microscopy

• Petri Plate Analysis: Count and identify different colony types.

• Colony Description: Size (mm), color, margin (edge), elevation, shape.

• Microscopy Observations: Cell shape, arrangement, Gram reaction, size, motility.

Additional info: Accurate descriptions aid in identification and reporting.

Summary Table: Media Types and Their Properties

Medium	Selective	Differential	Key Reaction/Indicator
Blood Agar	No	Yes	Hemolysis (clear, green, or no change)
Mannitol Salt Agar	Yes (salt)	Yes (mannitol)	Yellow color for fermentation
Baird-Parker Agar	Yes (tellurite)	Yes (lecithinase)	Black colonies, white precipitation
DNase Agar	No	Yes	Clear zone for DNase activity
VRB-MUG	Yes (bile salts)	Yes (MUG)	Fluorescence for E. coli
SMA	No	No	Total viable count