BIO221 Lab Material Review: Microbiology Laboratory Techniques and Concepts

Lab Safety, Equipment, and Biohazard Protocols

Personal Protective Equipment (PPE) and Discard Procedures

Proper lab safety is essential in microbiology to prevent contamination and ensure a safe working environment.

• PPE: Includes lab coats, gloves, safety goggles, and closed-toe shoes. These protect against chemical and biological hazards.

• Discard Procedures: Biological waste (e.g., used gloves, contaminated materials) must be disposed of in designated biohazard containers. Non-contaminated waste goes in regular trash.

• Glove Disposal: Used gloves are discarded in biohazard bins to prevent the spread of microorganisms.

• Emergency Protocols: In case of emergency, immediately notify the instructor and follow Purdue's emergency procedures (e.g., evacuation, spill response).

Biohazard Levels and HazCom Symbols

• Biohazard Levels: Indicate the risk associated with biological agents. Level 1 is minimal risk; Level 4 is high risk (e.g., dangerous pathogens).

• HazCom Symbols: Standardized icons that communicate hazards such as biohazard, chemical, or radiation risks.

Laboratory Equipment

• Common Equipment: Microscopes, Bunsen burners, pipettes, Petri dishes, and incubators are essential for microbiological experiments.

Microscopy

Microscope Parts and Usage

Microscopes are fundamental tools for observing microorganisms.

• Parts: Ocular lens (eyepiece), objective lenses, stage, coarse and fine focus knobs, light source, arm, base.

• Importance: Each part contributes to magnification, resolution, and proper handling of specimens.

• Carrying/Storage: Always carry the microscope with both hands (one on the arm, one under the base). Store with the lowest objective in place and cover to prevent dust.

Example:

• Using the 100x oil immersion objective for bacterial observation increases resolution.

Wet Mount Preparation and Cell Types

Wet Mounts and Cellular Characteristics

Wet mounts allow observation of living microorganisms and their motility.

• Purpose: To observe live cells, motility, and cell morphology without staining.

• Preparation: Place a drop of sample on a slide, cover with a coverslip, and observe under the microscope.

• Cellular Characteristics:

  • Prokaryotes: No nucleus, simple structure, includes Bacteria and Archaea.

  • Eukaryotes: Nucleus present, complex organelles, includes Fungi, Protozoa, Algae.

• Groups: Prokaryotes (bacteria, archaea); Eukaryotes (protozoa, fungi, algae).

Aseptic Technique

Maintaining Sterility and Tool Sterilization

Aseptic technique prevents contamination of cultures and the environment.

• Importance: Ensures pure cultures and accurate results.

• Maintaining Sterility: Disinfect work surfaces, minimize exposure of sterile items, use flame to sterilize tools.

• Tool Sterilization: Use a Bunsen burner to flame inoculating loops and needles before and after use.

Liquid Media Bacterial Growth Terminology

• Turbidity: Cloudiness indicates bacterial growth.

• Sediment: Cells settle at the bottom.

• Pellicle: Growth at the surface.

• Flocculent: Clumps of cells throughout the medium.

Bacterial Staining

Types of Staining and Morphologies

Staining enhances contrast and allows differentiation of bacterial types.

• Types: Simple stain, differential stain (e.g., Gram stain), negative stain, acid-fast stain.

• Morphologies: Cocci (spherical), bacilli (rod-shaped), spirilla (spiral), arrangements (chains, clusters).

• Gram Stain Problems: Over-decolorization, under-decolorization, old cultures may give inaccurate results.

• Gram Stain Steps:

  1. Crystal violet (primary stain)

  2. Iodine (mordant)

  3. Alcohol/acetone (decolorizer)

  4. Safranin (counterstain)

Example:

• Gram-positive bacteria retain crystal violet and appear purple; Gram-negative bacteria appear pink/red.

Colony Isolation and Morphology

Petri Plate Labeling and Storage

Proper labeling and storage are crucial for tracking and preserving microbial cultures.

• Labeling: Write on the bottom (agar side) of the Petri dish to ensure identification even if lids are switched.

• Storage: Store plates inverted (agar side up) to prevent condensation from dripping onto colonies.

Colony Morphologies

• Characteristics: Shape (circular, irregular), margin (smooth, wavy), elevation (flat, raised), color, texture.

Colony Isolation Techniques

T-Streak and Colony Description

Isolation techniques allow separation of individual bacterial species from mixed samples.

• Purpose: To obtain pure cultures by spreading bacteria over agar surface.

• T-Streak: A method dividing the plate into three sections, streaking each to dilute the sample.

• Describing Colonies: Use morphology characteristics (see above).

Enumerating Bacteria

Cell Counting Techniques and Pipetting

Enumeration quantifies bacterial populations in samples.

• Techniques: Direct count (microscope), viable plate count (serial dilution and plating), spectrophotometry.

• Pipette Use: Draw and dispense liquids accurately; avoid air bubbles.

• Calculating Dilutions: Use serial dilution equations:

$\text{Dilution Factor} = \frac{\text{Volume of sample}}{\text{Total volume}}$

$\text{CFU/mL} = \frac{\text{Number of colonies}}{\text{Volume plated} \times \text{Dilution factor}}$

Selective and Differential Media

Media Types and Examples

Specialized media help identify and differentiate microorganisms.

• Selective Media: Inhibits growth of some organisms while allowing others (e.g., MacConkey agar for Gram-negative bacteria).

• Differential Media: Distinguishes organisms based on biochemical properties (e.g., color change).

• Examples:

  • MacConkey Agar: Selective for Gram-negative, differential for lactose fermentation (pink colonies).

  • Snyder Agar: Used for oral bacteria; color change indicates acid production.

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

Table: Selective vs Differential Media

Media	Selective For	Differential For	Indicator
MacConkey Agar	Gram-negative bacteria	Lactose fermentation	Neutral red (pink colonies)
Mannitol Salt Agar	Staphylococci	Mannitol fermentation	Phenol red (yellow color)
Snyder Agar	Oral bacteria	Acid production	Bromcresol green (yellow/green)

Oxygen Growth Requirements

Classification and Indicators

Bacteria differ in their oxygen requirements, which can be tested using indicator dyes and specific media.

• Indicator Dye: Resazurin or methylene blue; color change indicates oxygen presence.

• Classifications:

  • Obligate aerobes: Require oxygen.

  • Obligate anaerobes: Cannot tolerate oxygen.

  • Facultative anaerobes: Can grow with or without oxygen.

  • Microaerophiles: Require low oxygen.

  • Aerotolerant anaerobes: Do not use oxygen but tolerate it.

• Inoculation: Use sterile technique to introduce bacteria into media.

• Electron Transport Chains: Aerobic bacteria use oxygen as terminal electron acceptor; anaerobic bacteria use other molecules.

Carbon Growth Requirements

Sugar Fermentation and Citrate Utilization

Tests for metabolic capabilities help identify bacteria.

• Gas Production: Indicates fermentation; detected by bubbles in Durham tubes.

• Sugar Fermentation Tubes: Contain a sugar, pH indicator, and Durham tube for gas detection.

• MR-VP Tubes: Used for methyl red and Voges-Proskauer tests; require specific reagents.

• Citrate Slant: Tests ability to use citrate as sole carbon source; color change (green to blue) indicates positive result.

Enzymatic Tests

Testing for Bacterial Enzymes

Enzymatic tests reveal specific metabolic activities.

• Tryptophanase Test: Uses SIM medium; addition of Kovac's reagent produces red color if indole is present.

• Phenylalanine Deaminase Test: Uses phenylalanine agar; addition of ferric chloride produces green color if positive.

Bloodstream and Hemolysis Tests

Hemolysins and Blood Agar

Blood agar is used to detect hemolytic activity of bacteria.

• Hemolysins: Enzymes that lyse red blood cells.

• Types of Hemolysis:

  • Alpha: Partial hemolysis, greenish color.

  • Beta: Complete hemolysis, clear zone.

  • Gamma: No hemolysis.

• Plasma Clotting Test: Used to determine coagulase production (e.g., by Staphylococcus aureus).

Example Table: Hemolysis Types

Type	Appearance	Significance
Alpha	Greenish zone	Partial RBC lysis
Beta	Clear zone	Complete RBC lysis
Gamma	No change	No RBC lysis

Additional info: Where original notes were brief, standard microbiology lab context and definitions have been added for completeness and clarity.