Microbiology Laboratory Techniques and Concepts: Lab Exam 1 Study Guide

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Microbiological Media and Culture Techniques

Preparation and Types of Microbiological Media

Microbiological media are essential for the cultivation and study of microorganisms. Media can be solid or liquid, depending on the presence of a solidifying agent such as agar.

Solid Media: Contains agar, used for isolating and culturing bacteria.
Liquid Media: Lacks agar, used for growing bacteria in suspension.
Sterilization: Media must be sterilized before use, typically by autoclaving at 121°C, 15 psi for 15 minutes.
Example: Sabouraud Dextrose Agar is a solid medium with a final pH of 5.6 and 4% dextrose concentration.

Inoculation and Aseptic Techniques

Aseptic technique is critical for preventing contamination during the transfer and inoculation of cultures. Different tools and methods are used depending on the type of culture.

Streak Plate Method: Used to isolate single colonies from a mixed culture.
Agar Slant: Used for maintaining pure cultures over extended periods.
Broth Culture: Used for growing bacteria in liquid form.
Describing Growth: Terms such as turbidity, pellicle, and sediment are used for broth cultures; form, elevation, and margin describe colonies on solid media.

Streak plate method steps Colony morphology: form, elevation, margin

Bacterial Staining Procedures

Gram Stain

The Gram stain is a differential staining technique used to classify bacteria based on cell wall structure.

Reagents (in order): Crystal Violet (primary stain), Iodine (mordant), Ethanol (decolorizer), Safranin (counterstain).
Gram Positive: Thick peptidoglycan layer retains crystal violet-iodine complex (purple).
Gram Negative: Thin peptidoglycan and lipid membrane lose primary stain after alcohol (red after counterstain).

Schaffer-Fulton Endospore Stain

Reagents: Malachite Green (primary stain), Safranin (counterstain).
Endospores: Stain green; vegetative cells stain red.

Acid-Fast Stain (Kinyoun Method)

Reagents: Carbol Fuchsin (primary stain), Acidified alcohol (decolorizer), Brilliant Green or Methylene Blue (counterstain).
Acid-Fast Cells: Appear red due to mycolic acid in cell wall.

Endospore stain: endospore and vegetative cell Gram positive spore-forming bacteria in tissue Anaerobic jar for culturing anaerobic bacteria Kirby Bauer antibiotic disk test

Medical Mycology

Histoplasma capsulatum

Histoplasma capsulatum produces tuberculated chlamydospores for reproduction and dispersal. The disease caused is histoplasmosis, acquired by inhalation of spores, commonly found in the Ohio and Mississippi River Valleys.

Histoplasma capsulatum spores

Aspergillus and Aflatoxin

Aspergillus is an opportunistic pathogen producing aflatoxin, which can cause liver necrosis and cancer. It primarily causes respiratory infections (aspergillosis).

Aspergillus fungus

Transmission of Microorganisms

Hand Washing and Disease Prevention

Hand washing removes transient microbes but does not eliminate all bacteria. Surgical scrubbing reduces contamination but does not sterilize skin. Blood agar is an enriched and differential medium used to culture fastidious organisms and observe hemolytic patterns. Transmission via fomites (e.g., drinking glass rim) is a common route for infection.

Bacterial Motility

Determining Motility

Bacterial motility can be observed using semisolid motility medium, flagella stains, or hanging drop slides. Most bacteria are motile via flagella, while spirochetes use axial filaments.

Semisolid motility medium: motile vs non-motile Peritrichous flagella arrangement Flagella arrangements: monotrichous, amphitrichous, lophotrichous, peritrichous Bacterial movement: run and tumble

Gram Positive Spore-forming Bacteria

Bacillus and Clostridium

Gram positive spore-forming bacteria include Bacillus (aerobic) and Clostridium (anaerobic). Diseases caused include anthrax, botulism, tetanus, gas gangrene, and antibiotic-resistant diarrhea. Anaerobic conditions are provided by Gas Pack jars, cooked meat medium, thioglycollate medium, and anaerobic incubators. Candle jars are used for capnophiles, not strict anaerobes.

Destruction and Control of Microorganisms

Physical and Chemical Methods

Microbial control methods include refrigeration (bacteriostatic), autoclaving (bacteriocidal), dry oven, and boiling. Endospores are resistant to heat and require autoclaving for destruction.

Procedure	Temperature (°C)	Time (min)	Pressure (psi)
Autoclave	121	15	15
Oven	160-170	120-180	--
Boiling	100	15-20	--

Antibiotic Susceptibility Testing

The Kirby Bauer Disk Susceptibility Test is used to determine bacterial sensitivity to antibiotics. Zones of inhibition are measured and compared to standard tables to interpret results as sensitive, intermediate, or resistant. The size of the zone depends on the diffusion rate of the antibiotic, not necessarily its effectiveness.

Kirby Bauer antibiotic disk test

Mycobacterium and Acid-Fast Staining

Kinyoun Acid Fast Stain Procedure

Mycobacteria are distinguished by their acid-fast cell walls containing mycolic acid. The Kinyoun method stains acid-fast cells red, while non-acid-fast cells are counterstained green or blue. Diseases include tuberculosis (Mycobacterium tuberculosis) and leprosy (Mycobacterium leprae).

Gram positive spore-forming bacteria in tissue

Summary Table: Colony Morphology Terms

Term	Description	Examples
Form	Overall shape of colony	Circular, Irregular, Filamentous, Rhizoid
Elevation	Side view appearance	Raised, Convex, Flat, Umbonate, Crateriform
Margin	Edge detail	Entire, Undulate, Filiform, Curled, Lobate

Colony morphology: form, elevation, margin

Additional info: Academic context and explanations have been expanded for clarity and completeness. All images included are directly relevant to the adjacent content and reinforce key laboratory concepts.