Microbiology Lab Study Guide: Techniques, Staining, and Microbial Enumeration

Hand Washing and Microbes in the Environment Labs

Purpose and Importance of Handwashing

Handwashing is a fundamental practice in microbiology labs to reduce the transmission of microbes. It primarily removes bacteria but can also remove some viruses and other microorganisms.

• Degerming: Physical removal of microbes from the skin, especially transient microbes.

• Fatty acids in soap: Mildly disrupt microbial membranes and viral envelopes.

• Environmental microbes: Microorganisms are found everywhere, including air, soil, water, and even the skin.

Sample Handling and Incubation

• Incubation Temperature: Bacteria from human samples are incubated at 37°C to mimic body temperature, which supports optimal growth.

• Colony Morphology: Includes shape, elevation, margin, size, pigmentation/color, texture, and other visible characteristics.

• Contamination Prevention: Students must minimize contamination of petri plates and other media by using aseptic techniques.

• Lab Safety: Protecting oneself from bacterial and airborne (aerosol) infections is crucial, especially during pandemics.

Compound Microscope Use

Microscope Operation and Objectives

Understanding the use of a compound light microscope is essential for observing microorganisms.

• Objectives: Different magnifications (4x, 10x, 40x, 100x oil immersion) are used for various levels of detail.

• Total Magnification: Calculated as the product of ocular and objective lens powers.

• Resolution: The ability to distinguish two points as separate; improved by shorter wavelengths and higher numerical aperture.

• Contrast: Enhanced by staining, sample preparation, and adjusting microscope settings.

• Dissecting Microscopes: Lower magnification (~40x), do not invert images.

Staining Techniques

Simple Stain

Simple stains use a single dye to increase contrast and visualize cell shape, structure, and arrangement.

• Basic Dyes: Positively charged, bind to negatively charged cell components (e.g., crystal violet, safranin, methylene blue).

• Acidic Dyes: Negatively charged, stain the background (e.g., Congo red, India ink).

• Heat Fixing: Adheres cells to the slide and shrinks them slightly.

• Capsule Stain: Used to determine the size/shape of capsules; do not heat fix.

Gram Stain

The Gram stain differentiates bacteria into Gram-positive and Gram-negative based on cell wall structure.

• Gram-positive: Thick peptidoglycan layer, retain crystal violet (purple).

• Gram-negative: Thin peptidoglycan, outer membrane, lose crystal violet, take up safranin (pink/red).

• Steps: Crystal violet (primary stain), iodine (mordant), alcohol (decolorizer), safranin (counterstain).

• Arrangement: Cocci (spheres), bacilli (rods), spirilla (spirals).

Endospore Stain

Endospore staining differentiates vegetative cells from endospores, which are highly resistant structures formed by some bacteria (e.g., Bacillus, Clostridium).

• Primary Stain: Malachite green, applied with heat to penetrate endospores.

• Counterstain: Safranin stains vegetative cells pink.

• Endospores: Resistant to desiccation, heat, chemicals, and radiation.

Acid-Fast Stain

The acid-fast stain identifies bacteria with mycolic acid in their cell walls (e.g., Mycobacterium, Nocardia).

• Primary Stain: Carbolfuchsin, applied with heat.

• Decolorizer: Acid-alcohol removes stain from non-acid-fast cells.

• Counterstain: Methylene blue stains non-acid-fast cells.

• Acid-fast cells: Remain red/pink; non-acid-fast cells are blue.

Quadrant Streak Plate

Purpose and Procedure

The quadrant streak plate is used to isolate pure colonies from a mixed culture by mechanically diluting bacteria across the surface of an agar plate.

• Colony: Arises from a single bacterium; different species may have distinct appearances.

• Aseptic Technique: Minimizes contamination during streaking.

• Steps: Sterilize loop, streak in quadrants, incubate plate.

Media Types and Bacterial Growth

Liquid Broth

• Supports rapid bacterial growth; cannot distinguish colony morphology.

• Facultative anaerobes grow throughout; obligate aerobes at the surface.

Slants

• Used for storage; take up less space and reduce contamination risk.

• Allow observation of surface growth and morphology.

Petri Plates

• Allow growth of bacteria on the surface of soft agar.

• Used for quadrant streak technique and colony isolation.

Eukaryotic Microbes (Microscopy)

Helminths and Fungi

• Helminths: Flatworms (Fasciola hepatica, Taenia), roundworms (Trichinella spiralis, Ascaris lumbricoides).

• Fungi: Aspergillus, Penicillium, Saccharomyces; recognize hyphae, mycelium, and mycoses (fungal diseases).

Pipette Use, Pour Plates, and Spread Plates

Dilutions and Colony Counting

Serial dilutions are used to reduce bacterial concentration for accurate colony counting.

• IDF (Individual Dilution Factor): Volume transferred / Volume in new tube.

• TDF (Total Dilution Factor): $\text{TDF}_n = (\text{IDF}_1) \times (\text{IDF}_2) \times \ldots \times (\text{IDF}_n)$

• Colony Forming Units (CFU): Each colony arises from a single bacterium.

• Spread Plates: Used to quantify bacteria by spreading diluted samples on agar.

• Pour Plates: Mix dilution with molten agar, pour into plates; colonies grow in and on agar.

• Sterilization: Spreaders are sterilized by flaming or using sterile packaging.

Colony Counting Table

Plate Type	Colony Location	Colony Count Range
Spread Plate	Surface only	25–250
Pour Plate	Surface and within agar	25–250

Viable Cell Count and Bacterial Growth

Bacterial Growth Phases

• Lag Phase: Little or no growth; cells adapt to environment.

• Exponential (Log) Phase: Rapid cell division by binary fission.

• Stationary Phase: Growth rate slows; nutrient depletion and waste accumulation.

• Death Phase: Cell death rate exceeds division rate.

Generation Time and CFU Calculations

• Generation Time: Time for a bacterial population to double.

• CFU/mL Calculation:

$\text{CFU/mL}_{\text{tube}} = \frac{\text{Number of colonies/mL plated}}{\text{TDF of the culture used for plating}}$

$\text{CFU/plate} = (\text{CFU/mL}_{\text{tube}}) \times (\text{mL plated})$

• Laws of Exponents: $10^x \times 10^y = 10^{x+y}$; $10^4 \times 10^6 = 10^{10}$; $1 \times 10^6 = 10^6$

• Plausibility of Results: Extremely high CFU/mL (e.g., $2 \times 10^{10}$) may not be plausible for overnight cultures.

Additional info:

• Some context and explanations have been expanded for clarity and completeness.

• Scientific names are italicized as per academic convention.