Microbiology Lab Exam 1 Study Guide: Core Concepts and Techniques

Scientific Notation and Metric Conversions

Scientific Notation

Scientific notation is a method of expressing very large or very small numbers in the form a × 10n, where a is a number between 1 and 10, and n is an integer.

• Example: 20,000 = 2 × 104; 0.0002 = 2 × 10-4

Metric Conversions

Understanding metric units is essential for laboratory measurements. Common conversions include:

• 1 millimeter (mm) = 1/1000 meter (m)

• 1 micrometer (µm) = 1/1,000,000 meter (m)

• 1 nanometer (nm) = 1/1,000,000,000 meter (m)

Laboratory Safety and Material Disposal

Proper Disposal of Materials

Correct disposal of contaminated materials is crucial for safety and preventing the spread of microorganisms.

• Dispose of contaminated slides, cultures, and media in designated biohazard containers.

• Follow lab protocols for sharp objects and glassware.

Microscopy: Use, Care, and Types

Parts of the Microscope and Their Functions

• Ocular lens (eyepiece): Magnifies the image, usually 10×.

• Objective lenses: Provide various magnifications (e.g., 4×, 10×, 40×, 100×).

• Stage: Holds the slide in place.

• Coarse and fine focus knobs: Adjust the focus of the image.

• Light source: Illuminates the specimen.

Proper Use and Care

• Carry the microscope with both hands.

• Clean lenses with lens paper only.

• Store with the lowest power objective in place.

Types of Microscopy

• Bright Field Microscopy: Uses visible light; best for stained specimens.

• Phase Contrast Microscopy: Enhances contrast in unstained, living cells.

Comparison Table: Bright Field vs. Phase Contrast Microscopy

Feature	Bright Field	Phase Contrast
Sample Type	Stained, dead	Unstained, living
Contrast	Low (unless stained)	High (internal structures visible)
Application	General morphology	Motility, internal detail

Oil Immersion Objective

• Immersion oil increases the resolving power by reducing light refraction.

• Used with the 100× objective lens for observing bacteria.

Key Definitions

• Magnification: The increase in apparent size of an object.

• Resolving Power: The ability to distinguish two points as separate; calculated as $d = \frac{0.5\lambda}{NA}$ where $d$ is the minimum distance, $\lambda$ is the wavelength, and $NA$ is the numerical aperture.

• Parfocal: Microscope remains in focus when switching objectives.

Microbial Media and Colony Morphology

Types of Nutrient Media (Based on Purpose)

• General Purpose Media: Supports growth of a wide variety of microbes (e.g., Nutrient agar).

• Selective Media: Inhibits some microbes, allows others (e.g., MacConkey agar).

• Differential Media: Distinguishes microbes based on biochemical reactions (e.g., Blood agar).

Types of Media (Based on Physical State)

• Solid: Contains agar; used for isolating colonies.

• Liquid: Broth; used for growing large numbers of cells.

• Semisolid: Lower agar concentration; used for motility tests.

Colony and Cultural Characteristics

• Colony: A visible mass of microbial cells arising from a single cell.

• Characteristics: Margin (edge), elevation, pigment, size, and texture.

Incubation and Labeling

• Incubate plates upside down to prevent condensation on agar surface.

• Label plates with sample info, date, and initials.

Staining and Microbial Morphology

Staining Mechanisms

• Positive Staining: Dyes bind to cell structures (e.g., crystal violet).

• Negative Staining: Dyes stain the background, not the cells (e.g., nigrosin).

Types of Dyes

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

• Acidic Dyes: Negatively charged; stain background (e.g., eosin, nigrosin).

Staining Techniques

• Simple Staining: Uses one dye to highlight cells.

• Differential Staining: Uses multiple dyes to distinguish cell types (e.g., Gram stain).

Smear Preparation

• Air dry before heat fixing to prevent cell lysis.

• Heat fixing adheres cells to slide and kills them.

Bacterial Shapes and Arrangements

Basic Shapes

• Coccus: Spherical

• Bacillus: Rod-shaped

• Spiral: Twisted or curved

Arrangements of Cocci

• Diplococci: Pairs

• Streptococci: Chains

• Tetrads: Groups of four

• Sarcinae: Cubes of eight

• Staphylococci: Clusters

Arrangements of Bacilli

• Single Bacillus

• Diplobacilli: Pairs

• Streptobacilli: Chains

Spiral Forms

• Vibrio: Comma-shaped

• Spirillum: Rigid, wavy

• Spirochete: Flexible, corkscrew

Yeast Morphology

• Yeasts are larger, oval-shaped, and may show budding.

• Distinguished from cocci by size and budding.

Measuring Microorganisms

• Use an ocular micrometer with oil immersion to measure size in micrometers (µm).

The Scientific Method in Microbiology

Key Terms

• Scientific Method: Systematic approach to investigation.

• Hypothesis: Testable explanation.

• Control: Standard for comparison.

• Variable: Factor that can change.

• Controlled Variables: Kept constant.

• Double Blind Study: Neither subject nor experimenter knows group assignments.

Designing Experiments

• Identify question, form hypothesis, design experiment with controls and variables, collect data, analyze results.

Cell Wall Structure and Gram Staining

Gram Stain Purpose and Mechanism

• Purpose: Differentiates bacteria into Gram-positive (purple) and Gram-negative (pink) based on cell wall structure.

• Mechanism: Crystal violet stains all cells; iodine forms complex; alcohol decolorizes Gram-negative; safranin counterstains Gram-negative.

Gram Reaction vs. Gram Type

• Gram Reaction: Result of staining (color observed).

• Gram Type: Structural classification based on cell wall.

KOH Method

• Gram-negative cells lyse in KOH, releasing DNA (stringy); Gram-positive do not.

Factors Affecting Gram Reaction

• Age of culture, type of bacterium, staining technique, presence of endospores.

Lysozyme, Penicillin, and Ampicillin

• Lysozyme: Enzyme that breaks down peptidoglycan, more effective on Gram-positive bacteria.

• Penicillin/Ampicillin: Inhibit cell wall synthesis, mainly affect Gram-positive bacteria.

Effect of Fever Temperature

• High temperature can enhance the effect of antibiotics and lysozyme, especially on Gram-negative bacteria.

Zones of Inhibition

• Measure in millimeters to assess antimicrobial effectiveness.

Predicting Gram Reaction

• Factors such as lysozyme, antibiotics, alcohol, age, temperature, and endospore formation can alter Gram reaction.

Enumeration of Bacteria: Calculations

Standard Plate Count Formula

To calculate the number of bacteria in a sample:

$\text{Cells per mL} = \frac{\text{Colony count on plate}}{\text{Total dilution of tube} \times \text{Amount plated (mL)}}$

Sample Calculations

• Example 1: 47 colonies, dilution = 1/100 × 1/10 × 2/10 × 1/10, amount plated = (as per handout) Result: $2.4 \times 10^6$ cells/mL

• Example 2: 50 colonies, dilution = 1/1000 × 1/1/100 × 1/10 Result: $5 \times 10^7$ cells/mL

• Example 3: 206 colonies (average), dilution = 1/100 × 1/1000 × 1/10 × 1 Result: $2.06 \times 10^8$ cells/mL

Note: Always use the correct dilution factor and volume plated for accurate results.

Summary Table: Key Laboratory Techniques and Their Purposes

Technique	Purpose	Key Points
Gram Stain	Differentiates bacteria by cell wall	Gram-positive: purple; Gram-negative: pink
Simple Stain	Visualize cell shape/arrangement	One dye used
Negative Stain	Visualize capsules/background	Background stained, cells clear
Plate Count	Quantify viable bacteria	Use dilution and colony count
Microscopy	Observe morphology	Use oil immersion for bacteria

Additional info: For all calculations and experimental designs, always include controls and replicate samples for accuracy. When interpreting Gram stains, consider possible sources of error such as over-decolorization or old cultures.