Microbiology Lab Review: Labs 1–6

Lab Safety and Aseptic Technique

Laboratory Safety Rules and Biosafety Levels (BSL)

Understanding and following laboratory safety rules is essential for preventing contamination and ensuring the safety of all personnel. Biosafety levels (BSL) classify laboratories based on the risk associated with the microorganisms handled:

• BSL-1: Non-pathogenic microbes

• BSL-2: Moderate-risk agents

• BSL-3: Indigenous/exotic agents with potential for aerosol transmission

• BSL-4: Dangerous/exotic agents with high risk of life-threatening disease (e.g., Ebola)

Example: Ebola virus is handled in BSL-4 laboratories due to its high pathogenicity and lack of treatment.

Key Safety Rule: Always wear appropriate personal protective equipment (PPE) and practice aseptic technique to prevent contamination.

Aseptic Technique

Aseptic technique refers to procedures used to prevent contamination of cultures, laboratory environment, and personnel. It is vital for accurate experimental results and safety.

• Purpose: Prevent contamination of the microorganism, the environment, and personnel.

• Application: Used in all microbiological procedures, including inoculation and media preparation.

Microscopy in Microbiology

Parts and Functions of the Microscope

The compound light microscope is a fundamental tool in microbiology for observing microorganisms. Understanding its parts and their functions is essential for proper use.

• Oculars (Eyepieces): Magnify the image, usually 10x.

• Objectives: Provide additional magnification (e.g., 4x, 10x, 40x, 100x).

• Stage: Holds the slide in place.

• Coarse and Fine Focus: Adjust the focus of the image.

• Condenser and Iris Diaphragm: Focus and regulate the amount of light.

• Light Source: Illuminates the specimen.

Microscope Terminology

• Total Magnification (TM): Calculated by multiplying the ocular lens magnification by the objective lens magnification. $\text{Total Magnification} = \text{Ocular Magnification} \times \text{Objective Magnification}$

• Parfocal: The ability of the microscope to stay nearly in focus when switching between objectives.

• Resolution: The ability to distinguish two points as separate entities.

Example Calculation: If the ocular lens is 4x and the objective is 20x, total magnification is $4 \times 20 = 80$.

Eukaryotic Microorganisms: Protozoa, Algae, and Fungi

Protozoan Characteristics

Protozoa are unicellular eukaryotes found in moist or aquatic environments. They lack cell walls and are enclosed by a plasma membrane or pellicle. Most are chemoheterotrophs, except Euglena, which is a photoautotroph. Protozoa reproduce mainly asexually and move by cilia, flagella, or pseudopodia.

Algae Characteristics

Algae are photosynthetic eukaryotes found in aquatic environments. They can be unicellular or multicellular, with cell walls typically composed of cellulose, though some have hard (silica) or soft (pectin) components. Most algae are photoautotrophs and reproduce sexually or asexually. Some are motile via flagella.

Fungal Characteristics

Fungi are found throughout the environment. Yeasts are unicellular, while molds are multicellular and composed of hyphae. Fungal cell walls contain chitin and may be septate or non-septate. Most fungi are saprotrophs, secreting enzymes to digest substrates externally before absorbing nutrients. Reproduction can be sexual or asexual.

• Hypha: Long, thread-like structure forming the body of molds.

• Chitin: Main component of fungal cell walls.

Microbial Culturing and Media

Five Basic Steps of Culturing Bacteria

1. Inoculation: Introduction of microbes into media.

2. Incubation: Providing optimal conditions for growth.

3. Isolation: Separating individual species.

4. Inspection: Observing colony morphology and characteristics.

5. Identification: Determining the species present.

Types of Culture Media

• Enriched Media: Contains nutrients to support fastidious organisms.

• Differential Media: Distinguishes between organisms based on biochemical reactions.

• Selective Media: Inhibits growth of some organisms while allowing others.

• Reduced Media: Supports growth of anaerobes by reducing oxygen.

Inoculation Methods

• Stabbing: Inoculating deep into agar to test for motility or oxygen requirements.

• Stroke/Fishtail: Surface inoculation for growth observation.

• Streak Plate Technique: Dilutes bacteria to isolate colonies.

• Lawn Culture: Evenly spreading bacteria for antibiotic testing.

• Quadrant Streaking: Isolates pure colonies from mixed cultures.

Colony Morphology (FACESS)

• Form

• Appearance

• Color

• Edges

• Size

• Surface

Mixed Culture: Contains two or more different microbial species.

Selective Media: Inhibits some bacteria while allowing others to grow.

Environmental Effects on Microbial Growth

Temperature Classification

• Psychrophiles: -10 to 20°C (optimum 15°C)

• Psychrotrophs: 0 to 30°C

• Mesophiles: 15 to 45°C (optimum 37°C)

• Thermophiles: 40 to 70°C (optimum 50°C)

• Extreme Thermophiles: 65 to 110°C

High temperatures denature enzymes; low temperatures solidify membranes and inhibit enzyme function.

Chromogenesis

Chromogenesis is the production of pigment by microbes, which depends on enzyme activity and optimal temperature. For example, Serratia marcescens produces orange pigment at 25°C but not at 37°C.

pH and Microbial Growth

• Acidophiles: pH 0–5.5

• Neutrophiles: pH 5.5–8

• Alkalinophiles: pH 7.5–14

pH affects enzyme activity and cell structure.

Osmotic Solutions

• Hypertonic: Water leaves the cell, causing plasmolysis (cell shrivels).

• Hypotonic: Water enters the cell, causing lysis (cell bursts).

• Isotonic: No net water movement; cell remains unchanged.

Oxygen Requirements

• Facultative Anaerobes: Can grow with or without oxygen.

• Obligate Aerobes: Require oxygen.

• Microaerophiles: Require low levels of oxygen.

• Obligate Anaerobes: Cannot tolerate oxygen.

• Aerotolerant: Do not use oxygen but tolerate its presence.

Staining Techniques in Microbiology

Types of Stains

• Basic Stains: Contain a positive chromogen (e.g., methylene blue, crystal violet, safranin).

• Acidic Stains: Contain a negative chromogen (e.g., nigrosin).

Staining Methods

• Simple Stain: Uses one dye to reveal cell shape, size, and arrangement.

• Differential Stain: Uses two dyes to distinguish cell types or structures (e.g., Gram stain, endospore stain, acid-fast stain).

• Special Stains: Capsule, flagellar, and negative stains for specific structures.

Simple Stain Example

Simple stains such as methylene blue allow visualization of bacterial morphology under the microscope.

Negative Stain

Negative staining uses an acidic dye (e.g., nigrosin) to stain the background, not the cells. It is useful for observing live bacteria that cannot withstand heat fixing and for determining size, shape, and arrangement.

Bacterial Shapes and Arrangements

• Cocci: Spherical

• Bacilli: Rod-shaped

• Spiral: Spiral-shaped

Arrangements include single, pairs (diplo-), chains (strepto-), clusters (staphylo-), etc.

Glossary of Key Terms

• Ubiquity: Presence of microbes everywhere.

• Chromogenesis: Pigment production by microbes.

• Colony Morphology: Observable characteristics of microbial colonies.

• Pure Culture: Culture containing only one species of microorganism.