BIO 150 Microbiology Lab Exam Study Guide

Lab Safety and Accident Response

Laboratory Safety Procedures

Understanding and following laboratory safety protocols is essential to prevent accidents and ensure a safe working environment in the microbiology lab.

• Accident Response: Know the appropriate actions for different types of accidents (chemical spills, broken glass, fire, biological exposure).

• Material Disposal: Properly discard contaminated materials in designated biohazard containers; glass in sharps containers; chemicals as instructed.

• Personal Protective Equipment (PPE): Always wear lab coats, gloves, and eye protection as required.

Microscopy

Microscope Operation and Components

The microscope is a fundamental tool for observing microorganisms. Understanding its parts and proper use is crucial for accurate observation.

• Starting the Microscope: Begin with the lowest objective lens, adjust the light, and focus using coarse and fine adjustments.

• Oil Immersion (100x Objective): Oil is used to increase resolution by reducing light refraction between the slide and lens.

• Diaphragm/Iris: Controls the amount of light reaching the specimen.

• Condenser: Focuses light onto the specimen for clearer images.

• Magnification: Total magnification = (Objective lens) × (Ocular lens). For example, $10\times \text{ocular} \times 100\times \text{objective} = 1000\times$ total magnification.

Identification of Microorganisms

Cyanobacteria

Cyanobacteria are photosynthetic prokaryotes found in various aquatic environments. Identification is based on morphology observed under the microscope.

• Nostoc: Forms bead-like chains of cells (trichomes) within a gelatinous matrix.

• Oscillatoria: Unbranched, filamentous cyanobacteria that move by oscillation.

• Gloeocapsa: Spherical cells in groups, surrounded by a thick mucilaginous sheath.

Fungi

Fungi are eukaryotic organisms that can be identified by their reproductive structures and spore types.

• Penicillium: Characterized by brush-like conidiophores bearing chains of conidia (asexual spores).

• Rhizopus (Zygospores): Sexual spores formed by the fusion of hyphae; appear as thick-walled, dark structures.

• Coprinus (Basidiospores): Basidiospores are produced on club-shaped basidia, typical of mushrooms.

• Pezziza (Ascospores): Ascospores are contained within sac-like asci, characteristic of ascomycetes.

Bacteria: Morphology and Arrangement

Bacteria are classified by shape and arrangement, which are key for identification.

• Bacillus: Rod-shaped bacteria.

• Coccus: Spherical bacteria.

• Arrangements:

  • Strepto-: Chains (e.g., Streptococcus).

  • Staphylo-: Clusters (e.g., Staphylococcus).

Aseptic and Culture Techniques

Aseptic Technique

Aseptic technique prevents contamination of cultures and the environment.

• Heating the Loop: Sterilize the inoculating loop by flaming until red-hot before and after use.

• Heating the Tube: Briefly flame the mouth of culture tubes before and after transferring organisms.

• Handling Caps: Hold caps with the little finger to avoid contamination.

Pure Culture Technique

Obtaining a pure culture is essential for studying a single microbial species.

• Quadrant Streaking: Method to isolate individual colonies by streaking bacteria over four quadrants of an agar plate.

• Incubation: Plates are incubated in an inverted position to prevent condensation from dripping onto the agar surface.

Smear Preparation

Smear preparation is the first step in staining procedures.

• From Broth: Place a loopful of broth on the slide, spread, and air dry.

• From Solid Media: Mix a small amount of colony with a drop of water on the slide, spread, and air dry.

• Heat Fixation: Pass the dried smear through a flame to fix cells to the slide.

Staining Techniques

Gram Staining

Gram staining is a differential staining technique that distinguishes bacteria based on cell wall structure.

• Chemicals Used (in order):

  1. Crystal violet (primary stain)

  2. Iodine (mordant)

  3. Alcohol or acetone (decolorizer)

  4. Safranin (counterstain)

• Principle: Gram-positive bacteria retain crystal violet due to thick peptidoglycan; Gram-negative bacteria lose it and take up safranin.

• Identification: Gram-positive appear purple; Gram-negative appear pink/red.

Spore Staining (Schaeffer-Fulton Method)

This method stains bacterial endospores, which are resistant structures formed by some bacteria.

• Primary Stain: Malachite green (applied with steam for 5 minutes to penetrate spores).

• Counterstain: Safranin stains vegetative cells.

• Result: Spores appear green; vegetative cells appear red.

Motility Determination

Motility tests determine whether bacteria can move.

• Media Used: Semi-solid agar (motility test medium).

• Interpretation: Motile organisms spread from the stab line; non-motile remain along the stab.

Oxygen Requirement

Bacteria differ in their oxygen requirements for growth.

• Aerobes: Require oxygen; grow at the top of the medium.

• Facultative Anaerobes: Can grow with or without oxygen; grow throughout the medium but best at the top.

Acid-Fast Staining (Ziehl-Neelsen Method)

This differential stain identifies bacteria with waxy cell walls, such as Mycobacterium species.

• Primary Stain: Carbol fuchsin (applied with steam to penetrate cell wall).

• Decolorizer: Acid-alcohol (stronger than Gram stain decolorizer).

• Counterstain: Methylene blue.

• Acid-Fast Bacteria: Retain red color (e.g., Mycobacterium tuberculosis); non-acid-fast appear blue.

• Cell Wall Substance: Mycolic acid is present in acid-fast bacteria.

Capsular Staining

Capsule stains highlight the presence of a gelatinous capsule surrounding some bacteria.

• Appearance: Capsules appear as clear halos around stained cells and background.

• Significance: Capsules enhance virulence by protecting bacteria from phagocytosis.

Purpose of Laboratory Tests

Understanding Test Objectives

Each laboratory test is designed to identify, classify, or understand the physiology of microorganisms.

• Staining: Differentiates bacteria by cell wall structure, presence of spores, capsules, or acid-fastness.

• Motility and Oxygen Tests: Assess physiological traits important for identification and classification.

• Cultural Techniques: Ensure isolation and study of pure microbial species.