lab exam 1 - BASIC LAB SKILLS

Lab Safety Precautions

Laboratory safety is essential to prevent contamination, infection, and accidents. Students must understand and follow all safety protocols.

• Personal Protective Equipment (PPE): Always wear lab coats, gloves, and safety goggles.

• Hand Hygiene: Wash hands before and after lab work.

• Disinfection: Clean work surfaces before and after experiments.

• Proper Disposal: Dispose of biohazardous materials in designated containers.

• No Eating or Drinking: Food and drinks are prohibited in the lab.

Lab Equipment and Tools

Familiarity with laboratory equipment is crucial for accurate and safe experimentation.

• Inoculating Loop: Used to transfer microorganisms.

• Bunsen Burner: Provides a flame for sterilization.

• Pipettes: Used for measuring and transferring liquids.

• Petri Dishes: Hold solid media for culturing microbes.

• Test Tubes: Contain liquid or solid media for microbial growth.

Aseptic Transfer and Pure Culture Techniques

• Aseptic Transfer: Methods to prevent contamination of cultures and the environment.

• Pure Culture: A culture containing a single species of microorganism.

• Streak Plate Method: Technique to isolate individual colonies from a mixed culture.

Pipetting Techniques

• Serological Pipette: Graduated to the tip; used for measuring and transferring liquids.

• Mohr Pipette: Graduated but not to the tip; delivers measured volumes.

• Micropipette: Used for precise measurement of small volumes (microliters).

EPIDEMICS

Types of Epidemics

• Common Source Epidemic: All cases arise from exposure to a single source (e.g., contaminated water).

• Propagated Epidemic: Disease spreads from person to person, resulting in successive waves of cases.

Basic Reproduction Number (R0)

• Definition: The average number of secondary cases produced by a single infection in a susceptible population.

• Calculation: If R0 = 2, one case leads to 2 new cases, then 4, then 8, etc.

Formula:

MICROSCOPY

Microscope Components and Functions

• Ocular Lens (Eyepiece): Magnifies the image, usually 10x.

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

• Stage: Holds the slide in place.

• Condenser: Focuses light onto the specimen.

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

Proper Use of the Microscope

• Start with lowest power objective.

• Use oil immersion (100x) for bacterial observation: Place a drop of immersion oil on the slide and carefully rotate the 100x objective into place.

• Clean lenses with lens paper only.

STAINS

Smear Preparation

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

• From Agar: Add a drop of water, mix in a small amount of colony, air dry, and heat fix.

Types of Stains and Their Steps

• Gram Stain: Differentiates bacteria into Gram-positive (purple) and Gram-negative (pink).

• Acid-Fast Stain: Identifies Mycobacterium species; acid-fast cells retain red dye.

• Endospore Stain: Detects endospores, which appear green within red/pink cells.

General Steps:

1. Primary Stain

2. Mordant (fixes dye)

3. Decolorizer (removes dye from some cells)

4. Counterstain (stains decolorized cells)

Interpreting Stain Results

• Positive Reaction: Indicates presence of target structure (e.g., thick peptidoglycan in Gram-positive).

• Negative Reaction: Indicates absence or different structure (e.g., thin peptidoglycan in Gram-negative).

• Theory: Based on differences in cell wall composition or presence of specific structures.

MICROBIAL GROWTH

Types of Media

• Defined Media: Exact chemical composition is known.

• Complex Media: Contains extracts and digests of yeasts, meat, or plants; composition varies.

Specialized Media and Their Purposes

Oxygen Requirements and Temperature Preferences

• Thioglycolate Broth: Tests oxygen requirements; gradient from aerobic (top) to anaerobic (bottom).

• Candle Jar: Creates microaerophilic conditions (reduced O2).

• Anaerobic Jar: Removes O2 to create anaerobic environment.

• Temperature Categories: Psychrophiles (cold), Mesophiles (moderate), Thermophiles (hot).

BIOCHEMICAL REACTIONS

Common Biochemical Tests

BACTERIAL PATHOGENS

Identification and Classification

• Correct Spelling: Always write Genus and species names in italics, with Genus capitalized and species lowercase (e.g., Escherichia coli).

• Visual Identification: Recognize pathogens by colony morphology, Gram reaction, and cell shape.

Key Features for Pathogen Identification

• Disease Caused: Know the main diseases associated with each pathogen.

• Transmission: Understand how each pathogen is spread (e.g., airborne, contact, vector-borne).

• Reservoirs and Vectors: Identify where the pathogen is maintained in nature and any organisms that transmit it.

• Body Site: Know the typical location in the human body where the pathogen is found.

• Gram Reaction and Shape: Gram-positive (purple) or Gram-negative (pink); cocci (spherical), bacilli (rod-shaped), etc.

• Unique Features: Presence of endospores, special arrangements (chains, clusters), motility, etc.

Example: Staphylococcus aureus is a Gram-positive coccus, forms clusters, causes skin infections, is transmitted by contact, and produces coagulase.

Additional info: For exam preparation, students should be able to apply these concepts to unknowns and case studies, integrating lab results with clinical information.