Organisms Used in Microbiology Labs 1–5

Introduction

This guide summarizes the key organisms and laboratory techniques introduced in the first five microbiology laboratory sessions. It focuses on microbial morphology, measurement, and basic laboratory skills, providing foundational knowledge for further study in microbiology.

Experiment 1 – Introduction to the Microscope

Common Laboratory Organisms and Their Morphology

• Staphylococcus aureus: Gram-positive cocci, arranged in clusters (staphylo-); used to learn cocci morphology and cluster arrangement.

• Shigella dysenteriae: Gram-negative bacillus (rod), single arrangement; used to learn rod morphology.

• Bacillus anthracis: Gram-positive bacillus, arranged in chains (streptobacilli); used to learn chain arrangement of rods.

• Treponema pallidum: Spirochete, single spiral; too thin for Gram stain; used to learn spiral morphology.

• Corynebacterium diphtheriae: Gram-positive, club-shaped bacillus, palisades or "Chinese letters" arrangement; used to learn palisade arrangement.

• Saccharomyces cerevisiae: Yeast (eukaryote), oval, single cells with budding; not Gram classified; used to compare eukaryotic cells with bacteria.

• Human blood smear: Erythrocytes (red blood cells), individual cells, ~7–8 μm; used to compare bacterial size to human cells.

Key Concepts:

• Recognize microbial shape (coccus, bacillus, spirochete, etc.).

• Recognize arrangement (clusters, chains, singles, palisades).

• Know which organism demonstrates each morphology.

• Focus on observation, not identification, in this lab.

Experiment 2 – Ocular Micrometer

Measuring Microbial Size

• Staphylococcus aureus: Used to measure cocci diameter.

• Corynebacterium diphtheriae: Used to measure rod length and width.

• Saccharomyces cerevisiae: Used to compare yeast size (diameter).

• Human blood smear: Used to compare bacterial vs. human cell size (diameter ~7–8 μm).

Key Concepts:

• Cocci: Measure diameter only.

• Bacilli: Measure both length and width.

• Yeast cells are much larger than bacteria.

• Human red blood cells are larger than most bacteria.

Example Calculation:

• To measure a cell using an ocular micrometer, calibrate the micrometer with a stage micrometer, then count the divisions spanned by the cell and multiply by the calibration factor.

Formula:

Experiment 3 – Ubiquity of Microorganisms

Environmental Microbial Diversity

• Collected environmental microorganisms from air, skin, soil, etc.

• Focus on observing colony characteristics, not memorizing specific organisms.

Key Colony Morphology Features:

• Colony size

• Pigmentation (color)

• Margin (edge shape)

• Elevation (profile)

• Opacity (transparency)

• Texture (surface appearance)

• Hemolysis (on blood agar)

Example: A yellow, smooth, convex colony with a clear zone on blood agar may indicate hemolytic bacteria.

Experiment 4 – Preparation of Broth and Agar

Media Preparation and Sterilization

• No organisms were inoculated in this experiment.

• Focus on preparing and sterilizing different types of media: agar, broth, slants, deeps.

• Learned about autoclaving and calculations for media preparation.

Key Concepts:

• Media must be sterile to prevent contamination.

• Agar is a solidifying agent; broth is liquid medium.

• Slants and deeps are used for different culturing techniques.

• Autoclaving uses high pressure and temperature to sterilize media.

Formula for Media Preparation:

Experiment 5 – Transfer Techniques

Aseptic Technique and Pigment Production

• Serratia marcescens: Gram-negative rod, single arrangement; produces red pigment (prodigiosin) at 20–25°C, white at 40°C.

• Used to practice aseptic transfer and observe temperature-dependent pigment production.

Key Concepts:

• Gram-negative rod morphology.

• Single bacillus arrangement.

• Red colonies at 20–25°C; white colonies at 40°C (pigment production stops at higher temperature).

Example: Incubating Serratia marcescens at room temperature yields red colonies, while incubation at 40°C yields white colonies.

Morphology Summary Table (Labs 1–5)

High-Yield Organism Facts

• Staphylococcus aureus: Gram-positive, cocci, clusters; used to recognize cocci morphology.

• Shigella dysenteriae: Gram-negative, rod, single; used to recognize rod morphology.

• Bacillus anthracis: Gram-positive, rod, chains; used to recognize chain arrangement.

• Treponema pallidum: Spirochete, very thin spiral, not visible with standard Gram stain; used to recognize spiral morphology.

• Corynebacterium diphtheriae: Gram-positive, club-shaped rods, palisades; used for shape, arrangement, and size measurements.

• Saccharomyces cerevisiae: Yeast (fungus), eukaryote, budding cells, larger than bacteria; used to compare eukaryotic and prokaryotic cells.

• Human blood smear: Red blood cells, ~7–8 μm in diameter; used as a size comparison for bacteria.

• Serratia marcescens: Gram-negative rod, produces prodigiosin (red at 20–25°C, white at 40°C); used to practice aseptic transfer and demonstrate temperature-dependent pigment production.

Lab Exam Connections

• Staphylococcus aureus: Appears in Gram stain, blood agar, mannitol salt agar (MSA), and colony morphology exercises.

• Corynebacterium diphtheriae: Used for cell morphology and detection of metachromatic granules.

• Saccharomyces cerevisiae: Used for eukaryotic cell comparison.

• Serratia marcescens: Used for aseptic technique and pigment production studies.

• Human blood: Used in blood agar concepts and cell size comparisons.

Note: Understanding the purpose of each organism in the experiment is as important as recognizing its morphology, as exam questions often focus on experimental rationale.