Microbiology Practical Review: Essential Laboratory Techniques and Concepts

Microbiology Laboratory Safety and Clean-Up

Lab Safety Guidelines

Proper laboratory safety is fundamental in microbiology to prevent contamination and ensure the safety of all personnel. These guidelines must be followed at all times:

• Lab coat: Wear at all times in the lab.

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

• Disinfection: Disinfect lab benches before and after lab.

• Personal protection: Cover all cuts and scrapes; tie back long hair.

• Microbe transfer: Avoid inhaling airborne microbes during transfers.

• Culture handling: Never lay culture tubes on the bench.

• Accident reporting: Report all spills and accidents immediately.

• Disposal: Infectious materials and gloves go in biohazard trash.

Clean-Up Procedures

• Petri dishes and small contaminated items: Dispose in biohazard trash.

• Broken glass: Use designated container (no paper towels).

• Slides: Clean oil off prepared slides with Kim wipes; plain glass slides with Bon Ami (wear goggles).

• Test tubes: Remove markings/tape, loosen caps, boil for 15 minutes, pour contents into biohazard trash, wash tubes thoroughly.

• Decontamination: Cover spills with disinfectant, inform neighbors and instructor, wait 15 minutes, then clean up.

Microbiological Equipment and Techniques

Bunsen Burner Usage

The Bunsen burner is essential for sterilization and aseptic technique. Oxygen flow is controlled by rotating the barrel:

• Clockwise: Less oxygen

• Counterclockwise: More oxygen

Key parts: barrel, collar, base, gas valve intake.

Preparing Agar Plates

Agar plates are used to culture microbes. Agar talls (solid media in test tubes) are melted, cooled, and poured into petri dishes. The agar solidifies in about 15 minutes.

Steam Slide Preparation

Steam is used for endospore and acid-fast staining. Slides are heat-fixed, placed on a steaming coffee can, covered with a paper towel, and stained for 10 minutes.

Microbiological Media and Their Uses

Types of Media

• Tryptic Soy Agar (TSA): General purpose, neutral pH, supports a wide range of microbes.

• Blood Agar Plate (BAP): TSA with sheep's blood, used to detect hemolysis.

• Rodac Plates: Raised agar for surface sampling.

Hemolysis Types

• Alpha hemolytic: Partial hemolysis, brownish discoloration.

• Beta hemolytic: Complete hemolysis, clearing of red color.

• Gamma hemolytic: No hemolysis, no change in media.

Bacterial Morphology and Colony Characteristics

Bacterial Cell Morphologies

Bacteria exhibit three basic shapes:

• Cocci: Spherical

• Bacilli: Rod-shaped

• Spirals: Curved or spiral-shaped (vibrios, spirilla, spirochetes)

Colony Morphology

Colony morphology is used to identify bacteria based on appearance:

• Shape: Circular, irregular, filamentous, etc.

• Margin: Entire, undulate, lobate, curled, etc.

• Elevation: Flat, raised, convex, umbonate, etc.

Aseptic Technique and Pure Culture Isolation

Aseptic Methods

Aseptic technique prevents contamination and exposure. Pure cultures contain only one organism, while mixed cultures contain multiple.

• Flame sterilization: Use the hottest part of the flame (tip of blue inner cone).

Streak Plate Method

The streak plate method is used to isolate pure cultures from mixed samples. It involves streaking bacteria across the plate in a specific pattern to separate individual colonies.

Microscopy in Microbiology

Bright-Field Microscope Structure

Microscopes are essential for observing microorganisms. Key parts include:

• Ocular/eyepiece: 10x magnification

• Objective lenses: 4x, 10x, 40x, 100x

• Stage, arm, mechanical stage adjustment, condenser

Total magnification: $\text{Total Magnification} = \text{Objective Magnification} \times \text{Ocular Magnification}$

Microscopes are parfocal, meaning focus remains consistent when switching objectives.

Omnipresence of Microorganisms

Microbes in the Environment

Bacteria are found everywhere. Rodac plates are used to sample surfaces, while TSA plates capture airborne microbes.

Human Microbiome and Bacterial Pigments

Human Microbiome

Humans host a variety of microorganisms, many of which are beneficial. TSA, TSB, and blood agar plates are used to culture skin and throat microbes, and to test for hemolytic bacteria.

Bacterial Pigments

Some bacteria produce pigments, aiding identification. Gram-positive bacteria often produce pigments regardless of conditions, while Gram-negative bacteria like Serratia marcescens depend on incubation conditions.

Bacterial Motility

Motility Studies

Motile bacteria can move, while Brownian motion is random movement caused by water molecules. Wet mounts help distinguish true motility from Brownian motion.

Motility Media and Swarming

Motility media deeps are stabbed with inoculation needles to test for motility. Highly motile bacteria exhibit swarming on solid media, forming concentric circles.

Staining Techniques in Microbiology

Simple Staining

Staining increases contrast for microscopic observation. Simple stains use one dye; direct stains color cells, indirect stains color the background.

Negative Staining and Capsule Detection

Negative staining (using India ink) highlights bacterial capsules. Counterstains (safranin or crystal violet) color the cell, leaving the capsule clear.

Gram Staining

Gram staining differentiates bacteria by cell wall type:

• Gram positive: Dark blue/purple (retains crystal violet)

• Gram negative: Light red/pink (retains safranin)

Steps: Primary stain (crystal violet), mordant (iodine), decolorizer (alcohol), counterstain (safranin).

Endospore Staining

Endospores are highly resistant structures. The Schaeffer-Fulton method uses steam and malachite green to stain endospores, with safranin as a counterstain.

Acid-Fast Staining

Acid-fast staining (Ziehl–Neelsen method) identifies bacteria with mycolic acid in their cell walls (e.g., Mycobacteria). Acid-fast cells appear red; non-acid-fast cells appear blue.

Summary Table: Colony Morphology

The following table summarizes key colony morphology characteristics:

Shape	Margin	Elevation
Circular, Irregular, Filamentous	Entire, Undulate, Lobate, Curled	Flat, Raised, Convex, Umbonate
Additional info: Colony morphologies are used for preliminary identification of bacterial species.	Additional info: Margins help distinguish between species.	Additional info: Elevation is often noted in lab reports.

Key Equations

• Total Magnification: $\text{Total Magnification} = \text{Objective Magnification} \times \text{Ocular Magnification}$