Microbiological Laboratory Techniques

Aseptic and Culture Techniques

Aseptic techniques are essential in microbiology to prevent contamination and ensure the integrity of experimental results. These methods are used during the preparation, transfer, and analysis of microbial cultures.

• Aseptic Technique: Procedures performed under sterile conditions to prevent the introduction of unwanted organisms or contaminants. Used when transferring microbes between environments.

• Transferring Microbes: Instruments such as transfer loops and needles are sterilized by flame or incinerator before and after use. Culture plates are held to minimize exposure, and test tubes are handled with care, including flaming the lip after removing the lid.

Definitions of Key Terms

• Sterilization: Complete destruction or removal of all microorganisms, achieved by heat, radiation, filtration, or chemical methods.

• Disinfection: Use of physical or chemical agents to destroy vegetative microbes and viruses on inanimate surfaces. Antiseptics are used for skin.

• Sanitization: Mechanical removal of microorganisms and debris to reduce contamination to safe levels, often using soap. Common in food industries.

• Degermination: Reduction of microbes on human skin by scrubbing or immersion in chemicals.

Culture Media

Culture media provide nutrients for microbial growth. The choice of media depends on the nutritional requirements of the organism and the purpose of the experiment.

• Physical State: Media can be liquid (broth), semisolid (agar or gelatin), or solid (agar plates).

• Chemical State: Media are classified as chemically defined (synthetic) or complex (nonsynthetic). Defined media have exact chemical formulas; complex media contain extracts and are not precisely defined.

• Functional Type: Media are categorized as general-purpose, enriched, selective, or differential.

Types of Media

• Liquid Media: Water-based solutions (broths) that do not solidify above freezing; growth occurs throughout the container.

• Semisolid Media: Clot-like at room temperature, contain agar or gelatin, used to determine bacterial motility.

• Solid Media: Provide a firm surface for growth and isolation of discrete colonies; contain agar, which is solid at room temperature and liquefies at boiling.

Chemical Classification of Media

• Chemically Defined (Synthetic): Pure compounds with specified formulas; standardized and reproducible.

• Complex (Nonsynthetic): Contains components not chemically defined, such as extracts from animals, plants, or yeast.

Functional Types of Media

• General-Purpose Media: Support growth of a broad spectrum of microbes without special requirements.

• Enriched Media: Contain complex organic substances (e.g., blood, serum, hemoglobin) for fastidious organisms. Examples: Blood agar, Chocolate agar.

• Selective Media: Contain agents that inhibit certain microbes, favoring the growth of others. Used for isolating specific microorganisms from complex samples.

• Differential Media: Allow growth of several types of microorganisms; differences appear as variations in colony size, color, or media color change.

Live Media

• Some viruses and bacteria require cell cultures or host animals for growth. Animal inoculations are rare in diagnostics; fluid or tissue suspensions may be injected into mice or hamsters for certain infections.

Example: Xylose Lysine Deoxycholate Agar

• Chemically defined, both selective and differential.

• Yellow color indicates carbohydrate xylose use; black indicates hydrogen sulfide production.

The Five I’s of Microbiology

These are the basic procedures for examining and characterizing microbes:

• Inoculation: Introducing microbes into a culture medium using sterile instruments.

• Incubation: Growing cultures at appropriate temperatures (usually 20–40°C; most at 37°C).

• Isolation: Obtaining pure cultures by separating colonies using streak plate, pour plate, or spread plate techniques.

• Inspection: Observing cultures for growth characteristics.

• Identification: Determining the identity of the microbe using morphological, physiological, biochemical, and genetic methods.

Fixation and Staining Techniques

Staining is essential for visualizing microbes under the microscope. Fixation kills and preserves organisms, while staining enhances contrast.

• Fixation: Smears are dried on glass slides and fixed by heat or chemicals.

• Staining: Makes fixed organisms visible under the light microscope.

Types of Stains

• Negative Stain: Uses acidic stains (e.g., India ink, nigrosin) to create a dark background; does not require fixation. Useful for capsules and spirochetes.

• Simple Stain: Single stain applied after fixation (e.g., methylene blue, crystal violet, safranin).

• Differential Stains: Use multiple stains to differentiate cellular components.

• Gram Stain: Four-step process; distinguishes gram-positive (purple) from gram-negative (red/pink) bacteria.

• Acid-Fast Stain: Carbolfuchsin stain followed by acid-alcohol; acid-fast bacteria remain red, non–acid-fast are blue.

• Special Stains: Used for capsules (negative), endospores, and flagella.

Fixation and Staining for Electron Microscopy

• Transmission Electron Microscopy (TEM): Uses metals to block electron transmission and increase contrast.

• Scanning Electron Microscopy: Uses chemical fixation and cryofixation; samples are dehydrated after fixation.

Microbial Identification Techniques

Microbes are identified using a combination of morphological, physiological, biochemical, serological, and genetic characteristics.

• Morphological Characteristics: Shape and arrangement of cells (bacilli, cocci, spirilla, vibrios, spirochetes, coccobacilli).

• Culture Characteristics: Appearance on plates, slants, gelatin stabs, and broths.

• Physiological/Biochemical Characteristics: Tests for hydrogen sulfide production, citrate use, phenylalanine deamination, IMViC tests, litmus milk reactions.

• Serologic Analysis: Immunologic techniques to detect antigens; antibody strength determined by titer.

• Genetic Analysis: Differentiation based on DNA/RNA sequences or restriction enzyme fragments; DNA probes and nucleic acid sequence analysis.

Culture Plates and Media Types

Different media are used for isolation, differentiation, and identification of microbes.

• Nutrient Agar: General growth medium.

• Tryptic Soy Agar (TSA): General purpose, base for other plates.

• Phenylethyl Alcohol Agar (PEA): Selective for gram-positive bacteria.

• Blood Agar Plates (BAP): Differentiate species based on hemolysin production.

• Chocolate Agar (CHOC): Used for difficult respiratory bacteria.

• Thayer-Martin Agar (TM): Isolation of Neisseria gonorrhea and N. meningitides.

• MacConkey Agar (MAC): Differentiates gram-positive and gram-negative bacteria.

• Xylose Lysine Deoxycholate (XLD) Agar: Used for stool samples; inhibits gram-positive, facilitates gram-negative growth.

• Mannitol Salt Agar (MSA): Differentiates organisms that ferment mannitol.

Growth Appearance on Slants

• Filiform: Uniform growth along the line of inoculation.

• Echinulate: Jagged margins.

• Beaded: Separate or semiconfluent colonies.

• Effuse: Thin, veil-like growth.

• Arborescent: Branched, treelike growth.

• Rhizoid: Rootlike appearance.

Gelatin Stabs and Liquefaction Types

• Gelatin stabs test for gelatinase production. Types of liquefaction include:

  • Crateriform: Saucer-shaped.

  • Napiform: Turnip-like.

  • Infundibular: Funnel-like or inverted cone.

  • Saccate: Tubular or cylindrical sac.

  • Stratiform: Liquefied at the upper region of the tube.

Growth Patterns in Broths

• Sediment: Growth at the bottom.

• Turbid: Growth throughout.

• Pellicle: Thick growth at the top.

Table: Comparison of Media Types

Table: Morphological Classification of Bacteria

Example: IMViC Tests

• IMViC stands for Indole, Methyl Red, Voges-Proskauer, and Citrate tests. Used to differentiate Enterobacteriaceae.

Example: Gram Stain Procedure

1. Crystal violet (primary stain)

2. Iodine (mordant)

3. Alcohol (decolorizer)

4. Safranin (counterstain)

• Gram-positive bacteria retain crystal violet and appear purple.

• Gram-negative bacteria lose crystal violet and take up safranin, appearing red/pink.

Additional info:

• Serologic assays are important for diagnosing infections and determining immune status.

• DNA probes and nucleic acid sequence analysis are increasingly used for rapid and specific identification of microbes.