Techniques in Microbiology: Culturing, Quantification, and Safety

Techniques in Microbiology

Introduction

This section covers essential laboratory techniques in microbiology, focusing on aseptic technique, culturing bacteria, quantifying microbial cell numbers, and laboratory safety. These methods are foundational for studying microbial growth, metabolism, and for industrial and clinical applications.

Direct and Indirect Observation of Bacteria

Microscopy and Staining

Microscopy and staining are direct observation techniques used to study bacteria.

• Tells you about:

  • Cell shape, size, arrangement

  • Motility

  • Presence/absence of certain structures

• Does not tell you about:

  • Metabolism

  • Gene presence/expression

  • Protein presence/expression

• Example: Gram staining reveals cell wall structure and arrangement.

Culture-Based Observation (Petri Plates, Test Tubes)

• Tells you about:

  • Colony shape, size, arrangement

  • Motility

  • Presence/absence of certain structures

  • Biochemistry

  • Metabolism (to an extent)

• Does not tell you about:

  • Gene presence/expression

  • Protein presence/expression

• Example: Blood agar plates can show hemolysis patterns.

Culture-Independent (Molecular Biology)

• Tells you about:

  • Motility

  • Presence/absence of certain structures

  • Metabolism

  • Gene presence/expression

  • Protein presence/expression

• Does not tell you about:

  • Cell shape, size, arrangement

• Example: PCR and gel electrophoresis can detect specific genes.

Key Definitions in Microbiology

Basic Terms

• Bacterium: Singular form of bacteria.

• Culture: A growing population in the lab.

• Pure culture: Population of a single strain of bacteria.

• Colony: Bacteria on an agar plate derived from the same mother cell.

• Medium/Media: Environmental conditions used to culture bacteria.

• Aliquot: Splitting one volume into smaller volumes.

• Broth: Liquid media for culturing bacteria.

• Agar: Gelatinous extract from red algae used to make solid media.

• Agarose: Purified polymer from agar/seaweed.

• Pouring a plate: Pouring molten agar into a petri dish.

• Spreading a plate: Spreading an aliquot of bacteria on a plate.

Taxonomic and Functional Terms

• Isolate: Microbe purified from a sample.

• Genus: Group of closely-related bacteria.

• Species: Group of near-identical bacteria.

• Strain: Group of identical bacteria.

• Motility: Movement of bacteria by their own action.

• Pathogenicity: Ability to cause infectious disease.

• Virulence: Degree/severity of pathogenicity.

Aseptic Technique

Principles and Practice

Aseptic technique is essential to prevent contamination by bacterial or fungal spores during laboratory work.

• Pre/Post-Lab: Wipe bench with disinfectant.

• Ongoing sterility: Use Bunsen burners or biosafety cabinets.

• Steps for transferring liquid culture:

  1. Flame loop to sterilize.

  2. Remove tube cap.

  3. Flame tube opening.

  4. Obtain inoculum with loop.

  5. Flame tube opening again.

  6. Replace cap and flame loop.

Using a Bunsen Burner

• Loops: Sterilize until bright orange.

• Thicker metal objects: Dip in 70% IPA, then pass through flame.

• Plastic: Not suitable for flaming.

• Flame adjustment: Blue outer flame with inner bright blue flame is optimal.

Biosafety Cabinets

• Class I: Similar to fume hood.

• Class II: Minimum for experiment protection; most common.

• Class III: Glove box; allows research on Risk Group 4 pathogens.

• HEPA filters, air flow, and containment protect worker, environment, and product.

Streak Plate Method

Quadrant Streak Technique

Used to isolate pure cultures by spreading bacteria over an agar plate to form discrete colonies.

• Purpose: Test whether a culture is pure.

• How to tell: Pure cultures yield uniform colony morphology.

Physical Sterilization Methods

Autoclave

• Steam under pressure: Yields temperature of 121°C without boiling.

• Kills endospores: 15 min (longer for large items).

• Pasteurization: Reduces microorganisms in liquids for consumption (e.g., milk at 71°C for 15 seconds).

Non-Autoclavable Items

• UV: Used for large flat surfaces; causes DNA mutations and cell death.

• Ionizing Radiation: Used for food items and heat-sensitive materials.

• Filter Sterilization: For solutions with heat-sensitive solutes; uses 0.45 μm or 0.22 μm pore size filters.

Quantifying Microbial Cell Numbers

Microscopic Cell Count

• Quick and easy but cannot distinguish live from dead cells without specific stains.

• Calculation: Count cells in defined squares and multiply by dilution factor.

Viable Cell Count (Plate Count)

• Each viable cell forms a colony (colony-forming unit, CFU).

• Serial dilutions are necessary for accurate counts.

• Methods:

  • Spread plate

  • Pour plate

  • Drop plate (preferred for high throughput)

• Sources of error: Media type, temperature, incubation duration, pipetting accuracy, sample mixing.

• Formula: $\text{CFU/mL} = \frac{\text{CFU} \times \text{Dilution Factor}}{\text{volume plated in mL}}$

Serial Dilutions

• Performed in microfuge tubes (e.g., 100 μL in 900 μL media = 1:10 dilution).

• Used to obtain countable colonies for CFU calculation.

Spread Plate and Pour Plate

• Spread plate: Sample spread over agar surface.

• Pour plate: Sample mixed with molten agar.

• Drop plating: Multiple dilutions on a single plate; preferred for efficiency.

Turbidimetric Measures

• Spectrophotometer: Measures optical density (OD) at 595 or 600 nm.

• OD vs. CFU: Must create a standard curve to relate OD to cell count.

• Limitations: High cell densities can underestimate counts due to light scattering; clumping and biofilms can distort readings.

OD(595) vs Plate Count Example

• Use 10-fold dilutions and drop plating to determine CFU/mL.

• Use 2-fold dilutions to measure OD.

• Plot OD vs. CFU/mL and generate a line of best fit.

• Allows estimation of CFU/mL from OD for future cultures.

Membrane Filtration

Technique and Applications

• Use 0.45 μm filters to pass 100 mL of solution.

• Transfer membrane to solid media, incubate, and count colonies.

• Coliforms: Indicator organisms for fecal contamination (e.g., E. coli).

• Use EMB agar to test for coliforms.

Most Probable Number (MPN)

Statistical Estimation of Cell Numbers

• Use specific media to detect pathogens or total counts.

• 3-tube or 5-tube method; more tubes increase accuracy.

• Incubate and count positive tubes; use MPN index table for estimation.

First dilution set	Second dilution set	Third dilution set	MPN index per 100 mL
0	0	0	0
1	0	0	3
2	1	0	9
3	2	1	23
3	3	3	1100
4	4	4	1600
Additional info:	Table values inferred for illustration; refer to full MPN tables for complete data.

Laboratory Safety: Risk Groups and Containment Levels

Classification of Pathogens and Labs

• Risk Group 1: Low risk to individuals and community; not regulated.

• Risk Group 2: Moderate individual risk, low community risk (e.g., Salmonella, Shigella).

• Risk Group 3: High individual risk, low community risk; effective treatments available.

• Risk Group 4: High individual and community risk; likely to cause serious disease or death.

• Containment Levels (CL): Labs must match or exceed the risk group of pathogens handled.

Industrial Uses of Bacteria

Applications

• Bacteria are used in food production, biotechnology, waste treatment, and pharmaceuticals.

• Examples include fermentation (yogurt, cheese), bioremediation, and synthesis of antibiotics.

Summary

Mastery of aseptic technique, culturing, quantification, and safety protocols is essential for microbiological research and industrial applications. Understanding the strengths and limitations of each method ensures accurate results and safe laboratory practices.