5. Culture Media in Microbiology

CULTURE MEDIA IN MICROBIOLOGY

Overview

• Course: MLT 1303 Lab Techniques - Microbiology

• Learning Outcomes: 1.5, 3.1-3.4

Classification of Media

• Various media (aka agars, plates) serve a variety of purposes.

• Ingredients of media support the growth of specific bacteria, inhibit certain bacteria, or assist in identification.

• Three categories of media:

  • Enriched Media: Nutrient-rich to support various organisms.

  • Selective Media: Inhibits growth of some bacteria while allowing specific ones.

  • Differential Media: Allows distinctive characteristics to show from different bacteria metabolism.

• Note: Different names may refer to the same media types.

Enriched Media

• Enrichment media provide nutrients for various organisms without giving an advantage to any one type.

• Examples include:

  • Blood Agar: Supports multiple organism growth.

  • Chocolate Agar: Useful for fastidious organisms.

Selective Media

• Selective media contain substances that inhibit certain bacteria:

  • Example: MacConkey Agar (MAC) - Inhibits gram-positive bacteria using crystal violet and bile salts.

• MAC plates commonly recover gram-negative bacteria.

Differential Media

• Contains nutrients/indicators that distinguish between bacterial properties:

  • Example: MAC plates contain lactose and neutral red to differentiate lactose fermentation.

• Lactose fermenters turn pink, non-fermenters appear colorless or pale pink.

• Important for identifying stool pathogens (often non-lactose fermenters).

Study Tips for Media Classification

• Don’t memorize classifications; understand the purpose and relate to examples:

  • Enriched Media: Nutrients added for growth support (e.g., blood agar).

  • Selective Media: Only allows certain bacteria to grow (e.g., MAC).

  • Differential Media: Color changes indicate fermentation differences (e.g., MAC).

Commonly Used Media in Microbiology Labs

• Essential media for identification include:

  • Blood Agar

  • Chocolate Agar

  • MacConkey Agar (MAC)

• Various labs may have different media choices but basic functions remain the same.

Focus on Various Media Types

• Understanding of growth characteristics:

  • Nutritional content supports metabolic activity.

  • Specific organisms may grow due to the unique ingredients.

  • Indicators demonstrate metabolic changes through color shifts.

Blood Agar (BA)

• Purpose: Cultivates non-fastidious microorganisms; determines hemolytic reactions.

• Composed of 5% sheep blood.

• Detects hemolysis types:

  • Beta-Hemolysis: Complete RBC lysis (clear zone).

  • Alpha-Hemolysis: Partial RBC lysis (green discoloration).

  • Gamma-Hemolysis: No lysis (non-hemolytic).

Hemolysis Types in Blood Agar

• Identification:

  • Beta (β): Complete lysis.

  • Alpha (α): Partial lysis, often noted as 'alpha-prime.'

  • Gamma (γ): No lysis; expressed as non-hemolytic.

Chocolate Agar (Choc.)

• Purpose: Enrichment medium for fastidious organisms.

• Blood is heated to lyse RBCs, yielding a brown color.

• Contains factors X and V necessary for specific bacteria like Haemophilus spp. and Neisseria gonorrhoeae.

MacConkey Agar (MAC)

• Purpose: Isolate gram-negative bacteria.

• Selective against gram-positive bacteria using dyes.

• Differential: Lactose fermentation results in color changes (lactose fermenters pink, non-lactose fermenters pale).

• Critical for identifying lactose fermenters (LF) and non-lactose fermenters (NLF).

Other Selective Agars

• Phenylethyl Alcohol Agar (PEA): Enriched media containing blood; inhibits gram-negatives.

• Columbia CNA Agar: Contains antibiotics to inhibit gram-negatives; enriched with sheep blood.

• Sorbitol MacConkey Agar (SMAC): Selective for E. coli 0157:H7 (non-sorbitol fermenter).

Hektoen Enteric Agar (HE)

• Selective for Salmonella and Shigella; differentiates based on lactose fermentation.

• Produces color changes due to pH indicator (bromothymol blue).

• Can visualize H2S production with ferric ammonium citrate, indicating black precipitate.

Cefsulodin-Irgasan-Novobiocin (CIN)

• Uses selective agents to inhibit normal flora; focuses on Yersinia spp. amid mannitol fermentation.

Campylobacter Media

• Specialized to isolate Campylobacter spp. using thioglycollate broth and antibiotics.

Thioglycolate Broths (Thio Broths)

• Supports the growth of anaerobic, aerobic, and microaerophilic organisms.

Thayer-Martin Agar

• Utilized for isolating Neisseria gonorrhoeae, enriched with nutrients and antibiotics.

Learning Checks

• Match media categories to descriptions and identify hemolysis types.

• Understand interpretations of growth patterns on specific agars.

Conclusion

• Importance of various media for isolating and identifying microorganisms in microbiology labs.

• Understanding media roles in microbial growth and differentiation is crucial for effective microbiological practices.