Lecture Notes: Selective and Differential Media (Part 2)

Overview

• Focus on selective and differential media used to identify bacteria based on biochemical properties.
• Media can be selective, differential, or both.
• This lecture covers media that are both selective and differential.

Types of Media Discussed

1. Eosin Methylene Blue (EMB) Agar

• Features:
  • Dark bluish-purple color.
  • Selective for Gram-negative bacteria; inhibits Gram-positive bacteria.
  • Differential for lactose fermentation.
• Differentiation:
  • Lactose positive (Lac+): Colonies turn purple to black.
  • Lactose negative (Lac-): Colonies have no color.
  • Weak lactose fermenters: Colonies appear pink.

2. MacConkey Agar

• Features:
  • Light pinkish-purple color.
  • Contains bile salts and crystal violet; inhibits Gram-positive bacteria, selects for Gram-negatives.
  • Differential for lactose fermentation using neutral red as a pH indicator.
• Differentiation:
  • Lac+ organisms: Turn red/pink as pH drops.
  • Lac- organisms: Media remains yellow as pH increases.

3. Mannitol Salt Agar (MSA)

• Features:
  • High salt concentration (7.5-10% NaCl).
  • Selects for Gram-positive bacteria; inhibits Gram-negatives.
  • Differential for mannitol fermentation using phenol red.
• Differentiation:
  • Mannitol fermenters (Mannitol+): Media turns yellow (low pH).
  • Mannitol non-fermenters (Mannitol-): Media turns dark pink (increased pH).

4. Bile Esculin Agar (BEA)

• Features:
  • Contains high bile salts; selects against Gram-positive bacteria.
  • Differential for Enterococcus and Streptococcus via esculin hydrolysis.
• Differentiation:
  • Enterococcus: Media turns black (esculin hydrolysis).
  • Streptococcus: Media remains beige.

5. Hektoen Enteric (HE) Agar

• Features:
  • Teal color; selective against most Gram-positive bacteria.
  • Differential for lactose fermentation and hydrogen sulfide production.
• Differentiation:
  • Lac+ organisms: Media turns orange (acidic).
  • Lac- organisms: Media remains blue.
  • Hydrogen sulfide producers: Colonies show black precipitate.

Summary

• Selective and differential media are crucial tools in microbiology for differentiating bacterial types based on growth and biochemical properties.
• Understanding these media helps in identifying specific bacterial species, especially in clinical diagnostics.