Lecture Notes: Eosin Methylene Blue (EMB) Agar

Overview

• EMB Agar: A bacterial growth medium
• Selective and Differential Characteristics:
  • Selective for gram-negative bacteria
  • Differential for fecal coliform bacteria

Selective Characteristic

• Targets Gram-negative Bacteria:
  • Contains dyes toxic to gram-positive bacteria
  • Dyes: Eosiny and Methylene Blue

Differential Characteristic

• Targets Coliform Bacteria:
  • Coliforms: Rod-shaped, gram-negative, non-spore forming, motile or non-motile
  • Ability to ferment lactose producing acid and gas
  • Presence of coliforms indicates contamination by fecal matter

Mechanism of Differential Action

• Dyes as pH Indicators:
  • Dyes turn dark purple at low pH
  • Lactose fermentation by coliforms lowers pH resulting in dark purple color
  • Differentiates lactose-fermenters from non-fermenters (e.g., E. coli vs. Salmonella/Shigella)
  • Lactose-fermenting colonies absorb more dye (nucleated appearance)

Color Indicators

• Dark Nucleated Centers:
  • Coliforms show dark centers due to dye absorption
  • Non-coliforms remain colorless
• Metallic Green Sheen:
  • Seen in strong fermenters like E. coli, some Citrobacter, and Enterobacter
  • Interaction with dyes and strong acid end-products
  • Motion from flagella may contribute
• Pink Colonies:
  • Weak fermenters produce light pink colonies

Applications

• Use in Microbiology:
  • Distinguishes types of bacteria based on color changes
  • Identifies strong, weak, and non-lactose fermenters

Additional Resources

• Further Learning:
  • Explore other selective and differential media like Mannitol Salt Agar or McConkey Agar in the bacterial growth media playlist.

---

End of lecture notes on EMB Agar.