Bacterial Identification: Selective and Differential Media

Bacterial Identification Using Culture Media

1. Define:

   1. Selective Medium

      A selective medium favors the growth of specific microorganisms while inhibiting others. Examples include:

      • MAC agar (MacConkey agar)
      • Mannitol salt agar
      • Eosin methylene blue agar (EMB)

2. 2. Differential Medium

      A differential medium distinguishes between microorganisms based on their biochemical reactions. Examples include:

      • MAC agar
      • Blood agar
      • Mannitol salt agar
      • Eosin methylene blue agar

MacConkey (MAC) Agar

• MacConkey agar selects for which group of bacteria? Explain.
  • Gram-negative bacteria.
• MacConkey agar media differentiates between enteric bacteria based on what biochemical reaction? Explain.
  • Lactose fermentation (presence or absence of lactase enzyme).
• If the organism is E. coli, what would you see on your plates? Explain.
  • E. coli ferments lactose, resulting in pink colonies and a pink-halo in the surrounding agar due to acid production.
• If the organism(s) is/are other enterics, what would you see on your plate? Explain.
  • Other enterics may show growth but with different color changes, indicating whether they are lactose-positive (often pink/red) or lactose-negative (colorless/tan).

Eosin Methylene Blue (EMB) Agar

• EMB agar is a nutrient medium that is both selective and differential. It selects for Gram-negative bacteria and inhibits Gram-positive bacteria. It differentiates Gram-negative bacteria based on their ability to ferment lactose.
• If the organism is E. coli, what would you see on your plates? Explain.
  • E. coli typically produces a metallic green sheen on EMB agar due to vigorous lactose fermentation.

Mannitol Salt Agar (MSA)

• MSA agar selects for which group of bacteria? Explain.
  • MSA selects for staphylococci, which can tolerate high salt concentrations (7.5% NaCl).
• MSA agar media differentiates staphylococcal species based on what biochemical reaction? Explain.
  • MSA differentiates based on mannitol fermentation.
• If the organism is Staphylococcus aureus, what would you see on your plates? Explain.
  • Staphylococcus aureus ferments mannitol, causing the phenol red indicator in the medium to turn yellow around the colonies.
• If the organism is other Staphylococcus epidermidis, what would you see on your plate? Explain.
  • Staphylococcus epidermidis typically does not ferment mannitol, so the medium remains pink around the colonies.