Egg Yolk Agar- Composition, Principle, Preparation, Results, Uses


Egg Yolk Agar is a complex medium that contains the following ingredients and their quantities per liter of distilled water:

  • Casein enzymic hydrolysate: 15 g
  • Papaic digest of soyabean meal: 5 g
  • Yeast extract: 3 g
  • Sodium chloride: 5 g
  • L-cystine: 0.5 g
  • Vitamin K1: 1 mg
  • Hemin: 5 mg
  • Agar: 16.41 g
  • Egg yolk emulsion: 100 ml

The final pH of the medium is adjusted to 7.5±0.2 at 25°C.

Each ingredient has a specific role in the medium:

  • Casein enzymic hydrolysate and Papaic digest of soyabean meal provide the essential nutrients along with carbonaceous and nitrogenous substances for the growth of anaerobic bacteria.
  • Yeast extract supplies B-complex vitamins and other growth factors.
  • Sodium chloride maintains the osmotic equilibrium of the medium.
  • L-cystine is an amino acid that also acts as a reducing agent to create an anaerobic environment.
  • Vitamin K1 and hemin are essential cofactors for some anaerobic bacteria, especially those belonging to the genus Clostridium.
  • Agar is a solidifying agent that gives the medium a firm consistency.
  • Egg yolk emulsion is the key ingredient that differentiates Egg Yolk Agar from other media. It contains lecithin, a phospholipid that can be hydrolyzed by lecithinase-producing bacteria, and free fats that can be hydrolyzed by lipase-producing bacteria. It also contains proteins that can be degraded by proteolytic bacteria.

The composition of Egg Yolk Agar makes it suitable for detecting various enzymatic activities of anaerobic bacteria, which can be used for their presumptive identification.