Here's a breakdown of the types of bacteria that grow on EMB:
* Lactose fermenters: These bacteria produce acid as a byproduct of lactose fermentation. The acid lowers the pH of the medium, causing the colonies to turn dark purple or black, with a metallic green sheen in some cases. Examples include:
* E. coli: Forms characteristic dark purple colonies with a metallic green sheen.
* Klebsiella pneumoniae: Produces large, mucoid, dark purple colonies.
* Enterobacter aerogenes: Forms large, mucoid, dark purple colonies, often with a less distinct green sheen.
* Non-lactose fermenters: These bacteria cannot ferment lactose and therefore do not produce acid. Their colonies appear colorless or pale pink on EMB agar. Examples include:
* Salmonella spp.: Forms colorless colonies with a black center (due to the presence of H2S production).
* Shigella spp.: Forms colorless colonies.
* Proteus spp.: Forms colorless colonies that may swarm.
Important Note: EMB agar is selective for Gram-negative bacteria due to the presence of eosin and methylene blue, which inhibit the growth of most Gram-positive bacteria. However, some Gram-positive bacteria may grow weakly on EMB.
Overall, EMB agar is a valuable tool for differentiating between different Enterobacteriaceae based on their lactose fermentation ability, helping with the identification of potential pathogens.
