EMB Agar or Eosin Methylene Blue Agar differential for isolation of gram-negative enteric bacilli from clinical & nonclinical samples. Primarily this medium was used for the detection and confirmation of coliforms. Medium contains Lactose and Sucrose, due to these carbohydrates sources medium can be differential in primary culture: salmonellas and shigellas which are lactose-negative can be differentiated from other lactose - negative but sucrose -positive organisms such as Proteus sp.
Peptone is the nitrogen source in EMB Agar. Agar is a solidifying agent. Dipotassium phosphate is the buffer. Eosin Y and Methylene Blue are the indicators. Methylene blue is also a selective agent. The accompanying micro flora which hinders the isolation of medically important organisms is inhibited by the dyes of the medium, especially gram -positives. The dyes in EMB agar, Eosin Y and M ethylene blue are pH indicators and inhibitors of gram -positive bacteria and at an acid pH combine to form a green -metallic precipitate. Colonies of lactose fermenters are blue -black with or without a green metallic sheen. Sheen production appears to be sensitive to changes in pH, and the lack of sheen production could be due to the alkalinity of mastitic milk interfering with the acidic requirement of EMB agar for production of the green metallic sheen. Colonies of non -lactose fermenting bacteria are colourless and translucent.