In your own words what is the role of lactose in emb agar

EMB agar or Eosin Methylene Blue agar is a selective culture medium for bacteria, commonly used to grow gram negative organisms and inhibit the growth of gram positive organisms. When a colony forms in the EMB agar, the researcher is sure that the cultured organism is a gram negative organism. 

The lactose in EMB agar is to further specify whether the gram-negative organism is a lactose fermenter (i.e. E. coli) or a non-lactose fermenter. When a lactose fermenter organism is cultured, the lactose in the agar will be fermented to acid (i.e. lactic acid, acetic acid, etc). The acid will then act on the methylene blue (the pH indicator) and will give the colony a characteristic purple color. If there is rapid fermentation of lactose, then there will be a classic green metallic sheen in the culture media.

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