When acids are produced by bacteria, the combination of the dyes (which serve as pH indicators in this medium) produces color variations in the colonies because of the acidity. Lactose-fermenting bacteria ( Escherichia coli and other coliforms) produce acid using lactose. It is the lactose that is the key to this medium's ability to differentiate coliform bacteria. EMB contains two types of sugars: lactose and sucrose. The two dyes in this medium, eosin and methylene blue, act as selective agents to inhibit Gram-positive bacteria, but allow for the growth of Gram-negative bacteria.ĮMB enables differentiation of bacterial species that are coliforms due to its dyes and its lactose content. EMB is a selective medium because it inhibits Gram-positive bacterial growth. \)Įosin-methylene blue (EMB) medium is a selective medium and a differential medium used for isolation of Gram-negative rods in a variety of specimen types, including being used frequently in clinical laboratories.
