Selective Properties: EMB agar is selective due to the presence of bile salts and dyes, such as eosin and methylene blue. Bile salts inhibit the growth of Gram-positive bacteria, making it selective for Gram-negative bacteria. The dyes can inhibit the growth of certain Gram-negative bacteria, such as Proteus and Providencia, while allowing Enterobacteriaceae to grow.
Differential Properties: EMB agar also exhibits differential properties based on the fermentation of lactose and the production of certain enzymes by the bacteria. Lactose is a fermentable sugar present in the medium. Bacteria that ferment lactose produce acid, which causes a color change in the medium. Colonies that ferment lactose appear as dark purple or black colonies with a metallic sheen due to the precipitation of insoluble complexes formed by the reaction between the acid produced and the dyes.
Non-lactose fermenters, such as Salmonella and Shigella, do not produce acid from lactose and instead appear as colorless or transparent colonies. Additionally, EMB agar contains indicators such as eosin Y and methylene blue, which allow for the differentiation between bacteria that produce certain enzymes. For example, bacteria that produce β-glucuronidase can hydrolyze a substrate in the medium, causing a blue-green coloration around the colonies.
Overall, EMB agar serves as a valuable tool in microbiology laboratories for the isolation and differentiation of Gram-negative bacteria, particularly those belonging to the Enterobacteriaceae family. By combining selective and differential properties, this medium allows for the identification and characterization of specific bacteria based on their growth characteristics, color changes, and enzyme production.
