Which organism produces dark colonies with a metallic sheen on eosin methylene blue agar?

• A. Escherichia coli
• B. Enterobacter cloacae
• C. Klebsiella pneumoniae
• D. Proteus mirabilis

Answer: (A)

On eosin methylene blue agar, the appearance of a colony reflects lactose fermentation combined with dye interactions. The metallic green sheen is a hallmark of vigorous lactose fermentation by the organism, which acidifies the surrounding medium enough to cause a distinct, shiny metallic appearance as the dyes precipitate with the acid products. Escherichia coli ferments lactose rapidly, producing lots of acid, so it forms dark colonies with this characteristic metallic sheen on EMB. The other organisms listed can ferment lactose but typically do not show the metallic sheen: Enterobacter cloacae and Klebsiella pneumoniae often produce dark or purple, mucoid-looking colonies without the metallic sheen, and Proteus mirabilis generally yields colorless or pale colonies due to less or no lactose fermentation on EMB.