metallic sheen colonies

Escherichia coli on Endo agar, cultivation in an aerobic atmosphere, 37°C, 24 hours. E.coli and some other bacteria from the family Enterobacteriaceae metabolize lactose with the production of aldehyde and acid. The aldehyde liberates fuchsin from the fuchsin-sulfite compound, the fuchsin then colors the colonies red. In the case of Escherichia coli, this reaction is so intense that the fuchsin crystallizes out giving the colonies a greenish metallic sheen.