Salmonella Shigella agar (SS-agar) is a selective and differential medium widely used to isolate Salmonella and Shigella from feces. H2S producing bacteria, like Salmonella in this picture, will make colonies with black centers. Escherichia coli is not a H2S producer and will not produce black colonies. E.coli is, however, lactose-positive and will therefore produce pink/red colonies on this type of agar. (Salmonella is a not a lactose fermenter).