S.S. Agar is a selective, differential medium used for the isolation of pathogenic enterobacteria, mainly Salmonella and Shigella, from food and other samples. Gram(+) positive microorganisms and some enterobacteriaceae do not grow on this medium due to the presence of specific S.S. Agar components. Neutral red is a pH indicator and those bacteria that ferment lactose produce pink colonies. Some species of Salmonella and Proteus produce colonies with a black center as a result of hydrogen sulfide production. Enterobacteria are differentiated by their ability to ferment lactose. Salmonella and Shigella species do not ferment lactose and produce colorless colonies on S.S Agar. Salmonella species that produce H2S show colonies with a black center. E. coli produces pink to red colonies. Proteus can grow on S.S. Agar, giving colonies with a grey to black center due to hydrogen sulfide production. Enterococcus faecalis is partially inhibited while its colonies are colorless.