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 components of S.S. Agar. 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. H2S-producing Salmonella species show colonies with a black center. E. coli produces pink to red colonies. Proteus can grow on S.S. Agar, giving colonies with a gray to black center due to hydrogen sulfide production. Enterococcus faecalis is partially inhibited, and its colonies are colorless.