Salmonella-Shigella Agar or SS Agar is a selective and differential medium for the isolation of pathogenic enteric bacilli, particularly Salmonella, Shigella and Yersinia, from clinical samples. The Salmonella and Shigella agar is a variant of the deoxycholate-citrate agar described by Leifson. It is recognized as a moderately selective medium based on the degree of inhibition of Gram-positive microorganisms and Enterobacteriaceae other than Salmonella and Shigella which it inhibits due to its bile salt, bright green and citrate content. Salmonella Shigella Agar, developed before Hektoen Agar, is misnamed. Indeed, initially used for both Salmonella and Shigella research, it is not very efficient for Shigella culture. Shigella grow much better on Hektoen agar. The presence of lactose and neutral red allows to know the lactose character of the bacteria. Sodium thiosulfate is a source of sulfur from which some bacteria form hydrogen sulfide (H2S).