One of several media designed for the differential selective isolation of Salmonella and Shigella spp. from clinical and environmental samples. Lactose is added to the medium together with Neutral Red indicator to assist in the differentiation of Lactose and Non-Lactose fermenting organisms. The medium is made selective by the inclusion of Sodium Deoxycholate and Sodium Thiosulphate, which will inhibit most Gram-positive organisms. The Sodium Thiosulphate is also broken down by the enzyme thiosulphate reductase to form Sodium Sulphite and Hydrogen Sulphide. The Hydrogen Sulphide, if present, will in turn react with the ferric ions to produce a black precipitate of Ferrous Sulphide and give rise to the classical black centre of most Salmonellae. It has been suggested that this medium may be more suitable for secondary post-enrichment isolation while the original formulation is more appropriate for the primary inoculation of samples.