For cultivation and differentiation of enteric bacteria and gram positive organisms
Peptic digest of animal tissue supplies the necessary amino acids and polypeptides, while infusions supply vitamins, coenzymes and minerals as well as additional nitrogen compounds. Lactose has been used at a concentration of 1% (wt./vol) to detect acidification against the alkalinization caused by peptone catabolism. The selective action of this medium is attributed to Sodium taurocholate, which are inhibitory to most species of gram-positive bacteria. In addition, this medium does not contain crystal violet allowing Staphylococcus, Enterococcus and Mycobacterium spp. to grow. This medium does not contain Sodium chloride and therefore provides a “low electrolyte medium” on which preventing of Proteus spp. From spreading(swarming). Gram-negative bacteria usually grow well on the medium and are differentiated by their ability to ferment lactose. Neutral red will change colour as the pH changes. Lactose fermenting strains grow as red or pink and may be surrounded by a zone of acid precipitated bile. The red colour is due to production of acid from lactose, absorption of Neutral red and a subsequent colour change of the dye when the pH of medium falls below 6.8. Lactose non-fermenting strains, such as Shigella and Salmonella are colourless and transparent and typically do not alter appearance of the medium. Yersinia enterocolitica may appear as small, non-lactose fermenting colonies after incubation at room temperature.