Bismuth Sulphite Agar Medium (Twin Pack) (In accordance with IP 1996)

Principle And Interpretation

Bismuth Sulphite Agar is recommended by various Associations (1, 2, 3, 4, 5, 6) for the isolation and preliminary identification of Salmonella Typhi and other Salmonellae from pathological materials, sewage, water, food and other products. It is a modification of Wilson and Blair medium.

Brilliant green and bismuth sulphite incorporated into the medium inhibit the intestinal gram-negative and gram-positive bacteria. Peptone and beef extract are rich source for supplying essential nutrients for growth of the organism. The fermentable source of carbohydrate in this medium is dextrose, which provides energy of enhanced microbial growth. Phosphates incorporated in the medium acts as a good buffering agent. The bismuth ions are reduced to metallic bismuth, which impart metallic sheen around the colonies. Sulphite is reduced to black ferric sulphide giving the black colour with release of H2S.

Salmonella Enteritidis and Salmonella Typhimurium typically grow as black colonies (rabbit eye colonies) with a surrounding metallic sheen. Salmonella Paratyphi A grow as light green colonies. This medium also favors use of larger inoculum and heavily contaminated samples as compared to other selective media, as it has unique inhibitory action towards gram-positive and coliform organisms. The medium may be inhibitory to some strains of Salmonella species and therefore should not be used as the sole selective medium for these organisms. Shigella species are mostly inhibited on this medium and also some Salmonellae like S. Sendai, S. Berta, S. Gallinarum, S. Abortus-equally are inhibited. Proteus species are inhibited but few strains give dull green or brown colonies with metallic sheen.