BISMUTH SULPHITE AGAR MODIFIED

Description

The Salmonellae constitute the most taxonomically complex group of bacteria among Enterobacteriaceae. Human Salmonella infections are most commonly caused by ingestion of food, water or milk contaminated by human or animal excreta. Humans are the only reservoirs of S. typhi. Four clinical types of Salmonella infections may be distinguished namely gastroenteritis, bacteremia or septicemia, enteric fever and a carrier state. Of the various media employed for the isolation and preliminary identification of Salmonellae, particularly Salmonella typhi; Bismuth Sulphite Agar is the most productive. Bismuth Sulphite Agar, Modified is a modification of the original formulation of Wilson and Blair Medium. It is also recommended for the isolation of Salmonella typhi and other Salmonella. S. typhi, S. enteritidis and S.typhimurium typically grow as black colonies with a surrounding metallic sheen resulting from hydrogen sulphide production and reduction of sulphite to black ferric sulphide. Salmonella Paratyphi A grows as light green colonies. Bismuth Sulphite Agar 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; exceptions being S. flexneri and S.sonnei and also some Salmonella like S.Sendai, S. Berta, S. Gallinarum, S.abortus-equi are inhibited. Also this medium favors use of larger inoculum as compared to other selective media, as it has unique inhibitory action toward gram-positive organisms and coliforms.