Pathogenic Escherichia coli O157:H7 and their detection.

Enterohaemorrhagic Escherichia coli O157:H7 is of major concern to the food industry due to high pathogenicity of this foodborne organism. For the detection of these bacteria a special agar medium with a fluorogenic substrate has been developed. The medium uses the characteristics of this E. coli serotype not to ferment sorbitol and not to produce beta-glucuronidase. In contrast, approximately 96% of all other strains of E. coli are sorbitol-positive and nearly all of them are beta-glucuronidase-positive. For discrimination between Proteus and E. coli O157:H7 which are both sorbitol- and beta-glucuronidase-negative, sodium thiosulphate and ferrio ammonium citrate were added. This leads to a brownish colour of the Proteus colonies due to their production of hydrogen sulphide. Growth of the gram-positive flora was inhibited by the addition of sodium deoxycholate.