Enterohemorrhagic strains of Escherichia coli and in particular the serotype O157:H7 have presented an increasing concern among food and clinical microbiologists since their role as a causative agent in two major food related outbreaks of hemorrhagic colitis was established in 1982. Although a variety of serotypes are known to produce verotoxins; a three-year study in England and Wales showed that 99% of isolates from patients suffering diarrhoea, bloody diarrhoea or hemolytic uraemic syndrome (HUS) belonged to serogroup O157¹.

• Selective: selective for Escherichia coli O157
• Differential: Escherichia coli O157:H7 appear as straw coloured colonies while other coli appear as blue/green coloured colonies

The intestinal tract of ruminants is the prime reservoir of Escherichia coli O157 and other enterohemorrhagic Escherichia coli (EHEC) strains, therefore meats derived from cattle, sheep, goat and deer can be expected to be contaminated. Foods implicated in human illness related to Escherichia coli O157 include meats, dairy products, vegetables, salads, apple juice and water².

The Sorbitol MacConkey Agar (SMAC) with BCIG formulation includes 5-bromo-4-chloro-3-indolyl- b-D-glucuronide (BCIG) as a substrate for b-glucuronidase, an enzyme which is not usually present in Escherichia coli O157 strains. The non-sorbitol-fermenting and b-glucuronidase-negative Escherichia coli O157 will appear as straw-coloured colonies. Organisms with b-glucuronidase activity will cleave the substrate and release indoxyl (or halogenated indoxyl) which is rapidly oxidized to the insoluble indigo or its analogue, leading to a distinct blue-green coloration of the colonies³.

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