Sorbitol MacConkey Agar (SMAC-BCIG) is for the isolation of Escherichia coli O157:H7 and is not intended for use in the diagnosis of disease or other conditions in humans. Pathogenicity of Escherichia coli O157:H7, the primary serovar associated with haemorrhagic colitis (HC) and haemolytic uraemic syndrome (HUS), is linked to the production of verocytotoxins (VT1 and VT2). However, it should be noted that not all strains of O157 produce verocytotoxins, and that strains from other serovars can be toxin producers (e.g. O26, O103, O111, O113, O145). E. coli O157 has been associated epidemiologically with food poisoning outbreaks involving beef burgers and cold cooked meats. This medium is a modification of Sorbitol MacConkey Agar (SMAC). The addition of the chromogenic substrate BCIG (5-bromo-4-chloro-3-indoxyl-ß-D-glucuronide) improves the specificity of the medium. E. coli O157:H7 is typically sorbitol negative and ß-glucuronidase negative producing pale translucent colonies on this medium. The majority of other E. coli strains are ß-glucuronidase positive and sorbitol positive (pink/red colonies). A small percentage of E. coli are ß-glucuronidase positive and sorbitol negative and thus appear as blue/green colonies on this medium. Consequently, this medium can distinguish between non-O157 sorbitol negative E. coli and the genuine toxigenic E. coli O157:H7. This reduces the number of unnecessary confirmation tests that are performed. The medium can be made more selective by the addition of Cefixime Tellurite supplement (X161) to prepare CT-SMAC. Most workers recommend the use of CT-supplemented medium alongside unsupplemented medium to ensure maximum isolation of E. coli O157. This medium can also be useful for the detection of other VTEC producing E. coli in conjunction with specifically targeted IMS beads (Captivate™).
