Diagnosis
A stool culture can detect the bacterium, although it is not a routine test and so must be specifically requested. The sample is cultured on sorbitol-MacConkey (SMAC) agar, or the variant cefixime potassium tellurite sorbitol-MacConkey agar (CT-SMAC). On SMAC agar O157 colonies appear clear due to their inability to ferment sorbitol, while the colonies of the usual sorbitol-fermenting serotypes of E. coli appear red. Sorbitol nonfermenting colonies are tested for the somatic O157 antigen before being confirmed as E. coli O157. Like all cultures, diagnosis is time-consuming with this method; swifter diagnosis is possible using PCR techniques. Newer technologies using fluorescent and antibody detection are also under development.
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