Comparison of sorbitol MacConkey agar and a two-step method which utilizesenzyme-linked immunosorbent assay toxin testing and a chromogenic agar to detect and isolate enterohemorrhagic Escherichia coli

Enterohemorrhagic Escherichia coli (EHEC) and specifically serotype O157:H7 are a significant cause of hemorrhagic gastrointestinal disease and the he molytic uremic syndrome. Methods currently used in clinical microbiology la bs, such as sorbitol-MacConkey (SMAC) agar, reliably detect only O157:H7. W e have evaluated a two-step method that has the potential to identify and i solate all EHEC serotypes, including serotype O157:H7. This method utilizes a chromogenic selective-differential medium for the isolation of E. coli t ogether with an enzyme-linked immunosorbent assay (ELISA) that detects the Shiga-like toxins Stx1 and Stx2. Both are commercially available and usable in a wide range of clinical microbiology laboratories. Compared to a Vero cell cytotoxic assay, SMAC had sensitivities of 23.5% for the identificatio n of all EHEC serotypes and of 50.0% for the identification of O157:H7 alon e. The two-step method had sensitivities of 76.5 and 100%, respectively. Th e ELISA alone had a sensitivity of 82.4% in the detection of Stx1 and Stx2. The specificity was 100% in all cases. Overall, 14 EHEC isolates were obta ined: 8 (58%) O157:H7, 2 (14%) O26, 2 (14%) O111:NM, 1 (7%) O103: H2, and 1 (7%) O121:H19. All but one were isolated during the months of May to Septe mber. The tao-step method was found to be considerably more expensive than SMAC for both positive and negative samples.