Comparison of Sorbitol MacConkey Agar and a Two-Step Method Which Utilizes Enzyme-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 hemolytic uremic syndrome. Methods currently used in clinical microbiology labs, such as sorbitol-MacConkey (SMAC) agar, reliably detect only O157:H7. We have evaluated a two-step method that has the potential to identify and isolate all EHEC serotypes, including serotype O157:H7. This method utilizes a chromogenic selective-differential medium for the isolation of E. coli together 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 identification of all EHEC serotypes and of 50.0% for the identification of O157:H7 alone. The two-step method had sensitivities of 76.5 and 100%, respectively. The 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 obtained: 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 September. The two-step method was found to be considerably more expensive than SMAC for both positive and negative samples.

肠出血性大肠埃希菌（Enterohemorrhagic Escherichia coli, EHEC），尤其是血清型O157:H7，是引发出血性胃肠道疾病与溶血性尿毒症综合征的重要病原体。目前临床微生物实验室常规使用的检测方法，如山梨醇-麦康凯（sorbitol-MacConkey, SMAC）琼脂，仅能可靠检出O157:H7血清型。本研究评估了一种两步法检测方案，其可实现包括O157:H7在内的所有EHEC血清型的识别与分离。该方案借助显色选择性鉴别培养基分离大肠埃希菌，并结合可检测志贺样毒素Stx1与Stx2的酶联免疫吸附试验（enzyme-linked immunosorbent assay, ELISA）。上述两种试剂均已实现商业化，可在各类临床微生物实验室中开展应用。与Vero细胞细胞毒性试验相比，SMAC琼脂对所有EHEC血清型的检测灵敏度为23.5%，对单独O157:H7的检测灵敏度为50.0%；而该两步法的对应检测灵敏度分别为76.5%与100%。仅采用ELISA检测Stx1与Stx2时，检测灵敏度为82.4%。所有检测场景下的特异性均为100%。本研究共获得14株EHEC分离株，其中8株（58%）为O157:H7，2株（14%）为O26，2株（14%）为O111:NM，1株（7%）为O103:H2，1株（7%）为O121:H19。除1株外，其余分离株均采集于5月至9月期间。经测算，该两步法无论针对阳性样本还是阴性样本，检测成本均显著高于SMAC琼脂。