[Isolation of enterohemorrhagic Escherichia coli (O157:H7) by an immunomagnetic separation method]

Three sporadic cases of enterohemorrhagic Escherichia coli (EHEC) O157 infection which occurred in Kanagawa in 1996 were investigated. In an attempt to determine sources of the infection, a novel method of immunomagnetic separation (IMS) was employed to isolate the bacterium from feces, foods, and other associated items. In the first case, strains of EHEC O157:H7 producing Vero toxin (VT) 2 were isolated from both feces of the patient and suspected food (cattle liver) kept at a restaurant, and the strains were found to be genotypically identical through an analysis of pulsed-field gel electrophoresis (PFGE). Subsequent investigation in the meat processing store, from which the above cattle liver had been retailed to the restaurants revealed that the store was contaminated with EHEC O157:H7 producing both VT1 and VT2. In the second case, a strain isolated from the patient was EHEC O157:H7 producing both VT1 and VT2 while strains isolated from the patient's family (without apparent symptom) and the suspected facility were O137:NM producing VT2. PFGE analysis indicated that the latter two strains were genotypically identical, suggesting that the facility thus contaminated with EHEC O157 caused the infection in question. In the third case, EHEC O157:NM producing VT2 was isolated from 4 out of 7 family members including the patient, and these strains were found to be genotypically identical by subsequent PFGE analysis. Source of the infection was, however, not determined due to lack of suspected food items. In this context, four slaughterhouses in Kanagawa Prefecture were investigated for presence of EHEC O157. As a result, strains of EHEC O157:H7 producing VT1 and VT2 were isolated from the contents of cattle's distal colon and surface of the skinned carcasses. Additional attempt was also made to determine a possibility of river water being contaminated with EHEC O157. The bacterium was, however, not isolated from water samples collected from 4 major rivers in the prefecture (at 10 collecting sites). Experiments were undertaken in order to evaluate the use of IMS in isolation of EHEC O157 from food items, with different pre-enrichment media and conditions. The results indicated that pre-enrichment by trypticase soy broth at 36 degrees C for 6 h followed by inoculating onto sorbitol MacConkey agar plate containing cefixime and tellurite was most appropriate to isolate EHEC O157 strains.