Isolation of Shiga Toxin-producing Escherichia coli O157: H7 from Processed Salmon Roe Associated with the Outbreaks in Japan, 1998, and a Molecular Typing of the Isolates by Pulsed-field Gel Electrophoresis

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Abstract
Shiga toxin-producing Escherichia coil (STEC) O157 were isolated from processed salmon roe which had been a suspected food item in sporadic infections which occurred in Japan in 1998. A total of 45 samples of the processed salmon roe were pre-enriched in trypticase soy broth (TSB) at 36°C for 6 h and novobiocin-supplemented modified EC broth (mEC-NB) at 42°C for 18 h. After the preenrichments, the cultures were examined for possible occurrence of STEC O157, using an immunomagnetic separation (IMS) method. From the examination, a total of 84 strains of STEC O157: H7 that were positive for both stx 1 and stx 2 genes were isolated. By applying the most-probablenumber technique, it was estimated that the number of STEC O157 was in the range of 0.73-1.5 per 10 g of the processed salmon roe. Subsequent analysis of the isolates by a pulsed-field gel electrophoresis (PFGE) revealed a pattern commonly seen in 82 isolates and another pattern in two isolates. Clinical isolates from 7 patients also showed an identical pattern to those of the 82 isolates and one isolate from a patient showed the other pattern identical to those of the two isolates. The isolates were found to belong to the phage type 14.

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