All the culture-positive cases of EHEC infection, irrespective of the serogroups isolated, are reported to the National Institute of Infectious Diseases. At present, the most dominant serotype is O157:[H7], followed by O26:[H11] and O111:H-. These three serotypes account for more than 95% of the EHEC isolated in Japan (5). Recent advances in microbial genome sequencing have enabled the establishment of new methods for subtyping bacterial isolates. Among these, MLVA is one of the most widely accepted and useful methods (6). MLVA was successfully used to elucidate beta-catenin signaling the molecular epidemiology of EHEC O157:[H7], and an MLVA system
involving nine genomic loci was established for this serogroup (7). However, JNK signaling pathway inhibitors this system has not yet been
applied for EHEC serogroups other than O157. In the present study, we first investigated whether the MLVA system for O157 can be applied to EHEC O26 and O111 and found that it cannot. Therefore, on the basis of the genome sequences of EHEC O26 and O111 (8), we developed an expanded MLVA system that is applicable not only to the O157 serogroup but also to the O26 and O111 serogroups. Furthermore, our study revealed that cluster analysis based on the MLVA profiles is comparable to that based on PFGE profiles in outbreak investigations. A total of 641 EHEC isolates (153 O157:H7/-, 355 O26:H11/-, and 133 O111:H- isolates) were examined in the present study. All these isolates were collected by the staff of local public health institutes between 2005 and 2007. Among these, 145 O26 and 39 O111 isolates had been collected during nine and three outbreaks, respectively, and were used to evaluate the efficacy of our new MLVA system in detecting outbreak-related strains. A strain set comprising 469 isolates (153 of O157, 219 O26, and 97 O111 isolates, referred to as ‘representative isolates’) was used to evaluate the discriminatory power of the MLVA system. This included isolates from apparently independent
sporadic cases and those representing each outbreak (one isolate from one outbreak). MLVA was carried out as described in our previous study (9). The genome sequences of four EHEC strains (two O157, one O26, and one O111 strain) were searched for tandem repeats in silico (8, 10, 11). Finally, 18 loci, including the nine loci used in the current MLVA system for O157, were used to analyze the isolates in the present study. The primers were designed so that amplification reactions could be carried out in two multiplex mixtures. The primers used in this study are shown in Table 1. The O157-9 reverse primer for O26 and O111 was different from the original primer for O157, because the sequence corresponding to the primer in O26 and O111 differed from that in O157, as described below. One primer of each primer pair was labeled at its 5′ end with 6-FAM, NED, VIC, or PET (Applied BioSystems, Foster City, CA).