Emergence of Enterohemorrhagic Escherichia coli Serovar O157 Strains in Clade 8 with Highly Similar Pulsed-Field Gel Electrophoresis Patterns

Abstract

Enterohemorrhagic Escherichia coli serovar O157 (O157) strains with highly similar pulsed-field gel electrophoresis (PFGE) patterns were isolated in Japan during 2007 and 2008. Several genetic features related to O157 evolution were investigated to indicate whether homoplasy might have contributed to the highly similar PFGE patterns in these strains. The O157 strains were classified in lineage I/II, as defined by a lineage-specific polymorphism assay-6 with an atypical allele in Z5935 (code: 231111). Analysis of the insertion sites of stx(2) phage in these strains showed that the sites were "occupied" in yehV and "intact" in wrbA, indicating that the strains were derived from "Cluster 1" of "Subgroup C." When a specific single-nucleotide polymorphism in ECs2357 in clade 8 strains was investigated, all of the strains in the present study were confirmed to be clade 8 strains. These results indicated that the O157 strains in this study had common genetic features, suggesting that the highly similar PFGE patterns of these strains were not due to homoplasy. Because no common source of these strains could be identified in 2007 to 2008 in Japan, these strains may have emerged from a unique O157 clade 8 clone and then spread by dissemination in Japan.

Enterohemorrhagic Escherichia coli O157 subclade 8b strains in Chiba Prefecture, Japan, produced larger amounts of Shiga toxin 2 than strains in subclade 8a and other clades

“…Epidemiologically unlinked O157 strains were selected for this study so we could analyze genetically diverse strains in subclades 8a and 8b. The genetic diversity of strains derived from a clone is very low [ 21 ], and such strains show very similar levels of Stx2 production. If strains derived from a clone were included in the analysis of subclade 8a and 8b strains, the Stx2 production levels obtained for subclade 8a and 8b strains would be biased.…”

“…However, in a recent study [ 20 ], strains in subclades 8a and 8b isolated in various areas of Japan in the 1990s were analyzed, although that study did not specify the detailed areas or years in which the strains were isolated. Strains of an O157 clone can often emerge and be disseminated in a particular area during a specific time period [ 21 ]. Most strains derived from an O157 clone have very similar genetic features [ 21 , 22 ] e.g., Stx2 production, IS 629 distribution, and possession of pathogenic factors.…”

“…Strains of an O157 clone can often emerge and be disseminated in a particular area during a specific time period [ 21 ]. Most strains derived from an O157 clone have very similar genetic features [ 21 , 22 ] e.g., Stx2 production, IS 629 distribution, and possession of pathogenic factors. For example, strains of a subclade 8a clone produced larger amounts of Stx2 than strains in all other subclade 8a clones, and strains derived from the subclade 8a clone emerged in other areas of Japan in the 1990s.…”

Clade analysis of enterohemorrhagic Escherichia coli serotype O157:H7/H- strains and hierarchy of their phylogenetic relationships

“…Analysis of stx phage insertion sites was done by the method described by Shaikh and Tarr (2003) as previously reported (Yokoyama et al, 2011a). From these results, the O157 strains were divided into subgroups and clusters as described by Leopold et al (2009).…”

Phylogeny, Prevalence, and Shiga Toxin (Stx) Production of Clinical Escherichia coli O157 Clade 2 Strains Isolated in Shimane Prefecture, Japan

“…For example, O157 strains with a unique genetic marker were isolated from patients, who were not epidemiologically linked, in various areas of Japan in 2007-2008. Yokoyama et al [41] reported that these strains had very similar pulsed-field gel electrophoresis patterns, indicating that these strains had low diversity.…”