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

Abstract

Enterohemorrhagic Escherichia coli O157 (O157) strains can be classified into clades (one of several phylogenetic groups) by single nucleotide polymorphisms (SNPs): these are clade 1, clade 2, clade 3, descendant and ancestral clades 4/5, clade 6, clade 7, clade 8, clade 9, and clade 12. Some recent studies showed that some O157 strains in clade 8 produced a larger amount of Shiga toxin (Stx) 2 than other strains. In this study, 1121 epidemiologically unlinked strains of O157 isolated in Chiba Prefecture, Japan were classified into clades during 1996–2014. Clade 8 strains were further classified into subclade 8a (67 strains) and subclade 8b (48 strains) using SNP analysis. In the absence of mitomycin C (MMC), subclade 8a strains in this study produced significantly greater amounts of Stx2 than subclade 8b strains. However, in the presence of MMC, the levels of Stx2 production in subclade 8b strains were significantly greater than subclade 8a strains. On the other hand, a recent study reported that the Stx2 production level in O157 strains was determined mainly by the subtypes of Stx2a phage (ϕStx2_α, β, γ, δ, ε, and ζ). Using O157 strains in this study, the Stx2a phages were classified into these subtypes. In this study, all strains of subclades 8a and 8b carried ϕStx2a_γ and ϕStx2a_δ, respectively. Some strains in clade 6 also carried ϕStx2a_δ. In the presence of MMC, subclade 8b strains produced significantly greater amounts of Stx2 than clade 6 strains carrying ϕStx2_δ. In this study, we propose that Stx2 production in subclade 8b strains in the presence of MMC might be enhanced due to genetic factors other than ϕStx2_δ.

Clarification of relationship between single-nucleotide polymorphism panels of Shiga toxin-producing <i>Escherichia coli</i> O157:H7/H- strains

“…Our results also corroborated the previous finding that clade 8 should be divided into two groups [ 4 , 5 , 12 ]. Clade 8 has been thought to have an important genotype because of its high likelihood of causing HUS [ 5 , 6 , 7 , 12 ].…”

“…Our previous study indicated that clade 7 includes two lineages, which are phylogenetic groups, and those lineages were redefined as separate clades, namely, clades 7 and 12 ( Supplementary Table 1 ) [ 3 , 15 ]. However, our subsequent studies [ 4 , 5 ] showed that clade 7 could be further divided into several clades. Clade 8 has also been reported to be divided into two groups [ 4 , 5 , 12 ].…”

“…However, our subsequent studies [ 4 , 5 ] showed that clade 7 could be further divided into several clades. Clade 8 has also been reported to be divided into two groups [ 4 , 5 , 12 ].…”

“…The discriminatory power of molecular epidemiological methods can be used to further revise Manning’s revised panel [ 3 , 10 ]. Our previous studies [ 4 , 5 ] showed that, using data from molecular epidemiological analysis, the classification of clades could be revised or clades could be subdivided. Some SNP panels other than Manning’s revised panel [ 3 , 10 ], which consist of a small number of SNPs detected in the whole genomes of O157, have been reported to be sufficient to differentiate the organism into separate genotypes [ 1 , 8 , 13 ].…”

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“…In the USA, clade 8 strains have been associated with more severe human disease, which led researchers to define it as hypervirulent [18]. Although clade 8 strains had higher Stx2a expression levels relative to strains from other clades, along with unique genetic features [11,21,22], to date the factors associated with the increased virulence and Stx2a production are not completely understood. Interestingly, the clade 8 EHEC O157:H7 isolates obtained from bovines in Argentina had a certain degree of genetic variability and displayed variable virulence in in vitro and in vivo assays [20].…”