Comparative analysis of virulence genes, genetic diversity, and phylogeny of Shiga toxin 2g and heat-stable enterotoxin STIa encoding Escherichia coli isolates from humans, animals, and environmental sources

An analysis for stx2 variants among the 2010 human stx2-positive Shiga toxin-producing Escherichia coli (STEC) strains from Germany collected at the National Reference Centre 1999–2008 revealed 0.6% to possess the recently described stx2g gene. Sequencing of the whole stx2g operons showed new alleles and pseudogenes. The further molecular, phenotypic, and phylogenetic comparison of 12 human stx2g-harbouring isolates with 12 stx2g-harbouring isolates from animals or environmental sources demonstrated that both groups are closely related, indicating the human infections as a potential zoonotic disease. Although originating from various different sources, the stx2g-containing strains belong to only 3 phylogenetic lineages, represented by 4 serovars belonging to 4 sequence types. In view of the huge diversity among other STEC, this suggests the emergence of the stx2g variant as a rather recent microevolutionary event. Interestingly, in the strains under investigation, Stx2g was not expressed. However, all of them contained the estIa gene which typically is associated with enterotoxin-producing E. coli and did express STIa. By this combination of virulence genes of different pathotypes of intestinal pathogenic E. coli, these strains represent a new, intermediate pathotype and emerging pathogens. Given a rising number of intermediate pathotypes becoming described among E. coli, a wider range of virulence markers should be included in the regular pathotype diagnostics.