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 stx(2) variants among the 2010 human stx(2)-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 stx(2g) gene. Sequencing of the whole stx(2g) operons showed new alleles and pseudogenes. The further molecular, phenotypic, and phylogenetic comparison of 12 human stx(2g)-harbouring isolates with 12 stx(2g)-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 stx(2g)-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 stx(2g) 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.