Chromosomal Regions Specific to Pathogenic Isolates of Escherichia coli Have a Phylogenetically Clustered Distribution

Abstract

We studied the ancestry of virulence-associated genes in Escherichia coli by examining chromosomal regions specific to pathogenic isolates. The four virulence determinants examined were the alpha-hemolysin ( hly ) loci hlyI and hlyII , the type II capsule gene cluster kps , and the P ( pap ) and S ( sfa ) fimbria gene clusters. All four loci were shown previously to be associated with pathogenicity islands of uropathogenic E. coli isolates. The hly , kps , sfa , and pap regions each have an unexpected clustered distribution among the E. coli collection of reference (ECOR) strains, but all these regions were absent from a collection of diarrheagenic E. coli isolates. Strains in the ECOR subgroup B2 typically had a combination of at least three of the four loci, and all strains in subgroup D had a copy of the kps and pap clusters. In contrast, only four strains in subgroup A had either hly , kps , sfa , or pap , and no subgroup A strains had all four together. Strains of subgroup B1 were devoid of all four virulence regions, with the exception of one isolate that had a copy of the sfa gene cluster. This phylogenetic distribution of strain-specific sequences corresponds to the ECOR groups with the largest genome size, namely, B2 and D. We propose that the pathogenicity islands are ancestral to subgroups B2 and D and were acquired after speciation, with subsequent horizontal transfer into some group A, B1, and E lineages. These results suggest that the hly , kps , sfa , and pap pathogenicity determinants may play a role in the evolution of enteric bacteria quite apart from, and perhaps with precedence over, their ability to cause disease.