Relationship between phylogenetic groups of Escherichia coli and Pathogenicity among Isolates from chickens with Colibacillosis and healthy chickens

Avian pathogenic Escherichia coli (APEC) is closely related to extraintestinal pathogenic E. coli, which are frequently assigned to specific phylo-genetic groups (phylogroups). Therefore, we investi-gated the association between phylogroups of E. coli isolates and those recovered from commercial broiler and layer chickens with colibacillosis. We used 104 E. coli isolates from chickens with colibacillosis (hereaf-ter referred to as "colibacillosis-related isolates"), 56 E. coli isolates obtained from fecal samples of clinically healthy broiler chickens, and 58 isolates obtained from environmental samples of layer chicken housing facili-ties where clinically healthy layer chickens were reared (hereafter referred to as "healthy chicken-related iso-lates"). The prevalence of phylogroup F among colibacil-losis-related isolates was significantly (P < 0.05) higher than that among healthy chicken-related isolates, while phylogroups A and B1 were more frequently distributed in healthy chicken-related isolates. Fifty-seven (87%) of 65 colibacillosis-related isolates belonging to phy-logroup F were defined as APEC based on the presence of virulence-associated genes according to a previously established criterion. In contrast, none of the healthy chicken-related isolates were defined as APEC. As evi-denced by the chicken embryo lethality assay, 87 of the 92 healthy chicken-related isolates tested had embryo lethality rates of <30% and were considered avirulent, whereas 59 of the 104 colibacillosis-related isolates were considered virulent. Nonetheless, among isolates exhib-iting embryo lethality rates of <30%, the mean lethality rate of embryos inoculated with colibacillosis-related isolates was significantly higher than that of embryos inoculated with healthy chicken-related isolates. These observations suggest that phylogroup F predicts coliba-cillosis among E. coli strains with virulence-associated genes.