Phylogroup B2

Escherichia coli segregates into phylogenetic groups, with group B2 containing both extraintestinal pathogenic (ExPEC) and enteropathogenic (EPEC) strains. Ten main B2 subgroups (I-X)/sequence type complexes (STcs), as well as EPEC lineages have been identified. In the current study, we characterized ExPEC and EPEC strains of E. coli phylogenetic B2 subgroups/STcs that colonize Swedish and Pakistani infants. Gut-commensal E. coli B2 strains, 120 from Swedish (N=87) and 19 from Pakistani (N=12) infants, were assigned to B2 subgroups. Carriage of the bundle-forming pili and intimin adhesin was examined in the EPEC lineages. The ExPEC virulence markers and time of persistence of the strains in the microbiota were previously determined. In total, 84% of the Swedish and 47% of the Pakistani strains belonged to one of the ten main B2 subgroups (p = 0.001). Among the Swedish strains, the most common B2 subgroups were IX/STc95 (19%), II/STc73 (17%), VI/STc12 (13%) and III/STc127 (11%), each carrying distinctive sets of ExPEC virulence markers. EPEC lineages with few ExPEC features constituted 47% of the Pakistani but only 7% in the Swedish B2 strains (p=0.0001). The subgroup distribution within phylogenetic group B2 strains colonizing the gut differs between Swedish and Pakistani infants. B2 subgroups with uropathogenic characteristics dominated the gut microbiota of Swedish infants, while EPEC-1 strains frequently colonized the intestines of Pakistani infants. Moreover, within the B2 subgroups, ExPEC virulence genes were more prevalent in Swedish than Pakistani strains. Thus, ExPEC traits exemplify the intestinal B2 strains from Western populations. IMPORTANCE The intestinal microbiota is an important reservoir for bacteria that cause extra-intestinal infections. Escherichia coli is found ubiquitously in the gut microbiota and it also causes urinary tract infections, infantile septicemia, and meningitis. Urinary tract infections are usually caused by E. coli that originate in the intestinal microbiota. E. coli also causes gastrointestinal infections, and is a major cause of diarrhea in infants worldwide. The abilities of certain E. coli strains to cause infections are attributed to their virulence factors, i.e., bacterial components that contribute to the development of different diseases. Our study shows that different subtypes of potentially pathogenic E. coli strains dominate in the gut microbiota of infants in different geographical areas and expands our knowledge on the interplay between bacterial commensalism and pathogenicity.