Escherichia coli strains belonging to phylogenetic group B2 have superior capacity to persist in the intestinal microflora of infants

Escherichia coli strains segregate into 4 phylogenetic groups, designated " A," " B1," " B2," and " D." Pathogenic strains belong to group B2 and, to a lesser extent, group D, which more frequently carry virulence-factor genes than do group A strains and group B1 strains. This study investigated whether the capacity of E. coli to persist in the human intestine is related to its phylogenetic type. Resident (n = 58) and transient (n = 19) commensal E. coli strains isolated during a longitudinal study of 70 Swedish infants and previously tested for virulence-factor-gene carriage were tested for phylogenetic type. Of the strains resident in the intestinal microflora, 60% belonged to group B2, compared with only 21% of the transient strains (P = .004). In logistic regression, group B2 type predicted persistence in the intestinal microflora, independent of carriage of all investigated virulence- factor genes, including genes for P fimbriae (P = .03). Thus, group B2 strains appear to possess yet unidentified traits that enhance their survival in the human intestine.