Escherichia coliStrains 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.