Large-Scale Population Structure of Human Commensal Escherichia coli Isolates

The study of several Escherichia coli intestinal commensal isolates per individual in 265 healthy human subjects belonging to seven populations distributed worldwide showed that the E. coli population is highly structured, with major differences between the tropical and temperate populations. Escherichia coli is a commensal inhabitant of the intestinal tracts of healthy humans and many animal species, but it can also cause a wide range of diseases, ranging from diarrhea to extraintestinal infections (8). As it has been proposed that pathogenic E. coli strains are derived from commensal strains by the acquisition of chromosomal or extrachromosomal viru-lence operons (19), identifying the factors that shape the ge-netic structure of commensal strains might help us understand the emergence of virulence. E. coli can be considered to have a clonal genetic structure with a low level of recombination (7, 10). Four main phylogenetic groups, A, B1, B2, and D, consti-tute the bulk of the species (15). A few authors have examine