Evolutionary relationships among pathogenic and nonpathogenic Escherichia coli strains inferred from multilocus enzyme electrophoresis and mdh sequence studies

Within the species Escherichia coli, there are commensal strains and a variety of pathogenic strains, including enteropathogenic E. coli (EPEC), enterohemorrhagic E. coli (EHEC), enterotoxigenic E. coli (ETEC), enteroinvasive E. coli (EIEC), and urinary tract infection (UTI) strains. The pathogenic strains are identified by serotype and by possession of specific virulence determinants (toxins and adhesions, etc.) encoded by either monocistronic genes, plasmids, or pathogenicity islands. Although there are studies on the relationships between selected pathogenic strains, the relatedness among the majority of the pathogenic forms to each other, to commensal E. coli, and to the genus Shigella (which has often been suggested to be part of E. coli) has not been determined. We used multilocus enzyme electrophoresis (MLEE) at 10 enzyme loci and the sequence of the mdh housekeeping gene to study the genetic relationships of pathogenic E. coli strains (including Shigella clones), namely, 5 EPEC strains (serotypes O111 and O55), 3 EHEC strains (serotype O157), 6 ETEC strains (serotypes O78, O159, and O148), 5 EIEC strains (serotypes O124, O28, and O112), and 13 Shigella strains representing clones Flexneri, Dysenteriae, Boydii, and Sonnei, to commensal E. coli strains. Both the MLEE and mdh sequence trees reveal that EPEC, EHEC, ETEC, EIEC, and UTI strains are distributed among the ECOR set groups, with no overall clustering of EPEC, ETEC, EIEC, or UTI strains. The genus Shigella is shown to comprise a group of closely related pathogenic E. coli strains. Six pathogenic strains, i.e., M502 (EIEC; O112ac:NM), M503 (EPEC; O111:H12), M526 (ETEC; O159:H4), M522 (EPEC; O111ac:H12), M524 (ETEC; O78:H11), and M506 (ETEC; O78:H11), were found to have mdh sequences identical to those of five ECOR group A strains (ECOR5, ECOR10, ECOR14, ECOR6, and K-12). All 11 strains are closely related by MLEE. The results indicate that pathogenic strains of E. coli do not have a single evolutionary origin within E. coli but have arisen many times. The results also suggest the possibility that any E. coli strain acquiring the appropriate virulence factors may give rise to a pathogenic form.