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 (EP EC), enterohemorrhagic E. coli (EHEC), enterotoxigenic E. coli (ETEC), enteroinvasive E. coli (EIEC), and urinary tract infection (UTI) stra ins. The pathogenic strains are identified by serotype and by possessi on of specific virulence determinants (toxins and adhesions, etc.) enc oded by either monocistronic genes, plasmids, or pathogenicity islands . Although there are studies on the relationships between selected pat hogenic 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 det ermined. We used multilocus enzyme electrophoresis (MLEE) at 10 enzyme loci and the sequence of the mdh housekeeping gene to study the genet ic relationships of pathogenic E. coli strains (including Shigella clo nes), namely, 5 EPEC strains (serotypes O111 and O55), 3 EHEC strains (serotype O157), 6 ETEC strains (serotypes O78, O159, and O148), 5 EIE C strains (serotypes O124, O28, and O112), and 13 Shigella strains rep resenting clones Flexneri, Dysenteriae, Boydii, and Sonnei, to commens al E. coli strains. Both the MLEE and mdh sequence trees reveal that E PEC, EHEC, ETEC, EIEC, and UTI strains are distributed among the ECOR set groups, with no overall clustering of EPEC, ETEC, EIEC, or UTI str ains. The genus Shigella is shown to comprise a group of closely relat ed pathogenic E. coli strains. Six pathogenic strains, i.e., M502 (EIE C; O112ac:NM), M503 (EPEC; O111:H12), M526 (ETEC; O159:H4), M522 (EPEC ; O111ac:H12), M524 (ETEC; O78:H11), and M506 (ETEC; O78:H11), were fo und to have mdh sequences identical to those of five ECOR group A stra ins (ECOR5, ECOR10, ECOR14, ECOR6, and K-12). All 11 strains are close ly related by MLEE. The results indicate that pathogenic strains off. 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 ris e to a pathogenic form.