EVOLUTION OF ENTEROHEMORRHAGIC ESCHERICHIA-COLI HEMOLYSIN PLASMIDS AND THE LOCUS FOR ENTEROCYTE EFFACEMENT IN SHIGA TOXIN-PRODUCING ESCHERICHIA-COLI

This study assessed the diversity of the enterohemorrhagic Escherichia coli (EHEC) hemolysin gene (ehxA) in a variety of Shiga toxin-produci ng E. coli (STEC) serotypes and the relationship between ehxA types an d virulence markers on the locus for enterocyte effacement (LEE). Rest riction fragment length polymorphism of the ehxA gene and flanking seq uences and of the E. coli attaching and effacing (eae) gene was determ ined for 79 EHEC hemolysin-positive STEC isolates of 37 serotypes. Two main groups of EHEC hemolysin sequences and associated plasmids, whic h corresponded to the eae-positive and the eae-negative isolates, were delineated. Comparisons of the ehxA gene sequences of representative isolates of each group showed that this gene and the rest of the EHEC hemolysin operon are highly conserved. Digestion of an ehxA PCR produc t with the restriction endonuclease TaqI showed a unique restriction p attern for eae-negative isolates and another one for isolates of serot ypes 0157:H7 and 0157:NM. A conserved fragment of 5.6 kb with four pot ential open reading frames was identified on the EHEC hemolysin plasmi d of eae-positive STEC. Phylogenetic analysis of a subset of 27 STEC i solates, one enteropathogenic E. coli isolate, and a K-12 reference is olate showed that eae-positive STEC isolates all belong to a single ev olutionary lineage and that the EHEC hemolysin plasmid and the ehxA ge ne evolved within this lineage without recent horizontal transfer. How ever, the eae gene and the LEE appear to have been transferred horizon tally within this STEC lineage on several occasions. The reasons for t he lack of transfer or maintenance of the LEE in other STEC lineages a re not clear and require further study.