Cytolethal distending toxin gene cluster in enterohemorrhagic Escherichia coli O157:H- and O157:H7: characterization and evolutionary considerations

We identified a cytolethal distending toxin (cdt) gene cluster in 87, 6, and 0% of sorbitol-fermenting (SF) enterohemorrhagic Escherichia coli (EHEC) O157:H(-), EHEC O157:H7, and E. coli O55:H7/H(-) strains, respectively. The toxin was expressed by the wild-type EHEC O157 strains and by a cdt-containing cosmid from a library of SF EHEC O157:H(-) strain 493/89. The cdt flanks in strain 493/89 were homologous to bacteriophages P2 and lambda. Our data demonstrate that cdt, encoding a potential virulence factor, is present in the EHEC O157 complex and suggest that cdt may have been acquired by phage transduction.