Characterization of the a-haemolysin determinant fromthe human enteropathogenicEscherichia coli O26 plasmid pEO5

The 157-kb conjugative plasmid pEO5 encoding a-haemolysin in strains of human enteropathogenic Escherichia coli (EPEC) O26 was investigated for its relationship with EHEC-haemolysin-encoding plasmids of enterohaemorrhagic E. coli (EHEC) O26 and O157 strains. Plasmid pEO5 was found to be compatible with EHECvirulence plasmids and did not hybridize in Southern blots with plasmid pO157 from the EHEC O157:H7 strain EDL933, indicating that both plasmids were unrelated. A 9227-bp stretch of pEO5 DNA encompassing the entire a-hlyCABD operon was sequenced and compared for similarity to plasmid and chromosomally inherited a-hly determinants. The a-hly determinant of pEO5 (7252 bp) and its upstream region was most similar to corresponding sequences of the murine E. coli a-hly plasmid pHly152, in particular, the structural a-hlyCABD genes (99.2% identity) and the regulatory hlyR regions (98.8% identity). pEO5 and a-hly plasmids of EPEC O26 strains from humans and cattle were very similar for the regions encompassing the structural a-hlyCABD genes. The major difference found between the hly regions of pHly152 and pEO5 is caused by the insertion of an IS2 element upstream of the hlyC gene in pHly152. The presence of transposonlike structures at both ends of the a-hly sequence indicates that this pEO5 virulence factor was probably acquired by horizontal gene transfer.