Common origin of plasmid encoded alpha-hemolysin genes in Escherichia coli

Abstract

Background

Alpha (α)-hemolysin is a pore forming cytolysin and serves as a virulence factor in intestinal and extraintestinal pathogenic strains of E. coli. It was suggested that the genes encoding α-hemolysin ( hlyCABD) which can be found on the chromosome and plasmid, were acquired through horizontal gene transfer. Plasmid-encoded α- hly is associated with certain enterotoxigenic (ETEC), shigatoxigenic (STEC) and enteropathogenic E. coli (EPEC) strains. In uropathogenic E. coli (UPEC), the α- hly genes are located on chromosomal pathogenicity islands. Previous work suggested that plasmid and chromosomally encoded α- hly may have evolved independently. This was explored in our study.

Results

We have investigated 11 α- hly plasmids from animal and human ETEC, STEC and EPEC strains. The size of α- hly plasmids ranges from 48-157 kb and eight plasmids are conjugative. The regulatory gene ( hlyR) located upstream of the hlyCABD gene operon and an IS 911 element located downstream of hlyD are conserved. Chromosomally-encoded α- hly operons lack the hlyR and IS 911 elements. The DNA sequence of hlyC and hlyA divided the plasmid- and chromosomally-encoded α-hemolysins into two clusters. The plasmid-encoded α- hly genes could be further divided into three groups based on the insertion of IS 1 and IS 2 in the regulatory region upstream of the α- hly operon. Transcription of the hlyA gene was higher than the housekeeping icdA gene in all strains (rq 4.8 to 143.2). Nucleotide sequence analysis of a chromosomally located α- hly determinant in Enterobacter cloacae strain indicates that it originates from an E. coli α- hly plasmid.

Conclusion

Our data indicate that plasmids encoding α- hly in E. coli descended from a common ancestor independent of the plasmid size and the origin of the strains. Conjugative plasmids could contribute to the spread of the α- hly determinant to Enterobacter cloacae. The presence of IS-elements flanking the plasmid-encoded α- hly indicate that they might be mobile genetic elements.