Isolation and characterization of Escherichia coli O157:H7 and shiga toxin-converting bacteriophages from strains of human, bovine and porcine origin

Abstract

Toxin-converting bacteriophages encoding the Stx2 gene were induced from strains of Escherichia coli O157:H7 isolated from sewage, bovine and porcine faeces. Toxin synthesis can be stimulated by the induction of integrated toxin-converting phages from the host E. coli O157:H7 organism by ultra-violet (UV) exposure. The UV-mediated DNA damage of E. coli O157:H7 triggers a bacterial SOS response resulting in phage release. Free ranging phages outside their E. coli O157:H7 hosts were detected but could not be isolated directly from environmental samples such as sewage and river water. E. coli O157:H7 colonies carrying the genes coding for Stx2 were isolated from 1 sewage sample (0.76% of positive samples), 8 cattle faecal samples (16.67% of positive samples) and 2 pig faecal samples (14.28% of positive samples). Characterization of E. coli O157:H7 was done by repetitive sequence analysis using ERIC-PCR to determine the relationships between the individual E. coli O157:H7 strains. The ERIC-PCR analysis revealed distinct patterns for all E. coli O157:H7 strains with some small differences between the strains. DNA sequencing of some of the E. coli O157:H7 positive isolates carrying the Stx2 genes were performed confirming the amplified DNA nucleotide sequences of Stx2. Electron microscopic analysis revealed, for the first time in South Africa, that Stx2-converting phages induced from E. coli O157:H7 have different morphologies to that of phage lambda which was previously described. The role of the induced integrated Stx2 phages in natural environments such as river and dam water remains unclear. With the induction of Stx2-converting phages from environmental E. coli O157:H7 isolates, it is now possible to determine the potential of these phages to convert non-pathogenic E. coli strains and other enterobacteriaciae into pathogenic strains.