Diversity of stx2 converting bacteriophages induced from Shiga-toxin-producing Escherichia coli strains isolated from cattle

摘要

The presence of bacteriophages encoding Shiga toxin 2 (stx2 phages) was analysed in 168 strains of Shiga-toxin-producing Escherichia coli (STEC) isolated from cattle. Following mitomycin C induction, strains carrying stx2 phages were screened by plaque blot and hybridization with an stx2A-probe. In the stx2-phage-carrying strains, the amounts of phage production, phage DNA extracted and Stx2 produced after induction were assessed. The induced stx2 phages were characterized morphologically and genetically. Assays to obtain lysogens from different strains were also carried out and phages induced from the lysogens were compared with those induced from the STEC isolates. Results indicated that 18?% of the strains carried an inducible stx2 phage. Most of them showed a direct relationship between phage induction and toxin production. Each strain carried only one inducible stx2 phage, although a few strains had two copies of the stx2 in the chromosome. The stx2 phages showed diverse morphology and a wide variability in their genome. Assays to obtain lysogens showed that not all the phages were transduced with the same frequency and only six lysogens were obtained. Phages in the lysogens were the same as those induced from their respective initial STEC host strains, although the induction and relative toxin production of the lysogens varied. Most phages carried the stx2 gene, while a few carried stx2 variants. Infectivity of the phages depended on the different hosts, although O157?:?H7 was preferentially infected by phages induced from O157 strains. The results show that inducible stx2 phages are common among STEC of animal origin and that they may enhance the spread of stx2.