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

¹ Department of Microbiology, University of Barcelona, Diagonal 645, E-08028 Barcelona, Spain
² Laboratorio de Referencia de E. coli (LREC), Departamento de Microbiología y Parasitología, Facultad de Veterinaria, Universidad de Santiago de Compostela, 27002 Lugo, Spain
³ Servei Microbiologia, Laboratori de l'Agència de Salut Pública de Barcelona, Drassanes 13, E-08001 Barcelona, Spain

Correspondence
Maite Muniesa
mmuniesa{at}ub.edu

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.

The GenBank/EMBL/DDBJ accession numbers of the stx₂ genes of phages detailed in this paper are AY633453–AY633473.

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