Diversity of stx 2 converting bacteriophages induced from Shiga-toxin-producing Escherichia coli strains isolated from cattle

Abstract

The presence of bacteriophages encoding Shiga toxin 2 (stx 2 phages) was analysed in 168 strains of Shiga-toxin-producing Escherichia coli (STEC) isolated from cattle. Following mitomycin C induction, strains carrying stx 2 phages were screened by plaque blot and hybridization with an stx 2 A-probe. In the stx 2 -phage-carrying strains, the amounts of phage production, phage DNA extracted and Stx 2 produced after induction were assessed. The induced stx 2 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 stx 2 phage. Most of them showed a direct relationship between phage induction and toxin production. Each strain carried only one inducible stx 2 phage, although a few strains had two copies of the stx 2 in the chromosome. The stx 2 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 stx 2 gene, while a few carried stx 2 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 stx 2 phages are common among STEC of animal origin and that they may enhance the spread of stx 2 .

Evaluation of Major Types of Shiga Toxin 2e-Producing Escherichia coli Bacteria Present in Food, Pigs, and the Environment as Potential Pathogens for Humans

“…It is therefore likely that the P27-like phages are present as cryptic prophages or phage remnants in most of the strains. This suggestion is also supported by the finding that stx 2e genes are generally not transducible from their host strains, which is in contrast to the case for other types of stx genes (29,31).…”

BaeSR, Involved in Envelope Stress Response, Protects against Lysogenic Conversion by Shiga Toxin 2-Encoding Phages

“…The effect, with some variations, appears to be common to all the Stx2 phages studied here. This group of phages show some variability in their sequences (14) but also share common traits.…”

“…Phage receptors are mainly represented by surface proteins, polysaccharides, and lipopolysaccharides (35). Short-tailed Stx phages, like the ones in this study (14), specifically recognize BamA (21,22). BamA (or YaeT) is a conserved protein essential for outer membrane protein assembly in Gram-negative bacteria (21).…”

“…933W is a reference phage isolated from a STEC strain (13), and the other seven Stx2 phages were induced from STEC strains isolated from cattle (14). All the phages were labeled with a cat gene (from plasmid pKD3) (Table 1) inserted into the stx 2 gene (15) (stx 2 ::cat phages) ( Table 1) and were used to assess lysogenic conversion of E. coli DH5␣ (Table 1), a recA-negative E. coli K-12 derivative.…”

“…DH5␣⌬baeSR with a pGEM vector without an insert was used as a negative control. The Stx2 phages used in this study differed genetically from each other, and some also had different insertion sites (14,20). Phage lysates containing 10 7 PFU/ml were used for at least three to five independent assays.…”