Characterization of Shiga toxin-encoding temperate bacteriophages

The production of Shiga toxin (Stx) is associated with strains of Shigella dysenteriae type 1 and several serotypes of Escherichia coli, such as enterohaemorrhagic E.coli O157:H7. S. dysenteriae produces the prototypical Stx, whereas different variants of Stx (designated Stx1 and Stx2) are produced by E. coli strains. In E. coli the genes encoding the different Stx types are associated with temperate lambdoid bacteriophages, which are integrated into the chromosome of the host bacterium. In S. dysenteriae no Stx-converting phages are produced, probably because of loss of essential phage genes, although the stx genes are flanked by phage related genes.In our laboratory we isolated a viable Stx-encoding bacteriophage from Shigella sonnei (1), and phages encoding the toxin variants Stx1c (2) and Stx2c (3) from E. coli strains. These phages can multiply and lyse their host bacteria and are able to infect Stx-negative E. coli strains, which are converted to STEC. We are currently determining the genomic sequences of the phages and the integration sites in the bacterial chromosome to explore their relationship to nontoxigenic lambdoid bacteriophages and other Stx-encoding phages. The aim is to identify phage specific gene sequences associated with toxin variants, like e.g. the q gene (late antiterminator) of the stx2c-encodinging phage, and to elucidate the ecological and genetic mechanisms which convert harmless prophages into vectors spreading potent pathogenicity markers by horizontal gene transfer.(1) Strauch, Lurz, Beutin. 2001, Infect. Immun. 69: 7588-95.(2) Koch, Hertwig, Lurz, Appel, Beutin. 2001, J. Clin. Microbiol. 39:3992-8.(3) Strauch, Schaudinn, Beutin. 2004, Infect. Immun. 72:7030-9.