The CI repressors of Shiga toxin-converting prophages are involved in co-infection of Escherichia coli strains, which causes a down regulation in the production of Shiga toxin 2

Department of Microbiology, Faculty of Biology, University of Barcelona, Diagonal 645, E-08028 Barcelona, Spain

^* To whom correspondence should be addressed. Email: mmuniesa{at}ub.edu.

	Abstract

Shiga toxins (Stx) are the main virulence factors associated with a form of Escherichia coli known as STEC (Shiga toxin-producing E. coli). They are encoded in temperate lambdoid phages located on the chromosome of STEC. STEC strains can carry more than one prophage. Consequently, toxin and phage production might have been influenced by the presence of more than one Stx prophage on the bacterial chromosome. To examine the effect of the number of prophages on Stx production, we produced E. coli K-12 strains carrying either one Stx2 prophage or two different Stx2 prophages. We used recombinant phages in which an antibiotic resistance gene (aph, cat or tet) was incorporated in the middle of the Shiga toxin operon. Shiga toxin was quantified by immunoassay and by cytotoxicity assay on Vero cells (CD₅₀). When two prophages were inserted in the host chromosome, Shiga toxin production as the rate of lytic cycle activation fell. cI repressor seems to be involved in the second prophage incorporation. Incorporation and establishment of lysogenic state of the two prophages, which lowers toxin production, could be regulated by the CI repressors of both prophages operating in trans. Although sequence of the cI genes of the phages studied differed, the CI protein conformation was conserved. Results indicate that the presence of more than one prophage in the host chromosome could be regarded as a mechanism to allow genetic retention in the cell, by reducing the activation of lytic cycle and hence the pathogenicity of the strains.