Bacteriophage control of Shiga toxin 1 production and release by Escherichia coli

Abstract

The stx genes of many Shiga toxin‐encoding Escherichia coli (STEC) strains are encoded by prophages of the λ bacteriophage family. In the genome of the Stx1‐encoding phage H‐19B, the stx₁AB genes are found ≈ 1 kb downstream of the late phage promoter, p_R′, but are known to be regulated by the associated iron‐regulated promoter, p_Stx1. Growth of H‐19B lysogens in low iron concentrations or in conditions that induce the prophage results in increased Stx1 production. Although the mechanism by which low iron concentration induces Stx1 production is well understood, the mechanisms by which phage induction enhances toxin production have not been extensively characterized. The studies reported here identify the factors that contribute to Stx1 production after induction of the H‐19B prophage. We found that replication of the phage genome, with the associated increase in stx₁AB copy number, is the most quantitatively important mechanism by which H‐19B induction increases Stx1 production. Three promoters are shown to be involved in stx₁AB transcription after phage induction, the iron‐regulated p_Stx1 and the phage‐regulated p_R and p_R′ promoters, the relative importance of which varies with environmental conditions. Late phage transcription initiating at the p_R′ promoter, contrary to previous findings in the related Stx2‐encoding phage φ361, was found to be unnecessary for high‐level Stx1 production after phage induction. Finally, we present evidence that phage‐mediated lysis regulates the quantity of Stx1 produced by determining the duration of Stx1 accumulation and provides a mechanism for Stx1 release. By amplifying stx₁AB copy number, regulating stx₁AB transcription and allowing for Stx1 release, the biology of the Stx‐encoding phages contributes greatly to the production of Stx, the principal virulence factor of STEC.