Lysogenisation of Shiga toxin-encoding bacteriophage represses cell motility

Bacteriophages are genetic elements that play key roles in the evolution and diversification of bacterial genomes. The Shiga toxin (Stx)-encoding phage plays an important role in the horizontal transfer of the stx gene. However, the influence of the Stx phage integration on the physiological properties and gene expression pattern of the host have not been clearly resolved. In this study, we constructed the Sp5 lysogen through lysogenisation of E. coli K-12 by Sp5, an Stx2 phage in enterohaemorrhagic E. coli (EHEC) O157:H7 Sakai, and examined the effect of the resulting lysogen on cell motility under various growth conditions. Sp5 lysogenisation decreased cell motility and the expression of fliC, which encodes flagellin, under anaerobic conditions at 37°C. Sp5 also lowered the expression of fliA, which encodes the FliA-sigma factor responsible for the transcription of fliC, and flhD, which facilitates the expression of fliA. Sp5 lysogenisation reduced the amount of FlhD and FlhC expressed from the araBAD promoter, suggesting that one or more genes present in Sp5 represses flhDC at the post-transcriptional level. Flagellin is highly antigenic and triggers an immune response in the host. Thus, Sp5 might enhance its viability by repressing the expression of the flagellar regulon to circumvent the immune response of host cells.