Nitric oxide inhibits Shiga-toxin synthesis by enterohemorrhagic Escherichia coli

Shiga-toxin (Stx) is the cardinal virulence factor of enterohemorrhagic Escherichia coli (EHEC). The genes encoding Stx are carried by a lambdoid phage integrated in the bacterial genome and are fully expressed after a bacterial SOS response induced by DNA-damaging agents. Because nitric oxide (NO) is an essential mediator of the innate immune response of infected colonic mucosa, we aimed to determine its role in Stx production by EHEC. Here we demonstrate that chemical or cellular sources of NO inhibit spontaneous and mitomycin C-induced stx mRNA expression and Stx synthesis, without altering EHEC viability. The synthesis of stx phage is also reduced by NO. This inhibitory effect apparently occurs through the NO-mediated sensitization of EHEC because mutation of the NO sensor nitrite-sensitive repressor results in loss of NO inhibiting activity on stx expression. Thus our findings identify NO as an inhibitor of stx expressing-phage propagation and Stx release and thus as a potential protective factor limiting the development of hemolytic syndromes.