ThePseudomonas aeruginosasensor RetS switches Type III and Type VI secretion via c-di-GMP signalling

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Acute bacterial infections are associated with motility and cytotoxicity via the type III secretion system (T3SS), while chronic infections are linked to biofilm formation and reduced virulence. InPseudomonas aeruginosa, the transition between motility and sessility involves regulatory networks including the RetS/GacS sensors, as well as the second messenger c-di-GMP. The RetS/GacS signalling cascade converges on small RNAs, RsmY and RsmZ, which control a range of functions via RsmA. AretSmutation induces biofilm formation, and high levels of c-di-GMP produce a similar response. In this study, we connect RetS and c-di-GMP pathways by showing that theretSmutant displays high levels of c-di-GMP. Furthermore, aretSmutation leads to repression of the T3SS, but also upregulates the type VI secretion system (T6SS), which is associated with chronic infections. Strikingly, production of the T3SS and T6SS can be switched by artificially modulating c-di-GMP levels. We show that the diguanylate cyclase WspR is specifically involved in the T3SS/T6SS switch and that RsmY and RsmZ are required for the c-di-GMP-dependent response. These results provide a firm link between the RetS/GacS and the c-di-GMP pathways, which coordinate bacterial lifestyles, as well as secretion systems that determine the infection strategy ofP. aeruginosa.

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