c-di-GMP is a key regulator of Pseudomonas aeruginosa response to UVA radiation

The intracellular signaling molecule bis-(3',5')-cyclic dimeric guanosine monophosphate (c-di-GMP) plays a crucial role in several bacterial processes, including biofilm formation. c-di-GMP levels depend on the balance between its production and breakdown, controlled by diguanylate cyclases (DGCs) and phosphodiesterases (PDEs), respectively. In this work, the role of c-di-GMP in the response of Pseudomonas aeruginosa to ultraviolet A radiation (UVA, 400-315 nm) was evaluated. Solar UVA radiation is a major environmental stressor for bacteria; its harmful effects on microorganisms, due mainly to oxidative damage, have been exploited for natural solar and commercial disinfection methods. It was demonstrated here that sublethal doses of UVA produce an early increase in c-di-GMP levels and in cell adhesion. qRT-PCR assays revealed that UVA modulates the expression of genes involved in c-di-GMP metabolism, by up-regulating the DGCs-encoding genes PA3177, yfiN, sadC and wspR, and down-regulating the PDEs-coding genes bifA and rbdA, indicating a regulation at the transcriptional level. Studies with mutants deficient for the genetic regulatory systems Stringent Response (SR) and Quorum Sensing (QS) demonstrated that the up-regulation of PA3177, yfiN, and sadC appears to depend on the UVA-induced SR-QS pathway, while the induction of wspR seems to be exclusively regulated by the SR. In contrast, the repression by UVA of bifA and rbdA do not seem to be linked to either of these regulatory systems. It was also demonstrated that c-di-GMP has a role in the survival of planktonic and biofilm cells under lethal UVA, possibly by promoting the production of Pel and/or Psl exopolysaccharides.