SagS Contributes to the Motile-Sessile Switch and Acts in Concert with BfiSR To Enable Pseudomonas aeruginosa Biofilm Formation

Abstract

The interaction of Pseudomonas aeruginosa with surfaces has been described as a two-stage process requiring distinct signaling events and the reciprocal modulation of small RNAs (sRNAs). However, little is known regarding the relationship between sRNA-modulating pathways active under planktonic or surface-associated growth conditions. Here, we demonstrate that SagS (PA2824), the cognate sensor of HptB, links sRNA-modulating activities via the Gac/HptB/Rsm system postattachment to the signal transduction network BfiSR, previously demonstrated to be required for the development of P. aeruginosa . Consistent with the role of SagS in the GacA-dependent HtpB signaling pathway, inactivation of sagS resulted in hyperattachment, an HptB-dependent increase in rsmYZ , increased Psl polysaccharide production, and increased virulence. Moreover, sagS inactivation rescued attachment but abrogated biofilm formation by the Δ gacA and Δ hptB mutant strains. The Δ sagS strain was impaired in biofilm formation at a stage similar to that of the previously described two-component system BfiSR. Expression of bfiR but not bfiS restored Δ sagS biofilm formation independently of rsmYZ . We demonstrate that SagS interacts directly with BfiS and only indirectly with BfiR, with the direct and specific interaction between these two membrane-bound sensors resulting in the modulation of the phosphorylation state of BfiS in a growth-mode-dependent manner. SagS plays an important role in P. aeruginosa virulence in a manner opposite to that of BfiS. Our findings indicate that SagS acts as a switch by linking the GacA-dependent sensory system under planktonic conditions to the suppression of sRNAs postattachment and to BfiSR, required for the development of P. aeruginosa biofilms, in a sequential and stage-specific manner.