SadB acts as a master regulator modulating Pseudomonas aeruginosa pathogenicity

Pseudomonas aeruginosa sadB mutants exhibit a hyper-swarming, biofilm defective phenotype. To acquire insights into the in vivo contribution of sadB to virulence in a mouse soft tissue infection model, we employed bioluminescence imaging and histopathology. Compared with the parent PAO1 strain, the {Delta}sadB mutant was highly attenuated and rapidly cleared from the infection site whereas genetic complementation conferring constitutive expression of sadB resulted in a much more persistent phenotype. Transcriptome analysis of exponential and stationary phase planktonic cells revealed that SadB modulates expression of diverse genes involved in biofilm development, quorum sensing (QS), secondary metabolite production, iron acquisition, virulence, protein secretion and anaerobiosis. In {Delta}sadB, we observed log phase induction of the rhl and pqs QS systems, increased production of siderophores and pyocyanin, differential regulation of genes involved in c-di-GMP signalling and a growth defect under static conditions. Since {Delta}sadB is a hyperswarmer and as swarming requires rhamnolipids, this suggested that deletion of sadB may impact on the timing and level of rhamnolipids produced. In {Delta}sadB, the rhlA and rhlB genes were induced in log rather than stationary phase resulting in overproduction of rhamnolipids. Since they also contribute to biofilm maturation and dispersal and can act as anti-adhesives, we deleted the rhlA in {Delta}sadB and observed that biofilm formation was restored offering mechanistic insight into the biofilm defective phenotype of {Delta}sadB. SadB clearly has a more global role than previously appreciated and acts as a master regulator of diverse genes involved in environmental adaptation, biofilm formation and virulence. IMPORTANCEBiofilms are characterised by their intrinsic tolerance to antibiotics, host immune defences and ability to cause persistent infections. In Pseudomonas aeruginosa, mutation of the surface attachment defect gene, sadB results in cells that are biofilm, defective, hyperswarmers. Here we describe an investigation of the contribution of SadB to virulence. Our study indicates that in P. aeruginosa a sadB deletion mutant is greatly attenuated in an acute mouse skin infection model. We further demonstrate that SadB acts as a pleiotropic regulator of many genes involved in biofilm development, quorum sensing, iron acquisition, protein secretion and anaerobiosis, uncovering a wider role in pathogenesis than formerly recognized, and that it has considerable potential as a novel protein target for antibacterial drug discovery.