Inverse Regulation of Biofilm Formation and Swarming Motility byPseudomonas aeruginosaPA14

Abstract

We previously reported that SadB, a protein of unknown function, is required for an early step in biofilm formation by the opportunistic pathogen Pseudomonas aeruginosa. Here we report that a mutation in sadB also results in increased swarming compared to the wild-type strain. Our data are consistent with a model in which SadB inversely regulates biofilm formation and swarming motility via its ability both to modulate flagellar reversals in a viscosity-dependent fashion and to influence the production of the Pel exopolysaccharide. We also show that SadB is required to properly modulate flagellar reversal rates via chemotaxis cluster IV (CheIV cluster). Mutational analyses of two components of the CheIV cluster, the methylaccepting chemotaxis protein PilJ and the PilJ demethylase ChpB, support a model wherein this chemotaxis cluster participates in the inverse regulation of biofilm formation and swarming motility. Epistasis analysis indicates that SadB functions upstream of the CheIV cluster. We propose that P. aeruginosa utilizes a SadB-dependent, chemotaxis-like regulatory pathway to inversely regulate two key surface behaviors, biofilm formation and swarming motility.Pseudomonas aeruginosa is an important model organism for the study of gram-negative biofilm development, yet little is known about the molecular mechanisms underlying the initial events leading to the surface interactions that characterize the early steps in bacterial biofilm formation. Microscopic observations (23,26,40,51) and genetic analyses (2) revealed two sequential events that lead to stable surface interactions. First, a bacterial cell pole contacts the surface in a process referred to as reversible attachment. This is a relatively unstable interaction, as reversibly attached bacteria can readily return to a planktonic existence. The second event is a transition from the polar association to one that is mediated by the long axis of the cell body, referred to as irreversible attachment. In P. aeruginosa, the only mutation known to block the transition from reversible to irreversible attachment is in the sadB gene (2).Another key aspect of biofilm formation by P. aeruginosa is the production of an extracellular matrix. In pseudomonads, this matrix is thought to be comprised of exopolysaccharides (EPS), DNA, and protein (19). The biofilm matrix has typically been credited with structuring the mature biofilm (4). Studies have identified the pel and psl loci as two sets of genes predicted to be involved in the production of the polysaccharide component of the matrix required for biofilm maturation by P. aeruginosa on abiotic surfaces, although only the pel gene cluster is found in P. aeruginosa strain PA14 (7,8,15,27), the focus of study in this report. Interestingly, recent studies suggest that the pel locus also plays a role in early biofilm formation. A pel mutant of P. aeruginosa PAK shows a strong attachment defect in a strain lacking type IV pili (48) and P. aeruginosa PAO1 with a mutation in the psl locus has a block in biofilm initiat...

Sticky Situations: Key Components That Control Bacterial Surface Attachment

“…The mutant was also deficient in polysaccharide production and displayed altered reversal patterns while swimming in high-viscosity medium, two behaviors proposed to influence biofilm formation and swarming motility. Similarly, SadB was found to inversely regulate biofilm formation and swarming motility via its ability to both modulate flagellar reversals in a viscosity-dependent fashion and influence the production of the Pel exopolysaccharide (19,20). The role of SadB in the transition to irreversible attachment was further confirmed by microscopic examination that revealed an increased number of sadB mutant cells adhering via the cell pole.…”

Electron-shuttling antibiotics structure bacterial communities by modulating cellular levels of c-di-GMP

“…Cumulative reports in the literature have revealed trends in the relationships between a strain's matrix production during colony growth, ability to form a biofilm at the air-liquid interface of a static culture (pellicle), and capacity for swarming (motility over semisolid surfaces) (24,25,27,28). We note that the Δphz mutant is unusual in this context because it shows increased matrix production in the colony morphology assay but also exhibits swarming behavior comparable to that of the WT in motility assays; typically, P. aeruginosa mutants with altered c-di-GMP signaling show an inverse correlation between matrix production and swarming.…”

Global Regulatory Pathways and Cross-talk ControlPseudomonas aeruginosaEnvironmental Lifestyle and Virulence Phenotype

“…Lastly, SadB is predicted to be involved in the transmission of the signal to Pel machinery and components of the chemotaxis machinery in order to regulate biofilm formation and swarming motility (Caiazza, 2004(Caiazza, , 2007. These findings suggest that the nearly 40 P. aeruginosa DGC/PDE enzymes generate specific phenotypic outputs by altering subcellular c-di-GMP pools.…”

“…SadB/C/BifA pathway Studies performed by George O'Toole's group have identified several factors that are involved in inversely regulating surface-associated behaviors (Caiazza, 2004(Caiazza, , 2007Kuchma and O'Toole, 2007;. Previously, a genetic screen identified surface attachment-defective (sad) mutants of P. aeruginosa PA14, defined by their inability to form biofilms in a microtiter dish biofilm assay (O'Toole, 1998(O'Toole, , 2000.…”