SadC Reciprocally Influences Biofilm Formation and Swarming Motility via Modulation of Exopolysaccharide Production and Flagellar Function

Abstract

Pseudomonas aeruginosa has served as an important organism in the study of biofilm formation; however, we still lack an understanding of the mechanisms by which this microbe transitions to a surface lifestyle. A recent study of the early stages of biofilm formation implicated the control of flagellar reversals and production of an exopolysaccharide (EPS) as factors in the establishment of a stable association with the substratum and swarming motility. Here we present evidence that SadC (PA4332), an inner membrane-localized diguanylate cyclase, plays a role in controlling these cellular functions. Deletion of the sadC gene results in a strain that is defective in biofilm formation and a hyperswarmer, while multicopy expression of this gene promotes sessility. A ⌬sadC mutant was additionally found to be deficient in EPS production and display altered reversal behavior while swimming in high-viscosity medium, two behaviors proposed to influence biofilm formation and swarming motility. Epistasis analysis suggests that the sadC gene is part of a genetic pathway that allows for the concomitant regulation of these aspects of P. aeruginosa surface behavior. We propose that SadC and the phosphodiesterase BifA (S. L. Kuchma et al., J. Bacteriol. 189:8165-8178, 2007), via modulating levels of the signaling molecule cyclic-di-GMP, coregulate swarming motility and biofilm formation as P. aeruginosa transitions from a planktonic to a surface-associated lifestyle.

Genetic reductionist approach for dissecting individual roles of GGDEF proteins within the c-di-GMP signaling network in Salmonella

“…However, a c-di-GMP component that controls this process also exists, because the individual presence of AdrA or HmsT in ⌬XII blocked motility in a manner that depended on the presence of an active GGDEF site. The latter finding is in agreement with previous studies that correlate high levels of c-di-GMP and sessility (12,(35)(36)(37)(38).…”

Sticky Situations: Key Components That Control Bacterial Surface Attachment

“…SadC has been demonstrated to contribute to attachment. However, while inactivation of sadC by transposon mutagenesis in P. aeruginosa PA14 was shown to result in increased attachment and pellicle formation (86), deletion of sadC in the same parental strain rendered the mutant impaired in biofilm formation (104). It is likely that such inconsistencies arise from differences in parental strains or in the manner in which mutants were generated or from differences in growth conditions and experimental setup or that it may simply be a question of when over the course of the biofilm developmental process the biofilm formation capabilities were analyzed.…”

“…In P. aeruginosa, the proteins SadB, SadC, and BifA are required for the transition from reversible to irreversible attachment by regulating several surfaceassociated behaviors, including swarming, EPS production, and modulation of flagellar reversal rates via the chemotaxis cluster IV in a c-di-GMP-dependent manner. Merritt et al (104) demonstrated that deletion of sadC, encoding an inner membrane-localized DGC, results in a hyperswarming strain defective in biofilm formation. 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.…”

“…Using primarily glass or plastic surfaces, microscopic studies as well as genetic analyses have shown that the initial surface attachment phase by many bacteria proceeds in two distinct steps (2,20,104,124,140), which are referred to as the reversible and irreversible attachment stages. In the first step, known as reversible attachment, cells are loosely attached via a single pole and may readily detach and return to the planktonic phase.…”

LapD is a bis-(3′,5′)-cyclic dimeric GMP-binding protein that regulates surface attachment by Pseudomonas fluorescens Pf0–1

“…Although c-di-GMP appears to be a conserved signal, the outputs it regulates are diverse and vary among different systems. These outputs include regulation of extracellular polysaccharide production (5,6), adhesin secretion and localization (7), flagellar function (6,8), and transcription of genes that direct attachment (9). Although c-di-GMP signaling has been established as a conserved modality in biofilm regulation, the specific mechanisms by which the signal is received and acted upon are largely unknown.…”