Role of Specific Quorum-Sensing Signals in the Regulation of Exopolysaccharide II Production within Sinorhizobium meliloti Spreading Colonies

Abstract

BackgroundQuorum sensing (QS) in Sinorhizobium meliloti involves at least half a dozen different N-acyl homoserine lactone (AHL) signals. These signals are produced by SinI, the sole AHL synthase in S. meliloti Rm8530. The sinI gene is regulated by two LuxR-type transcriptional regulators, SinR and ExpR. Mutations in sinI, sinR and expR abolish the production of exopolysaccharide II (EPS II).Methodology/Principal FindingsThis study investigated a new type of coordinated surface spreading of Rm8530 that can be categorized as swarming. Motility assays on semi-solid surfaces revealed that both flagella and EPS II are required for this type of motility. The production of EPS II depends on AHLs produced by SinI. Of these AHLs, only C16:1- and 3-oxo-C16:1-homoserine lactones (HSLs) stimulated swarming in an ExpR-dependent manner. These two AHLs induced the strongest response in the wggR reporter fusions. WggR is a positive regulator of the EPS II biosynthesis gene expression. The levels of the wggR activation correlated with the extent of swarming. Furthermore, swarming of S. meliloti required the presence of the high molecular weight (HMW) fraction of EPS II. Within swarming colonies, a recombinase-based RIVET reporter in the wggR gene was resolved in 30% of the cells, indicating an enhanced regulation of EPS II production in the subpopulation of cells, which was sufficient to support swarming of the entire colony.Conclusions/SignificanceSwarming behavior of S. meliloti Rm8530 on semi-solid surfaces is found to be dependent on the functional QS regulatory cascades. Even though multiple AHL signals are produced by the bacterium, only two AHLs species, C16:1- and 3-oxo-C16:1-HSLs, affected swarming by up-regulating the expression of wggR. While EPS II is produced by Rm8530 as high and low molecular weight fractions, only the HMW EPS II facilitated initial stages of swarming, thus, suggesting a function for this polymer.

Entropy-driven motility of Sinorhizobium meliloti on a semi-solid surface

“…One is a surface-swarming motion of hyperflagellated cells, described in detail by Soto et al [6]. The other is an unusual type of surface spreading or translocation that has been observed by several groups [5,6,9] and was recently described in detail by Gao et al [8] and Nogales et al [7]. A newly inoculated colony that spreads by this mechanism first acquires a wet appearance as it secretes a nearly transparent mucoid fluid or slime.…”

“…Sinorhizobium meliloti is capable of at least two forms of motility on semi-solid agar surfaces [5][6][7][8][9]. One is a surface-swarming motion of hyperflagellated cells, described in detail by Soto et al [6].…”

“…The cells travel outward with the mucoid layer until the colony is dispersed over a final area as large as approximately 1 cm 2 . During this spreading, the colony typically develops a feathered or curdled, patterned appearance as shown in figure 1 and in the images in references [7][8][9]. The mechanism of this spreading has been unclear.…”

“…Although some authors have described it as a form of swarming or sliding motility, it is uncharacteristic of those behaviours in several respects. First, while swarming cells are normally hyperflagellated, the spreading S. meliloti possess only two to four flagella per cell [7,8]. In fact, this spreading does not absolutely require flagella [7]; flagellumdeficient strains form spreading mucoid layers like wild-type, although they do not disperse as efficiently throughout those layers and their colonies lack the distinctive feathered or curdled patterns [8].…”

Rhizobial homologs of the fatty acid transporter FadL facilitate perception of long-chain acyl-homoserine lactone signals

“…gene regulation by ExpR has been explored in great detail (19,20), and in a genomic search we identified a single fadL homolog in S. meliloti. Thus, we chose this rhizobial species to test our hypothesis that FadL is able to facilitate long-chain AHL uptake.…”