ExpR Is Not Required for Swarming but Promotes Sliding in Sinorhizobium meliloti

Abstract

Swarming is a mode of translocation dependent on flagellar activity that allows bacteria to move rapidly across surfaces. In several bacteria, swarming is a phenotype regulated by quorum sensing. It has been reported that the swarming ability of the soil bacterium Sinorhizobium meliloti Rm2011 requires a functional ExpR/Sin quorum-sensing system. However, our previous published results demonstrate that strains Rm1021 and Rm2011, both known to have a disrupted copy of expR, are able to swarm on semisolid minimal medium. In order to clarify these contradictory results, the role played by the LuxR-type regulator ExpR has been reexamined. Results obtained in this work revealed that S. meliloti can move over semisolid surfaces using at least two different types of motility. One type is flagellum-independent surface spreading or sliding, which is positively influenced by a functional expR gene mainly through the production of exopolysaccharide II (EPS II). To a lesser extent, EPS II-deficient strains can also slide on surfaces by a mechanism that is at least dependent on the siderophore rhizobactin 1021. The second type of surface translocation shown by S. meliloti is swarming, which is greatly dependent on flagella and rhizobactin 1021 but does not require ExpR. We have extended our study to demonstrate that the production of normal amounts of succinoglycan (EPS I) does not play a relevant role in surface translocation but that its overproduction facilitates both swarming and sliding motilities.

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.…”

“…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.…”

“…A newly inoculated colony that spreads by this mechanism first acquires a wet appearance as it secretes a nearly transparent mucoid fluid or slime. The mucoid layer then expands radially outward across the agar surface, at speeds of ≃0.04-0.4 mm s 21 , over a period of 2-3 days [7,8]. The cells travel outward with the mucoid layer until the colony is dispersed over a final area as large as approximately 1 cm 2 .…”

“…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].…”

Effect of the plant flavonoid luteolin on Ensifer meliloti 3001 phenotypic responses

“…Therefore, the purpose of the present study was to provide an extensive investigation on the effects of luteolin on the phenotypic changes occurring in E. meliloti, to get at an interpretative framework for modeling luteolin-induced metabolic switches. The model of our investigation was the E. meliloti 3001 (expR + ) strain (Nogales et al 2012), which is derived from E. meliloti 2011 (expR − ) and harbors a complete ExpR/Sin QS system, enabling us to investigate the effect of luteolin on QS related phenotypes.…”