Contribution of the two flagellar systems of Bradyrhizobium diazoefficiens USDA 110 to chemotaxis and swimming

C. JIMENEZ SANCHEZ; J.I. QUELAS; A.A. MELGAREJO; V.I. MARCONI; M.J. ALTHABEGOITI; J.J. ORTEGA CALVO; A.R. LODEIRO

Tenerife

Conferencia; 11th European Nitrogen Fixation Conference; 2014

Bradyrhizobium diazoefficiens USDA 110 has two different flagellar systems: a subpolar and constitutive flagellum (Fli), and a lateral flagellum (Laf), whose expression depends on the carbon source of the medium (1). Since the role of each flagellum is unclear, we evaluated their contributions to motility and chemotaxis with mutant USDA 110 strains deleted of the subpolar flagellins (fliC) or the lateral flagellins (lafA) (1). Mean swimming speed in liquid medium (v) of lafA was indistinguishable from the wild-type, in the order of 25-30 m s?1. Mean time between changes of swimming direction () of these strains was also similar, around 0.53-0.57 s. However, angles of changes of direction (α) were 50.95.9o for the wild-type and 70.714.1o for lafA, indicating that the lateral flagellum stabilized swimming direction. By contrast, in fliC mutants v=11.52.5 m s?1, =0.210.04 s, and α=91.11.2o, indicating that the lateral flagellum may not respond to chemotaxis. To further address this question, we evaluated chemotaxis with a modification of the Adler?s method (2) and by an ad-hoc method based on crystal violet staining of bacteria attracted to agar droplets containing the attractant. In agreement with the above characterization of swimming, the wild-type and the lafA mutant were proficient for chemotaxis, while the fliC mutant was deficient. Seed exudate was the best attractant followed by succinate, while mannitol, arabinose and glutamate were poorer attractants. To assess the motility of these bacteria in non-homogeneous media like soil, we simulated the displacement of the three strains in media with obstacles of different size and disposition (3). We found that fliC was the most retarded, while lafA and the wild-type moved farther. Despite the inefficacies of fliC for motility and chemotaxis, it was proficient in viscous medium. When motility was assayed in minimal medium with agar 0.3% and its viscosity increased by the addition of 7% polyvynyl pyrrolidone, fliC moved slightly less than the wild-type, while lafA did not move. We concluded that the subpolar flagellum is the main responsible for swimming and chemotaxis, while lateral flagella do not contribute significantly to chemotaxis. However, lateral flagella stabilize swimming direction and serve for swimming in viscous media, all of which might be important for displacement in soils at field capacity. Supported by ANPCyT, UNLP and CONICET (Argentina) and Spanish Ministry of Science and Innovation (Spain). 1. Covelli J.M. et al. 2013. Res. Microbiol. 164: 136-144. 2. Jiménez Sánchez C. et al. 2012. Environ Sci Technol 46: 6790-6797. 3. Berdakin I. et al. 2013. Phys. Rev. E 87: 052702.