Starvation-induced changes in motility and spontaneous switching to faster swarming behavior of Sinorhizobium meliloti.

摘要

Motility and chemotaxis responses to starvation were studied in Sinorhizobium meliloti. Cells lost motility in a strain-specific pattern within, 8 h to 4 d after transfer to starvation buffer. A transient, 2- to 6-fold increase in chemotactic responsiveness toward attractants was observed. Nonmetabolizable chemoattractants could prevent motility loss, thus interactions of chemoreceptors with attractants appear to affect motility independently of nutrient availability. Most nonmotile cells retained flagella, indicating that deactivation of flagellar motors was the first important response of S. meliloti to nutrient deprivation.;Several Tn5 mutants of S. meliloti which swarmed twice as faster as the parent in semi-solid agar, moist sand and viscous liquid were identified. The faster swarming (FS) mutants outgrew the wild type 30- to 40-fold within 2 d in soft agar plates, and had a significant growth advantage in all circumstances where nutrient gradients were present. The mutants had higher percentages of motile and flagellated cells, and longer and more flagella than the wild type. Thus FS behavior is likely a result of derepression of flagellar synthesis. Spontaneous variants that behaved exactly like the Tn5 FS mutants were obtained at a frequency of about 1 per 15,000 cells. These FS variants reverted to wild-type behavior at low, but variable frequencies. The FS mutants and variants produced less exopolysaccharide (EPS) than the parent. Various swarm rates, swim patterns, EPS phenotypes, and restriction patterns observed in different FS mutants indicate that multiple genetic configurations could result in FS-EPS behavior. Preliminary molecular characterization has shown that the Tn5 insertion sites in all three FS mutants were all in the same location, but the sequences flanking the Tn5 did not complement the mutant behavior. Complementing sequences have been isolated and are being characterized. Possible molecular mechanisms for FS-EPS phenotype and for switching are discussed, and future directions presented.;Our study on Pseudomonas aeruginosa mutants defective in AHL synthesis or perception indicates that AHL signalling plays a significant role in growth at low cell densities, in root colonization, and in the development of a stress resistant subpopulation.