Abstract
BackgroundSwarming is a multicellular phenomenom characterized by the coordinated and rapid movement of bacteria across semisolid surfaces. In Sinorhizobium meliloti this type of motility has been described in a fadD mutant. To gain insights into the mechanisms underlying the process of swarming in rhizobia, we compared the transcriptome of a S. meliloti fadD mutant grown under swarming inducing conditions (semisolid medium) to those of cells grown under non-swarming conditions (broth and solid medium).ResultsMore than a thousand genes were identified as differentially expressed in response to growth on agar surfaces including genes for several metabolic activities, iron uptake, chemotaxis, motility and stress-related genes. Under swarming-specific conditions, the most remarkable response was the up-regulation of iron-related genes. We demonstrate that the pSymA plasmid and specifically genes required for the biosynthesis of the siderophore rhizobactin 1021 are essential for swarming of a S. meliloti wild-type strain but not in a fadD mutant. Moreover, high iron conditions inhibit swarming of the wild-type strain but not in mutants lacking either the iron limitation response regulator RirA or FadD.ConclusionsThe present work represents the first transcriptomic study of rhizobium growth on surfaces including swarming inducing conditions. The results have revealed major changes in the physiology of S. meliloti cells grown on a surface relative to liquid cultures. Moreover, analysis of genes responding to swarming inducing conditions led to the demonstration that iron and genes involved in rhizobactin 1021 synthesis play a role in the surface motility shown by S. meliloti which can be circumvented in a fadD mutant. This work opens a way to the identification of new traits and regulatory networks involved in swarming by rhizobia.
Highlights
Swarming is a multicellular phenomenom characterized by the coordinated and rapid movement of bacteria across semisolid surfaces
In contrast to the wild type strains GR4 and Rm1021, 1021FDC5 like QS77 could not grow on minimal medium (MM) plates containing oleate as sole carbon source, a phenotype that was restored after introduction of the pBBRD4 construct harbouring
The results reveal that the physiology of S. meliloti cells growing on the surface of agar media is significantly different from that of cells growing in broth, with the differential expression of more than a thousand genes
Summary
Swarming is a multicellular phenomenom characterized by the coordinated and rapid movement of bacteria across semisolid surfaces. Swarming is a type of bacterial motility generally dependent on flagella and is characterized by a rapid and coordinated population migration across solid surfaces. Signals and signalling pathways controlling swarm cell differentiation are largely unknown. Recent genome-scale approaches performed in model bacteria such as Salmonella typhimurium, Escherichia coli and Pseudomonas aeruginosa, indicate that swarmer differentiation represents much more than a motility phenotype as substantial alterations in metabolic pathways and gene expression have been observed [5,6,7,8,9]. A large number of genes involved in several metabolic activities, iron acquisition, regulatory proteins, chaperones, and biosynthesis of cell surface components have been demonstrated to be important for this multicellular migration [7,8]
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