Biofilm formation assessment in Sinorhizobium meliloti reveals interlinked control with surface motility

BMC Microbiology, Mar 2015

Background Swarming motility and biofilm formation are opposite, but related surface-associated behaviors that allow various pathogenic bacteria to colonize and invade their hosts. In Sinorhizobium meliloti, the alfalfa endosymbiont, these bacterial processes and their relevance for host plant colonization are largely unexplored. Our previous work demonstrated distinct swarming abilities in two S. meliloti strains (Rm1021 and GR4) and revealed that both environmental cues (iron concentration) and bacterial genes (fadD, rhb, rirA) play crucial roles in the control of surface motility in this rhizobial species. In the current study, we investigate whether these factors have an impact on the ability of S. meliloti to establish biofilms and to colonize host roots. Results We found that strain GR4, which is less prone to translocate on solid surfaces than strain Rm1021, is more efficient in developing biofilms on glass and plant root surfaces. High iron conditions, known to prevent surface motility in a wild-type strain of S. meliloti, promote biofilm development in Rm1021 and GR4 strains by inducing the formation of more structured and thicker biofilms than those formed under low iron levels. Moreover, three different S. meliloti mutants (fadD, rhb, and rirA) that exhibit an altered surface translocation behavior compared with the wild-type strain, establish reduced biofilms on both glass and alfalfa root surfaces. Iron-rich conditions neither rescue the defect in biofilm formation shown by the rhb mutant, which is unable to produce the siderophore rhizobactin 1021 (Rhb1021), nor have any impact on biofilms formed by the iron-response regulator rirA mutant. On the other hand, S. meliloti FadD loss-of-function mutants do not establish normal biofilms irrespective of iron levels. Conclusions Our studies show that siderophore Rhb1021 is not only required for surface translocation, but also for biofilm formation on glass and root surfaces by strain Rm1021. In addition, we present evidence for the existence of control mechanisms that inversely regulate swarming and biofilm formation in S. meliloti, and that contribute to efficient plant root colonization. One of these mechanisms involves iron levels and the iron global regulator RirA. The other mechanism involves the participation of the fatty acid metabolism-related enzyme FadD.

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Biofilm formation assessment in Sinorhizobium meliloti reveals interlinked control with surface motility

Amaya-Gmez et al. BMC Microbiology Biofilm formation assessment in Sinorhizobium meliloti reveals interlinked control with surface motility Carol V Amaya-Gmez 1 Ann M Hirsch 0 Mara J Soto 1 0 Department of Molecular , Cell , and Developmental Biology and Molecular Biology Institute, University of California-Los Angeles , Los Angeles, CA 90095-1606 , USA 1 Departamento de Microbiologia del Suelo y Sistemas Simbioticos, Estacion Experimental del Zaidin , Consejo Superior de Investigaciones Cientificas (CSIC), Profesor Albareda 1, 18008 Granada , Spain Background: Swarming motility and biofilm formation are opposite, but related surface-associated behaviors that allow various pathogenic bacteria to colonize and invade their hosts. In Sinorhizobium meliloti, the alfalfa endosymbiont, these bacterial processes and their relevance for host plant colonization are largely unexplored. Our previous work demonstrated distinct swarming abilities in two S. meliloti strains (Rm1021 and GR4) and revealed that both environmental cues (iron concentration) and bacterial genes (fadD, rhb, rirA) play crucial roles in the control of surface motility in this rhizobial species. In the current study, we investigate whether these factors have an impact on the ability of S. meliloti to establish biofilms and to colonize host roots. Results: We found that strain GR4, which is less prone to translocate on solid surfaces than strain Rm1021, is more efficient in developing biofilms on glass and plant root surfaces. High iron conditions, known to prevent surface motility in a wild-type strain of S. meliloti, promote biofilm development in Rm1021 and GR4 strains by inducing the formation of more structured and thicker biofilms than those formed under low iron levels. Moreover, three different S. meliloti mutants (fadD, rhb, and rirA) that exhibit an altered surface translocation behavior compared with the wild-type strain, establish reduced biofilms on both glass and alfalfa root surfaces. Iron-rich conditions neither rescue the defect in biofilm formation shown by the rhb mutant, which is unable to produce the siderophore rhizobactin 1021 (Rhb1021), nor have any impact on biofilms formed by the iron-response regulator rirA mutant. On the other hand, S. meliloti FadD loss-of-function mutants do not establish normal biofilms irrespective of iron levels. Conclusions: Our studies show that siderophore Rhb1021 is not only required for surface translocation, but also for biofilm formation on glass and root surfaces by strain Rm1021. In addition, we present evidence for the existence of control mechanisms that inversely regulate swarming and biofilm formation in S. meliloti, and that contribute to efficient plant root colonization. One of these mechanisms involves iron levels and the iron global regulator RirA. The other mechanism involves the participation of the fatty acid metabolism-related enzyme FadD. FadD; Iron; Rhizobium; RirA; Root colonization; Siderophore; Swarming - Background Swarming motility and biofilms are two different and opposite behaviors displayed by bacteria living on surfaces. Biofilms are sessile assemblages of microorganisms embedded in a self-produced polymeric matrix that adhere to a surface or are associated with interfaces [1,2]. By contrast, swarming is a mode of surface translocation * Correspondence: 1Departamento de Microbiologa del Suelo y Sistemas Simbiticos, Estacin Experimental del Zaidn, Consejo Superior de Investigaciones Cientficas (CSIC), Profesor Albareda 1, 18008 Granada, Spain Full list of author information is available at the end of the article that depends on rotating flagella and is characterized by the rapid and coordinated movement of multicellular groups of bacteria [3]. Several studies have revealed the existence of a link between swarming and biofilm formation: i) both are surface-associated multicellular processes in which cell-cell communication and quorum sensing play important roles; ii) in both processes, the participation of the same cell surface-associated structures such as flagella, a polysaccharide matrix, and biosurfactants has been reported; and iii) swarming bacteria, similar to bacteria in biofilms, show increased resistance to several antimicrobial agents [3-8]. Studies performed in Pseudomonas aeruginosa, Vibrio cholerae, or V. parahaemolyticus show that the two lifestyles are inversely regulated by a common pathway, which is modulated by the intracellular second messenger cyclic di-GMP [9-14]. Swarming motility and biofilm formation have been studied almost exclusively in pathogenic bacteria. However, little is known about these multicellular surfaceassociated responses in rhizobia, soil-dwelling bacteria, which induce nitrogen-fixing nodules on the roots of legume plants following a complex and continuous molecular dialogue that co-ordinates bacterial infection with nodule organogenesis [15]. Sinorhizobium meliloti, the alfalfa symbiont, forms biofilms on both abiot (...truncated)


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Carol V Amaya-Gómez, Ann M Hirsch, María J Soto. Biofilm formation assessment in Sinorhizobium meliloti reveals interlinked control with surface motility, BMC Microbiology, 2015, pp. 58, 15, DOI: 10.1186/s12866-015-0390-z