Role of exopolysaccharides on biofilm formation in Sinorhizobium meliloti

RINAUDI, L., ZORREGUIETA, A., GIORDANO W

Rosario

Congreso; V Congreso Argentino de Microbiología General; 2008

SAMIGE

Bacterial surface polysaccharides are crucial for the establishment of a successful symbiosis between rhizobia and legumes. They were shown to be also critical for biofilm formation in several bacterial species, which is defined as a bacterial community surrounded by a polysaccharide matrix usually attached to a surface. S. meliloti is able to produce two exopolysaccharides, succinoglycan and EPS II. Both polymers are produced as high- and low-molecular-weight (HMW and LMW, respectively) fractions; however, only the LMW forms of either succinoglycan or EPS II are active in nodule invasion. Succinoglycan is normally produced by S. meliloti Rm1021; however, EPS II synthesis only occurs under phosphate limitation, in the presence of an intact copy of expR or when mucR is mutated. mucR is a transcriptional regulator of exp genes (responsible for the production of EPS II) and an activator of the biosynthesis of succinoglicano. Therefore, mucR regulates the production of both exopolysaccharides in S. meliloti. S. meliloti is able to sense nutritional and environmental onditions in the culture medium and respond abandoning the planctonic state and forming biofilms. Bacterial adhesion is modulated according to environmental changes through the regulation of exopolysaccharide biosynthesis. However, mucR expression (evaluated by using a lacZ transcriptional fusion) was not changed by addition of sucrose (0.3 M), NaCl (0.015 M), phosphate (25 mM) and calcium (7 mM) to the RDM medium, conditions in which biofilm formation is increased (Rinaudi et al., 2006). mucR expression is reduced 44% in biofilms compared with planctonic cells in exponential phase growing in Rhizobium Defined Medium (RDM). Additionally, we have shown that in a minimal medium low in phosphate (0.1 mM) the Sin/ExpR quorum sensing system controls biofilm formation through EPS II production. Moreover, synthesis of succinoglycan is not essential for biofilm formation but it would slightly affect its structure during the first days post-inoculation (Rinaudi et al., 2008 III ASM Conference in Cell-Cell Communication in Bacteria). The mucR mutant, which is able to produce succinoglycan and only HMW EPS II, showed the same low levels of biofilm formation compared with the Rm1021 strain. By CLSM we observed that the mucR mutant formed small channels in the culture chamber, but failed to develop microcolonies, a phenotype analogous to the one observed in the non-EPS II-producing parental strain Rm1021. Taken together, our results suggest that i) under various environmental conditions, MucR would not play an important role in biofilm formation on an abiotic surface by S. meliloti and ii) while the LMW-EPS II, would be necessary for attachment to abiotic surfaces, succinoglycan biosynthesis leads to unstable biofilms and it may play a role on biofilm dispersal but not in biofilm formation.