Pleiotropic effects of a mutation in lpsB gene of Sinorhizobium meliloti Rm 8530 strain.

NIEVAS, F., SORROCHE, F., NOCELLI, N., GIORDANO, W., BOGINO, P.

Congreso; XIII Congreso Argentino de Microbiología General (SAMIGE); 2018

Bacterial surface molecules such as exopolysaccharides (EPSs), lipopolysaccharides (LPSs) and capsular polysaccharides (KPSs), are crucial for adherence properties, colonization of surfaces, and as a barrier for defense against stressful environmental factors. For rhizobial bacteria those molecules are also relevant for the development of a successful rhizobia-legume symbiosis. Lipopolysaccharides (LPSs) are the most important structural components of the outer membrane of Gram-negative bacteria contributing to their structural properties and acting as a permeability barrier. Because of their position at the contact zone with the external environment, the LPSs of many bacterial species are the main determinants of interaction with biotic or abiotic surfaces. LPSs contribute to the establishment of the symbiotic relationship through suppressing host defenses and facilitating rhizobial entry into root hairs, infection thread formation, and eventually bacteroid differentiation. The Medicago symbiont Sinorhizobium meliloti produces a heterogeneous population of LPSs: LPS-1 which includes the O-antigen (S-LPS) and LPS-2 which lacks the O-antigen (R-LPS). The lpsB gene codes for a type I glycosyltransferase involved in the synthesis of the LPS core. In the present study, we evaluated the pleiotropic effects of a mutated lpsB gene in S. meliloti Rm 8530 strain. This mutation was examined alone and combined to deficiency of EPS II (exopolysaccharide II). We studied the mutated LPSB strain in cell-cell and cell-surface interactions, motility and symbiotic parameters with the host plant Medicago sativa. The LPSB mutant, which has a defective core portion of LPS, exhibited a reduction in biofilm formation on abiotic surfaces compared to the wild type strain. However, this ability in the in the LPSB mutant was not so reduced when compared to EPS II-defective mutant strains. Cell aggregation studies clearly showed that the LPSB mutant strain formed a greater number of higher cell aggregates compared to wild type strain. Moreover, autoaggregation experiments carried out with LPS and EPS mutant strains showed that both polysaccharides had an impact on the cell-cell adhesive interactions of planktonic bacteria. The lpsB mutation had also a marked effect in reducing the motility of strains carrying the mutation. In spite of the effects on several important physiological mechanisms caused by the lpsB mutation in this bacterium, the symbiotic process was not altered. In this sense the number and efficiency of nodules as well as the biomass parameters were not reduced because of the lpsB mutation. On the other hand, symbiosis was negatively affected in a LPS and EPS II double mutant. Taken into account the results obtained, this work shows that S. meliloti interactions with biotic and abiotic surfaces as well as the development of a successful symbiosis could depend on the interplay between LPS and EPS II.