The low-molecular-weight fraction of the exopolysaccharide II from Sinorhizobium meliloti is involved in autoaggregation of planktonic cells.

SORROCHE, F., RINAUDI, L.V, BANCHIO, E., ZORREGUIETA A., GIORDANO, W.

San Miguel de Tucumán

Congreso; VI congreso Argentino de Microbiología General (SAMIGE); 2009

Physical cell-cell interactions determine the aggregative behavior in many bacteria, and leads to the formation of multicellular conglomerates that show an increased resistance to unfavorable conditions. Sinorhizobium meliloti, a Gram negative soil bacterium, has the potential to establish a symbiotic chronic infection in Medicago spp., inducing the formation of nitrogen-fixing root nodules. Succinoglican (EPS I) and galactoglucan (EPS II) are two extracellular polysaccharides that play an important role in the symbiotic association. Specifically, the low molecular weight (LMW) fractions of both EPS have been shown to have a symbiotic relevance. ExpR is a LuxR type regulator that plays multiple roles in S. meliloti physiology, including  EPS II synthesis. Rm1021, a sequenced wild type reference strain, has an altered, non-functional expR locus. Although it is able to produce EPS I, it does not synthesize EPS II under normal conditions. The autoaggregation of Rm1021 was greatly reduced as compared to Rm8530, an Rm1021-derived strain which carries a functional allele of the expR gene and produces EPS II. Moreover, Rm2011 and 102F34, two wild type reference strains that carry deleterious expR alleles, also showed poorly autoaggregative phenotypes. These observations indicate that an intact, functional copy of the expR locus is required for an optimal expression of the autoaggregative phenotype in S. meliloti. Autoaggregation of washed cells of Rm8530, was drastically inhibited, indicating that an extracellular factor may be responsible for the aggregative behavior in this strain. The introduction of the mutant allele exoY210::Tn5 in Rm8530 abolished the production of EPS I, but had no significant effect on the autoaggregation phenotype. However, Rm8530 expA::Tn5-233, which does not synthesize EPS II, showed a strongly reduced autoaggregation, suggesting that EPS II is the polymer responsible for the aggregative phenotype of S. meliloti. The autoaggregation of the regulatory mutant Rm1021 mucR31::Tn5, which synthesizes HMW EPS II and traces of  EPS I, showed a typical poorly aggregative phenotype, indicating that HMW EPS II is not mediating the cell-cell interactions leading to autoaggregation. In fact, complementation of autoaggregation was demonstrated when cell-free culture supernatants from strains Rm8530 and Rm8530 exoY (containing LMW EPS II) were able to promote a significant aggregation of Rm1021, Rm8530 expA, Rm8530 expA exoY and Rm1021 mucR  (unable to produce LMW EPS II). These results show that the low molecular weight fraction of EPS II is a significant determinant in the establishment of adhesive interactions between planktonic S. meliloti cells.