EPS II from Sinorhizobium meliloti as an adhesive molecule in multi-species bacterial aggregates

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

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

Sinorhizobium meliloti is a rhizobium able to establish a nitrogen fixing symbiosis with the Medicago legume. This bacterium produces a mix of exopolysaccharides (EPSs) identified as succinoglycan or EPS I and galactoglucan or EPS II. EPSs play a major role in the bacterial processes of cell aggregation, flocculation, and biofilm formation. Those physiological events are critical for the initial steps of the interaction mechanism between bacteria and eukaryotic hosts. Bacterial aggregation is a highly specific process which involves interaction between molecules from bacterial surfaces that act as adhesins and complementary receptors, which are carbohydrates and proteins respectively. The bacterial aggregative mechanism can occur between bacteria belonging to the same species (autoaggregation) or between bacteria of different genus (co-aggregation). Since the process requires protein-saccharide interactions on the surfaces of both partners, aggregation can be modulated by the regulation of EPSs synthesis. Co-aggregation interactions contribute to the initial development of biofilms through specific recognition and adhesion of single, genetically different bacteria in suspension and the subsequent adhesion of previously co-aggregated cells. The aim of the present work was to study co-aggregation between S. meliloti strains and different rhizospheric bacteria. The co-aggregative phenotype was analyzed in the wild type S. meliloti Rm 8530 strain (complete EPS producer) and in their derivative mutants in the EPS synthesis: exoY strain (EPS I defective), expA strain (EPS II defective) and exoY-expA double mutant strain (DM, EPS defective). Each S. meliloti strain was tested for co-aggregation with microorganisms that share the same ecological niche, including Pseudomonas fluorescens, Azospirillum brasilense and Burkholderia sp. In general, the strains producing EPS II (wt, exoY) were shown to produce more co-aggregation compared to those strains incapable of producing EPS II (expA, DM). Co-aggregation assays were also carried out by adding to washed cells different sources of EPSs, i.e. either from supernatants of single cultures or purified EPS. It was always detected that the co-aggregative percentage was increased by providing a source of EPS II. Accordingly, it is concluded that EPS II from S. meliloti play an important role in the co-aggregation process with different rhizospheric bacteria. This exopolysaccharide may be a key factor for microorganisms to start the development of biofilms in nature. Moreover, this molecule could be considered as a possible universal connector with several relevant properties at ecological, biotechnological and agro-productive levels.