Roles of Extracellular Polysaccharides and Biofilm Formation in Heavy Metal Resistance of Rhizobia

NOCELLI, NATALIA; BOGINO P; BANCHIO, E; GIORDANO W

Materials

MOLECULAR DIVERSITY PRESERVATION INTERNATIONAL-MDPI

Lugar: Basel; Año: 2016 vol. 9

Bacterial surface components and extracellular compounds, particularly flagella,lipopolysaccharides (LPSs), and exopolysaccharides (EPSs), in combination with environmentalsignals and quorum-sensing signals, play crucial roles in bacterial autoaggregation, biofilmdevelopment, survival, and host colonization. The nitrogen-fixing species Sinorhizobium meliloti(S. meliloti) produces two symbiosis-promoting EPSs: succinoglycan (or EPS I) and galactoglucan(or EPS II). Studies of the S. meliloti/alfalfa symbiosis model system have revealed numerous biologicalfunctions of EPSs, including host specificity, participation in early stages of host plant infection,signaling molecule during plant development, and (most importantly) protection from environmentalstresses. We evaluated functions of EPSs in bacterial resistance to heavy metals and metalloids, whichare known to affect various biological processes. Heavy metal resistance, biofilm production, andco-culture were tested in the context of previous studies by our group. A range of mercury (Hg II)and arsenic (As III) concentrations were applied to S. meliloti wild type strain and to mutant strainsdefective in EPS I and EPS II. The EPS production mutants were generally most sensitive to themetals. Our findings suggest that EPSs are necessary for the protection of bacteria from either Hg (II)or As (III) stress. Previous studies have described a pump in S. meliloti that causes efflux of arsenicfrom cells to surrounding culture medium, thereby protecting them from this type of chemical stress.The presence of heavy metals or metalloids in culture medium had no apparent effect on formationof biofilm, in contrast to previous reports that biofilm formation helps protect various microorganismspecies from adverse environmental conditions. In co-culture experiments, EPS-producing heavymetal resistant strains exerted a protective effect on AEPS-non-producing, heavy metal-sensitivestrains; a phenomenon termed ?rescuing? of the non-resistant strain.