Investigations of Azospirillum brasilense biofilm formation

NIEVAS, F.L.; SORROCHE, F.G.; RINAUDI, L.V.; BOGINO, P.C.; BANCHIO, E.; GIORDANO, W.F.

Villa Carlos Paz, Córdoba

Congreso; VI Congreso Argentino de Microbiología General; 2009

SAMIGE

Azospirillum brasilense is considered to be an important plant growth promotive rhizobacteria that can improve the growth and yield of many agriculturally important crops, including wheat, corn, and rice. Azospirilla colonize the root surface and may significantly promote plant growth and crop yield, properties that make them attractive candidates for the development of biological fertilizers for these crops. The ability of Azospirillum to attain significant population’s surfaces of the host is essential for its beneficial effect on plant growth and requires that the bacteria come in close contact with the roots. Initial attachment of soil bacteria to plant cells have been shown to depend on superficial or surface associated polymers, such as lipopolysaccharide (LPS), exopolysaccharide (EPS), capsular polysaccharide (CPS) as well as flagella and outer membrane proteins. Some of these molecules are involved in biofilm formation, which are defined as structured communities surrounded by a self-produced polymeric matrix and adhered to a living or inert surface. In this work we investigated the influence of nutritional factors and the effect of mutations in genes encoding for surface components such as LPS, EPS and CPS, in the biofilm formation ability of A. brasilense. We found that three wild-type strains of A. brasilense (Az39, Cd, Sp7), are able to form biofilms on a hydrophobic inert surface. In particular, A. brasilense Sp7 was able to form relatively more robust biofilms compared with the other strains studied. The effect of the media composition on biofilm formation was also investigated in the Sp7 strain. In a nutritional-limited minimal medium, biofilm biomass was increased, whereas in a complex rich one, biofilm formation was strongly inhibited, indicating that A. brasilense is able to sense and differentially respond to the nutritional status of the medium. Three mutants of A. brasilense Sp7 showed alterations in biofilm formation. The exoC or exoB mutants have an altered composition of LPS and produce only the low-molecular-weight fraction of EPS. These mutants were severely affected in the adhesion to an inert surface, as compared to the wild type. In contrast a phbC mutant unable to produce PHB, (an intracellular storage compound), overproduces EPS, CPS, and shows an increased motility, revealed an increased sessile-biomass accumulation in comparison with the wild type. Interestingly, exoB and exoC mutants, deficient in biofilm formation, have previously been shown to be impaired in the attachment to roots. In contrast, the biofilm-overproducing mutant phbC, was found to be more proficient in the adhesion to the plant host. These results suggest a correlation between root surface attachment and biofilm formation.