Resistance of Biofilms Formed by the Soil Bacterium Azospirillum brasilense to Osmotic Stress

— Polyethylene glycol (PEG 6000) was used to establish osmotic stress conditions during growth of the type strain Azospirillum brasilense Sp7 and its spontaneous variants Sp7.4 and Sp7.8, because it causes a stable decrease in the water potential and thus makes it possible to simulate the effect of drought on the bacterial population. While PEG suppressed the motility of azospirilla, it had no effect on the ability of strains Sp7 and Sp7.8 to form biofilms, as well as on the metabolic activity of the biofilms formed in the absence of stress. PEG 6000-caused osmotic stress promoted biofilm formation in Sp7.8. While the biofilms of the Sp7.4 variant were those most sensitive to the negative effect of the water stress, the growth variables of the planktonic culture of this variant under stress conditions exceeded the values for both Sp7 and Sp7.8. In biofilms, strains Sp7, Sp7.4, and Sp7.8 produced polysaccharides and the plant hormone IAA; desiccation-resistant cell forms emerged. The variants Sp7.4 and Sp7.8, similarly to Sp7, formed biofilms during plant root colonization and affected the morphology of the root system of wheat seedlings. Our results show that spontaneous variants of strain Sp7 may be of interest for further research directed at selection of promising Azospirillum strains to enhance the drought resistance of plants.