Due to the primary localization of both epiphytic and endophytic plant growth-promoting rhizobacteria on the surface of the plant root system, biofilm formation is an adaptive trait for these microorganisms. Under conditions of nitrogen limitation in liquid media, nitrogen-fixing Azospirillum brasilense strains switch mainly to the biofilm mode of growth. Overall ultrastructural similarities of the cells within A. brasilense biofilms were revealed, and their resistance to desiccation and oxidative stress was characterized. In strains Sp7, Cd, and Sp245, several types of single and undivided cells were revealed, as well as cystlike cells with pronounced morphological diversity. Resistance to desiccation and to oxidative stress was higher in the biofilm populations of these strains than in planktonic cultures. Dormant forms remained viable in dry biofilms of strains Sp245, Sp7, and Cd formed in a nitrogen-free medium after storage for 120 days. Viability of the biofilms of the same strains formed in the presence of nitrogen was retained for 120, 90, and 60 days, respectively. The minimal inhibitory concentration of H₂O₂ for biofilms was 1.0% for strains Sp7 and Cd and 0.1% for strain Sp245. Both the dormant forms and biofilms of strain Sp245 were more sensitive to H₂O₂ than those of strains Sp7 and Cd. Peroxidase activity was not previously reported in Azospirillum biofilms.

The previously obtained insertion mutants of Azospirillum brasilense Sp245 in the genes mmsB1 and fabG1 (strains SK039 and Sp245.1610, respectively) were characterized by impaired flagellation and motility. The putative products of expression of these genes are 3-hydroxyisobutyrate dehydrogenase and 3-oxoacyl-[acyl-carrier protein] reductase, respectively. In the present work, A. brasilense strains Sp245, SK039, and Sp245.1610 were found to have differences in the content of 3-hydroxyhexadecanoic, hexadecanoic, 3-hydroxytetradecanoic, hexadecenoic, octadecenoic, and nonadecanoic acids in their lipopolysaccharide preparations, as well as in cell hydrophobicity and hemagglutination activity and dynamics of cell aggregation, in biomass amount, and in the relative content of lipopolysaccharide antigens in mature biofilms formed on hydrophilic or hydrophobic surfaces.