Adaptive physiological and membrane lipid responses of Bradyrhizobium sp. SEMIA 6144 to osmotic stress

Abstract Aims The effectiveness of rhizobial inoculants depends on their ability to survive environmental stresses associated with water-limiting conditions. This study aimed to evaluate the physiological and lipid responses of the membrane of Bradyrhizobium sp. SEMIA6144, a peanut (Arachis hypogaea) symbiont, under PEG-induced osmotic stress. Methods and Results Bacterial growth, viability, cell morphology, membrane lipid composition, fatty acid profile, and membrane fluidity were analyzed in Bradyrhizobium sp. SEMIA6144 under osmotic stress induced by polyethylene glycol (PEG 6000, 15 mmol.l⁻¹). Osmotic stress significantly reduced bacterial growth and viability, resulting in a 42% decrease in dry biomass and a prolonged generation time. Fluorescence microscopy revealed increased cell mortality under stress conditions. Lipid analyses based on radiolabeled acetate incorporation and GC–MS showed that phospholipids remained the predominant lipid class; however, osmotic stress altered neutral lipid distribution, decreasing free fatty acids and increasing fatty acid methyl esters. The phospholipid profile shifted toward higher phosphatidylcholine and cardiolipin contents, accompanied by reduced phosphatidylethanolamine and phosphatidylglycerol levels, indicating a trend toward membrane stabilization. Fatty acid composition was markedly modified, with an increase in the unsaturated 18:1Δ11 fatty acid and a decrease in 16:0, 18:0, and 16:1Δ9 species, resulting in a higher unsaturated/saturated fatty acid ratio. Despite these compositional changes, membrane fluidity remained unchanged, reflecting efficient homeostatic regulation under osmotic stress conditions. Conclusions These results demonstrate that Bradyrhizobium sp. SEMIA6144 adapts to osmotic stress through coordinated physiological and biochemical adjustments that preserve membrane functionality. These adaptive characteristics are relevant to understanding the persistence of rhizobia under water scarcity conditions and could contribute to the development of more robust inoculant formulations for agricultural applications.