Here, we estimate fast changes in the fluidity of Sinorhizobium meliloti membranes submitted to cyclic temperature changes (10°C–40°C–10°C) by monitoring the fluorescence polarization (P) of DPH and TMA-DPH of the whole cell (WC) as well as in its outer (OM) and inner (IM) membranes. Additionally, the long-term response to thermal changes is demonstrated through the dynamics of the phospholipid and fatty acid composition in each membrane. This allowed membrane homeoviscous adaptation by the return to optimal fluidity levels as measured by the PDPH/TMA-DPH in WC, OM, IM, and multilamellar vesicles of lipids extracted from OM and IM. Due to probe-partitioning preferences and membranes’ compositional characteristics, DPH and TMA-DPH exhibit different behaviors in IM and OM. The rapid effect of cyclic temperature changes on the P was the opposite in both membranes with the IM being the one that exhibited the thermal behavior expected for lipid bilayers. Interestingly, only after the incubation at 40°C, cells were unable to recover the membrane preheating P levels when cooled up to 10°C. Solely in this condition, the formation of threads and nodular structures in Medicago sativa infected with S. meliloti were delayed, indicating that the symbiotic interaction was partially altered but not halted.

Currently, bacterial inoculant technology focuses on improving long-term storage conditions to ensure adequate rhizobia numbers and their effectiveness as plant growth promoters. This study aimed to investigate whether storage at 4 °C for four years of alginate beads immobilizing Bradyrhizobium sp. SEMIA6144 maintains bacterial vitality, efficacy in growth promotion, and ability to establish early interactions with Arachis hypogaea L. The recovery of viable SEMIA6144 cells decreased over time (10% at six months, 1% at one year, and 0.01% at four years), while cell vitality remained high at 94.1%, 90.2%, and 93.4%, respectively. The unsaturated/saturated fatty acid ratio declined during storage, reducing membrane fluidity and metabolic activity. Mobility and root adhesion of SEMIA6144 decreased after one and four years. However, growth promotion in peanuts inoculated with SEMIA6144 beads was observed through increased biomass, total chlorophyll, leaf number, leaf area, and decreased chlorophyll fluorescence compared to non-inoculated plants. Although nodulation was low in plants inoculated with four-year-old beads, leghemoglobin levels were maintained. These results demonstrate that Bradyrhizobium sp. SEMIA6144 can be stored for four years in alginate beads at 4 °C, maintaining its vitality and ability to establish a symbiosis that stimulates early peanut growth. Understanding these physiological changes could be valuable for the future improvement of long-lasting inoculants.