Optimisation of a Bacterial Co-culture for Sustainable Bioinoculant Development to Mitigate Drought Effects in Colombian Grasslands

Climate change has intensified desertification and droughts, directly impacting agriculture and livestock. To promote sustainability in Colombian grasslands, AGROSAVIA developed a bioinoculant comprising a synthetic bacterial consortium of three rhizobacteria (Herbaspirillum sp. AP21, Azospirillum brasilense D7, and Rhizobium leguminosarum T88) with plant growth-promoting properties and drought resilience. This study evaluated the co-culture fermentation of these strains in M10 medium, testing different carbon source (SC) concentrations relative to the baseline (T0): T0, T1 (-30%), T2 (-10%), T3 (+10%), and T4 (30%). Fermentations were conducted at 150 rpm and 30°C. Co-culture was initiated with strain T88, followed 24 h later by the addition of strains D7 and AP21. To monitor growth dynamics, the cell viability (CFU/mL) of each strain and the mixed culture was quantified after 24 h and 48 h on LMA agar (30 °C, 72 h incubation). Once the optimal carbon concentration was identified, the co-culture was scaled up in an 8 L stirred-tank bioreactor (STR). The bioreactor results showed enhanced growth, with cell counts exceeding 10⁹ CFU/mL, confirming the feasibility of co-culture fermentation for the development of a sustainable bioinoculant to mitigate drought impacts.