Spore production optimization of a biofertilizer based on Bacillus cabrialesii HB7 for enhancing plant growth under saline stress.

Bacillus cabrialesii HB7, a novel halotolerant strain isolated from Qatari soil, can effectively promote tomato plant growth and mitigate saline stress and has a great biofertilizer potential. Due to its promising agronomical and biotechnological potential, high spore densities should be achieved, to extend its use. In this study, culture conditions were optimized with different methods to improve HB7 spore production. Firstly, three culture media were tested to find the best medium for high spore density. When using the semisynthetic medium supplemented with 15 g.L^-1 NaCl, high spore production was achieved compared to complex medium and G10 media with and without NaCl addition. Secondly, key fermentation parameters were determined by Plackett-Burman design, and the optimum levels of six components (yeast extract, KH₂PO₄, casein hydrolysate, (NH₄)₂SO₄, NaCl concentrations, and pH) were optimized with a hybrid design, achieving 1.82 × 10⁹ spore.mL^-1 and a threefold spore production improvement, when compared to the basal medium. Moreover, HB7 exhibited faster cell growth (0.97 h^-1 vs. 0.53 h^-1) and glucose uptake (1.09 h^-1 vs. 0.89 h^-1) after optimization. These findings may play a pivotal role in the synthesis of an effective biofertilizer based on B. cabrialesii HB7 for sustainable agriculture and biotechnological advancements.