Optimization of a by-product-based culture medium for Bacillus subtilis biocontrol: a circular economy approach

The use of chemical pesticides has been facing obstacles due to tightening regulatory restrictions and rapidly emerging disease resistance. In this context, bio-based products are an effective alternative to be used in biocontrol. Lipopeptides are biosurfactants with powerful antimicrobial activity and are produced by different microorganisms, including those of the genus Bacillus. In this work, a biosurfactant-producing Bacillus subtilis strain exhibited inhibitory activity against the phytopathogens Rhizoctonia solani, Corynespora cassiicola, Colletotrichum truncatum, Sclerotinia sclerotiorum, and Aspergillus flavus, qualifying it as a potential microorganism for biocontrol. First, the kinetics of bacterial growth was investigated in flasks to monitor the basic parameters of the bioprocess. With these data, different nitrogen and carbon sources were tested as a mean to optimize bacterial spore formation and biosurfactant production. Yeast extract and glucose or sugarcane molasses presented the best results. With the new optimized medium, the process was carried out in a 10 L bioreactor and reached 4.99 × 109 CFU/mL, 1.73 × 109 spores/mL, and crude biosurfactant concentration of 2.80 g/L. LC–ESI–MS analysis confirmed the production of lipopeptides by B. subtilis DEBB B-328, with detected m/z values consistent with fengycin homologues (m/z 1489.8–1491.8) and a surfactin isoform (m/z 1022.6). These results indicate that the product based on B. subtilis DEBB B-328 is a promising alternative to chemical pesticides, with healthier and environmentally friendlier features.