Production of poly-gamma-glutamic acid and spore formation in Bacillus velezensis 83 are affected by acidic growing conditions and glucose uptake rate

BACKGROUND: Bacillus velezensis 83 is a biocontrol agent and plant growth promoting bacteria that produces antifungal compounds (surfactin and bacilomycin D), plant growth stimulation metabolites such as acetoin and butanediol and the polymer poly-gamma-glutamic acid (gamma-PGA), which is essential for bacterial plant colonization. Bacillus sporulation is an important trait in order to obtain long shelf-life formulations for biocontrol purposes. gamma-PGA production and sporulation are closed-related processes controlled by glucose availability and modulated by quorum sensing. High spore production (and its recovery) in B. velezensis 83 cultures is limited by the high viscosity oxygen-limited broths resulting from gamma-PGA accumulation. This work studied the influence of acidic growing conditions and glucose specific uptake rate on growth, quorum sensing-related metabolites production and sporulation of B. velezensis 83.RESULTS: In batch cultures developed at pH = 5, no bacterial sporulation was detected and quorum sensing related metabolites as gamma-PGA (not detected) and surfactin and bacillomycin were significantly decreased. Instead, acetoin and butanediol were the main products. In contrast, in continuous cultures developed at pH = 5, spores were detected at D = 0.06 h(-1). The results indicate that sporulation at pH = 5 is not inhibited by affectations on quorum sensing signals but could be due to a low availability/transport of energy sources (i.e., glucose or acetoin/butanediol) or an affectation on Rap-Phr system.CONCLUSION: The findings in this study allow to establish a two-stage process for Bacillus velezensis 83 spore production: bacterial growth at pH = 5 and sporulation at pH = 6.8. (c) 2023 Society of Chemical Industry (SCI).