Consolidated bioprocessing in engineered B. subtilis lab strains: γ-PGA production from biomass

Previous article in issueNext article in issue The soil bacterium B. subtilis, the model organism for Gram positive bacteria, is the best characterized member of the Bacillus genus, which includes several highly exploited industrial species. Besides industrial enzymes, Bacillus spp. can synthesize poly-γ-glutamic acid (γ-PGA), a nontoxic, biodegradable, highly anionic biopolymer made up of multiple d-/l-glutamic acid monomers joined by amide linkages between the α-NH2 and γ-COOH groups. Thanks to several valuable characteristics, γ-PGA is applied in an expanding range of biotechnological fields (Ogunleye et al., 2015), including applications as drug carrier, gene delivery and scaffold material for tissue engineering (Luo et al., 2016). γ-PGA production is also an ideal model system to develop cost-competitive feedstocks for B. subtilis aerobic fermentations. Despite B. subtilis possessing a wide array of complex-carbohydrates degrading enzymes, direct transformation of biomass into biocommodities has not yet been reported for this microorganism. The aim of this work was to obtain economic γ-PGA production using a waste biomass as feedstock. Rice straw is one of the most abundant biomass resources, not in competition with food, for which there are no effective valorisation strategies. In this study, the cellulolytic capabilities of B. subtilis JH642 were maximized through self-cloning procedures, and a cheap and simple pretreatment to facilitate straw saccharification was developed. The engineered strain grew efficiently on treated straw. Moreover, by transferring the mutations supporting γ-PGA biosynthesis (Scoffone et al., 2013) into the cellulolytic strain, direct production of γ-PGA from biomass was obtained, definitely proving the applicability of Consolidated Bioprocessing concepts to B. subtilis.