Enhanced production of antifungal lipopeptides by Bacillus amyloliquefaciens for biocontrol of postharvest disease

Abstract

Food security to sustain increasing populations is a global concern. A major factor threatening food security is crop spoilage during postharvest storage. Reduction of postharvest spoilage has mainly been addressed by the application of synthetic chemicals. Bacillus lipopeptides, specifically lipopeptide homologues exhibiting antifungal efficacy, offer an alternative environmentally benign protocol for reduction of postharvest phytopathogens. This work is directed towards Bacillus lipopeptide production for biocontrol of postharvest phytopathogens in general and fungal phytopathogens in particular. Bacillus amyloliquefaciens DSM 23117 was identified as an organism with superior potential for lipopeptide production, via screening of 4 Bacillus candidates, in terms of antifungal lipopeptide concentration, yield, productivity and preferred homologue ratio. Efficacy of B. amyloliquefaciens lipopeptides against Botrytis cinerea substantiated appropriateness of this Bacillus species. Subsequent process modification of B. amyloliquefaciens cultures demonstrated that the concentration and ratio of the lipopeptides were significantly influenced by process conditions and further, distinguished nitrate and oxygen availability as key parameters defining optimal lipopeptide production. Discrete B. amyloliquefaciens cultures supplied with 4, 8, 10 and 12 g/L NH4NO3 demonstrated optimal lipopeptide concentration, yield and productivity, with respect to both total and antifungal lipopeptides, in the culture containing 8 g/L NH4NO3. Enhancement of total and antifungal lipopeptide kinetics similar to those quantified on increasing the nitrate from 4 to 8 g/L NH4NO3 were exhibited in B. amyloliquefaciens cultures when the oxygen in the sparge gas was increased from 21 to 30 mol%. The enhancement of lipopeptide production under conditions of increased nitrate and increased oxygen supply is explained in terms of increased availability of nitrogen for synthesis. This work has highlighted key parameters for maximisation of Bacillus lipopeptide production and manipulation of antifungal/surfactin ratios for optimum efficacy and informs on future development of process strategies towards production optimisation of antifungal lipopeptides as a green alternative to synthetic chemicals.