Co-culture of Aspergillus sydowii and Bacillus subtilis induces the production of antibacterial metabolites.

The co-culture strategy, which mimics natural ecology by constructing an artificial microbial community, is a useful tool to activate the biosynthetic gene clusters to generate new compounds. However, without optimization of fermentation conditions, the antagonism between the microbes often interferes with the production of secondary metabolites. In this study, the fermentation conditions of co-culture of Aspergillus sydowii and Bacillus subtilis were optimized by response surface methodology to increase the production of active metabolites against Staphylococcus aureus. After optimization, the inhibitory rate of the co-culture extract was 74.62%, which was 29.20% higher than that of the initial conditions. Meanwhile, a total of 15 newly biosynthesized metabolites were detected only in optimized co-culture, occupying 13.2% of all detected metabolites. The structures of the 12 metabolites with high variable importance in projection score were elucidated by the established LC-MS/MS approach integrated with various metabonomic tools. Among them, 7 metabolites were newly induced and the content of other 5 metabolites increased by 1.1-2.4 folds in optimized co-culture. The bioassay of metabolites in co-culture against S. aureus indicated that compounds (-)- (7S)- 10-hydroxysydonic acid, serine sydonate and macrolactin U' contributed much to the increment of antibacterial activity. This study demonstrated that optimizing the fermentation conditions of co-culture was beneficial to changing the metabolite profile and effective to induce the biosynthesis of active metabolites.