Indole-3-acetic acid-mediated self-rescue in Bacillus licheniformis against Saccharomyces cerevisiae stress

Microorganisms have evolved complex metabolic adaptations to environmental stresses in intricate microbial communities. However, the adaptive strategies of bacteria against the competitive stress from fungi are still poorly understood. Here, we reported an adaptive strategy adopted by Bacillus licheniformis in response to Saccharomyces cerevisiae stress. The growth of B. licheniformis in the presence of S. cerevisiae was reduced at 24 h, followed by a complete recovery after 48 h, coupled with a full-process reduction in biofilm formation. Meantime, genes involved in carbohydrate metabolism were significantly up-regulated, and those involved in biofilm formation were significantly down-regulated in B. licheniformis at 24 h. When B. licheniformis was cultured in S. cerevisiae conditioned medium, differentially up-regulated metabolites after 24h of incubation were primarily enriched in tryptophan metabolism pathway, with significant accumulation of indole-3-acetic acid (IAA) and its precursors. Supplementation with IAA suppressed biofilm formation in a concentration-dependent manner, and promoted biomass recovery at later fermentation stages, as well as up-regulation of genes involved in carbohydrate metabolism and down-regulations of genes involved in biofilm formation at 24 h. This study revealed that IAA acted as a bacterial signalling molecule via reducing biofilm formation to promote the growth recovery against the stress from fungi.