Antifungal activity of Lactiplantibacillus plantarum isolated from fruit and vegetables and detection of novel antifungal VOCs from fungal-LAB co-cultures

Undesired microbiota, such as filamentous fungi, negatively impacts the food sector leading to huge losses of food produce. For this reason, preservatives are often used to reduce microbial contamination. In this context, the use of food-grade antagonistic microorganisms has gained international relevance as a promising alternative to synthetic additives. Lactic acid bacteria (LAB) with antifungal activity might have the strain-specific ability to control decay agents and their application as bio-control tools is growing worldwide. In this work, eight Lacti- plantibacillus plantarum strains were selected and their antifungal activity against Aspergillus niger, one of the main fungal contaminants of foods, were deeply investigated. The overlay method was used for an initial fast screening, then the presence of antifungal metabolites was evaluated in the cell-free supernatants (CFSs) by HPLC/DAD and in the volatilome by GC–MS analysis. Contribution of volatile organic compounds (VOCs) to antifungal activity was assessed in a plate-on-plate method without physical contact between moulds and LAB. In order to determine the presence of inducible antifungal VOCs, fungal-LAB co-cultures were carried out in glass tubes for direct GC-MS analyses. Antifungal VOCs, such as acetic acid, 2-Nonanone and 2-Undecanone are produced constitutively by all strains. Whereas fourteen VOCs appeared only in the fungal-LAB co-cultures. Six VOCs out of fourteen identified in co-cultures, and the three VOCs previously mentioned were individually tested for antifungal activity against A. niger in airtight contact-less petri dishes assay. Among those, trans-2-Octenal and 2-Nonanol had the higher activity against A. niger. These results showed that antifungal VOCs produced by L. plantarum could effectively inhibit the growth of A. niger when tested in vitro. In addition, different antifungal VOCs were detected only in fungal-LAB co-cultures, pointing to metabolism modulation mediated by VOCs that results in an enhanced antifungal activity.