Fermentates of consortia of lactic acid bacteria and a cyanobacterium are effective against toxigenic fungi contaminating agricultural produces

The objectives of this study were to (i) test the in vitro inhibitory activity of fermented consortia of the cyanobacterium Arthrospira platensis (syn. Spirulina platensis) and two selected strains of the Lactiplantibacillus plantarum (formerly Lactobacillus plantarum) against a set of toxigenic fungal plant pathogens, including Alternaria alternata, Aspergillus niger, A. flavus, Fusarium graminearum, Giberella zeae, Penicillium commune, and P. expansum, (ii) assess the efficacy of consortia fermentates in preventing infections of F. graminearum and P. expansum on maize cobs and lemon fruits, respectively, and (iii) identify and quantify antifungal compounds and secondary metabolites produced by these consortia. A noticeably inhibitory activity of the fermented consortia, as determined by the agar diffusion test, was observed for all the fungal pathogens tested. Fusarium graminearum and G. zeae were the most sensitive, while A. flavus and A. niger were the least sensitive. Cell-free supernatants (CFSs) of fermentates showed the highest antifungal activity after 24 and 48 h of incubation. Their MIC and MFC values ranged from 3.13 (G. zeae) to 25 g L−1 (A. niger) and from 3.13 (G. zeae) to 100 g L−1 (Aspergillus and Penicillium species), respectively. CFSs of fermentates were also effective in preventing infections by two very common toxigenic fungi, F. graminearum and P. expansum, in maize cobs and lemon fruits, respectively. Results of chemical analyses suggest the antifungal activity of fermentates depend, at least in part, on the presence of high levels of lactic acid and significantly higher concentrations of some phenolic compounds, including DL-3-phenallactic acid, benzoic acid and 3–4-dihydroxy hydrocinnamic acid. Among the 66 secondary metabolites detected in fermentates by UHPLC-Q-TOF-MS analysis, only benzoic acid, leucine, oleic acid, and proline possess antifungal activity. Overall, results of this study reveal the potential of fermented microbial consortia as BCAs to prevent post-harvest fungal diseases and contamination by mycotoxins of food and agricultural produces.