Biocontrol mechanisms of a karstic cave-derived Streptomyces sp. GCAL-9 bioactive extract against Colletotrichum musae: insights into ergosterol pathway involvement and metabolomic profiling.

Anthracnose is a fungal disease associated with Colletotrichum members responsible for one of the major diseases that critically deteriorate fruit quality. Herein, we aimed to evaluate the biocontrol mechanism of the Streptomyces sp. GCAL-9 bioactive extract (BE9) against Colletotrichum musae. We also determined alterations in cell morphology, changes in membrane stability, and the effects on the expression of ergosterol biosynthesis-related genes. Additionally, we analyzed genomic and phylogenomic information. The strain GCAL-9 was initially isolated from a cave and evaluated for its antifungal activity against C. musae, and additionally against several phytopathogenic fungi. The minimal inhibitory concentration (MIC) of BE9 against C. musae conidial germination was 48 µg/mL, while the minimal fungicidal concentration (MFC) was 96 µg/mL. With an MFC/MIC ratio of 2, BE9 reduced fungal mycelial growth by 80% and spore germination by 75%. Regarding the effect of BE9 on the expression of ergosterol biosynthesis-related genes, it downregulated the transcription of Cmhmg1, the gene encoding a key enzyme in the mevalonate pathway. This downregulated effect on enzymatic biosynthesis of ergosterol suggest a disruption of the membrane integrity and viability of conidial cells, inducing morphological shrinkage and a contracted appearance. Furthermore, application of BE9 on banana surfaces infected with C. musae conidia prevented anthracnose progression, similarly to the chemical fungicide carbendazim. Untargeted metabolomic profiling of BE9 exposed the presumptive presence of enokipodin A, a sesquiterpene metabolite previously reported in fungal species. This study showed the identification of a potential novel species capable of producing extracellular secondary metabolites to control anthracnose-causing fungi that infect postharvest fruits.