Optimization of the antifungal metabolite production in Streptomyces libani isolated from northern forests soils in Iran

Background and Purpose: Soil bacteria have extreme population diversity among natural sources and are able to produce a wide array of antifungal metabolites. This study aimed to isolate and identify the bioactive metabolite-producing bacteria from forest soils and evaluate their antimicrobial potent against some pathogenic organisms. Materials and Methods: In this study, soil samples were screened for antifungal activity against Aspergillus fumigatus on glucose-yeast extract (GY) agar using a visual agar plate assay method. All growing bacteria were examined for antifungal activity, and antagonistic bacteria were identified based on 16S ribosomal RNA sequence analysis. For optimization of the production of antifungal bioactive metabolites, inhibitory bacteria were cultured on different culture conditions, including media, pH, temperature, and incubation time. Results: In total, 110 bacterial strains were isolated from the forest soils and four species with high antifungal activity were identified as Streptomyces libani, Streptomyces angustmyceticus, Bacillus subtilis, and Sphingopyxis spp. on the basis of 16s ribosomal RNA sequencing. Dichloromethane extract of the starch casein broth culture filtrate of the S. libani (incubated at 30° C for five days) showed strong antifungal activity against A. fumigatus, Aspergillus niger, and Aspergillus flavus. Conclusion: Based on the results, forest soils contain organisms with antifungal activity and could be considered as a good source for novel antifungal metabolites as effective and safe therapeutics.