Antimicrobial potential of actinomycetes from high altitude Nepalese soils

Nepal's unique geography and diverse biodiversity present a significant opportunity to discover new actinomycetes species that may lead to developing novel compounds with pharmaceutical applications. Fifty-nine actinomycetes strains were isolated from fifty-six soil samples collected from different altitudes and locations in Nepal. These actinomycetes strains were classified through cultural, morphological, partial 16 S rRNA gene sequence analysis, and phylogenetic analysis and were identified as belonging to the genera Streptomyces, Kitasatospora, Nocardia, Nocardiopsis, Nocardioides, Arthrobacter, Amycolatopsis, Promicromonospora, and Rhodococcus. The genus Streptomyces appeared to be the most abundant, comprising 44 isolates. Promicromonospora was isolated for the first time from high-altitude soils. Of the 59 isolated actinomycetes, 29 strains exhibited antimicrobial activity against the tested microorganisms. The highest activity was observed against fungi and Gram-negative bacteria, followed by Gram-positive bacteria. The crude extracts of Streptomyces olivaceus (13081) and Nocardia thailandica (13105) showed broad-spectrum antimicrobial activity against the tested bacteria and fungi. Ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) analysis of these two extracts revealed several antimicrobial compounds such as Neoaspergillic acid, Okaramine D, Antimycin A3, Tricycloalternarene A, Nocardimicin B, 4-O-methylmelleolide, Flocculosin, and Microcolin A. The study revealed the culture-based diversity of actinomycetes in Nepal and provided insights into their potential to produce bioactive compounds.