BackgroundActinobacteria are often known to be great producers of antibiotics. The rapid increase in the global burden of antibiotic-resistance with the concurrent decline in the discovery of new antimicrobial molecules necessitates the search for novel and effective antimicrobial metabolites from unexplored ecological niches. The present study investigated the antimicrobial producing actinobacterial strains isolated from the soils of two microbiologically unexplored forest ecosystems, viz. Nameri National Park (NNP) and Panidehing Wildlife Sanctuary (PWS), located in the Eastern Himalayan Biodiversity hotspot region.ResultsA total of 172 putative isolates of actinobacteria were isolated, of which 24 isolates showed strong antimicrobial bioactivity. Evaluation of the ethyl acetate extracts of culture supernatants against test microbial strains revealed that isolates PWS22, PWS41, PWS12, PWS52, PWS11, NNPR15, NNPR38, and NNPR69 were the potent producers of antimicrobial metabolites. The antimicrobial isolates dominantly belonged to Streptomyces, followed by Nocardia and Streptosporangium. Some of these isolates could be putative novel taxa. Analysis of the antimicrobial biosynthetic genes (type II polyketide synthase and nonribosomal peptide synthetase genes) showed that the antimicrobial metabolites were associated with pigment production and belonged to known families of bioactive secondary metabolites. Characterization of the antimicrobial metabolites of Streptomyces sp. PWS52, which showed lowest taxonomic identity among the studied potent antimicrobial metabolite producers, and their interaction with the test strains using GC-MS, UHPLC-MS, and scanning electron microscopy revealed that the potential bioactivity of PWS52 was due to the production of active antifungal and antibacterial metabolites like 2,5-bis(1,1-dimethylethyl) phenol, benzeneacetic acid and nalidixic acid.ConclusionsOur findings suggest that the unexplored soil habitats of NNP and PWS forest ecosystems of Northeast India harbor previously undescribed actinobacteria with the capability to produce diverse antimicrobial metabolites that may be explored to overcome the rapidly rising global concern about antibiotic-resistance.Electronic supplementary materialThe online version of this article (10.1186/s12866-018-1215-7) contains supplementary material, which is available to authorized users.

A Streptomyces sp. strain named 5K10, isolated from soil samples of Kaziranga National Park, Assam, India was found to secrete an antimicrobial proteinaceous compound, showing in vitro antagonistic effect against some pathogenic bacteria, yeasts, dermatophytes and filamentous fungi. Capacity to produce extracellular antimicrobial substances by the strain 5K10 was assessed by conventional spot inoculation method in agar media against test microorganisms as well as fungal spore germination inhibition assay by microscopic observations. The fermented culture broth with ammonium sulfate precipitation revealed a biologically active proteinaceous compound and molecular weight estimated to be about 14.3 kDa on SDS-PAGE gel. Using Candida albicans MTCC 227 as the indicator strain, antagonistic activity of the proteinaceous compound in the region of the SDS-PAGE gel was confirmed. Bioactivity of the extracellular protein was sensitive to proteinases and partially susceptible to prolonged heat treatment. The results suggest that Streptomyces sp. 5K10 might be an important bioresource of lead molecules for developing antimicrobial agents to control bacterial and fungal infections.