Abstract
Microorganisms associated with medicinal plants are of interest as the producers of important bioactive compounds. To date, the diversity of culturable endophytic actinomycetes associated with medicinal plants is in its initial phase of exploration. In this study, 42 endophytic actinomycetes were isolated from different organs of seven selected medicinal plants. The highest number of isolates (n = 22, 52.3%) of actinomycetes was isolated from roots, followed by stems (n = 9, 21.4%), leaves (n = 6, 14.2%), flowers (n = 3, 7.1%), and petioles (n = 2, 4.7%). The genus Streptomyces was the most dominant among the isolates (66.6%) in both the locations (Dampa TRF and Phawngpuii NP, Mizoram, India). From a total of 42 isolates, 22 isolates were selected for further studies based on their ability to inhibit one of the tested human bacterial or fungal pathogen. Selected isolates were identified based on 16S rRNA gene analysis and subsequently the isolates were grouped to four different genera; Streptomyces, Brevibacterium, Microbacterium, and Leifsonia. Antibiotic sensitivity assay was performed to understand the responsible antimicrobials present in the isolates showing the antimicrobial activities and revealed that the isolates were mostly resistant to penicillin G and ampicillin. Further, antimicrobial properties and antibiotic sensitivity assay in combination with the results of amplification of biosynthetic genes polyketide synthase (PKS-I) and non-ribosomal peptide synthetase (NRPS) showed that the endophytic actinomycetes associated with the selected medicinal plants have broad-spectrum antimicrobial activity. This is the first report of the isolation of Brevibacterium sp., Microbacterium sp., and Leifsonia xyli from endophytic environments of medicinal plants, Mirabilis jalapa and Clerodendrum colebrookianum. Our results emphasize that endophytic actinomycetes associated with medicinal plants are an unexplored resource for the discovery of biologically active compounds.
Highlights
Endophytic microorganisms reside in the internal tissues, and subsist in symbiotic or mutualistic association with their host plant without causing apparent symptoms of infection
Thirteen isolates were from starch casein nitrate agar (SCNA) medium, 10 isolates from actinomycetes isolation agar (AIA) medium, 10 from tap water yeast extract agar (TWYE) medium, 6 from glycerol asparagines agar medium (ISP5) and 3 isolates were obtained from malt yeast extract agar medium (ISP2)
Endophytic actinomycetes from different medicinal plants are reported as major source of natural products with potential antimicrobial activity (Cao et al, 2004; Castillo et al, 2007)
Summary
Endophytic microorganisms reside in the internal tissues, and subsist in symbiotic or mutualistic association with their host plant without causing apparent symptoms of infection. Actinomycetes are aerobic, gram positive bacteria comprising a group of branching unicellular microorganism, which play a significant role in the breakdown of organic matter into more obtainable nutrients. They are known for the production of many secondary metabolites including various antibiotics, antitumor and plant growth hormones, which are important for pharmaceutical and agricultural industries (Fiedler et al, 2008). There is increasing evidences for the existence of new endophytic actinomycetes within various tissues of medicinal plants, and some produce bioactive compounds with novel chemical structures (Qin et al, 2008; Nimnoi et al, 2010). Information is scarce on the tissue distribution and biodiversity of endophytic actinomycetes associated with traditional medicinal plants from unique environments
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