Objective: Isolation and molecular identification of actinobacterial endophytes from Zingiber nimmonii (J. Graham) Dalzell., Polygonum chinense L., and Justicia wynaadensis Heyne, important ethnomedicinal plant species of the ‘Western Ghats’, a hotspot location in southern India and characterization of secondary metabolites for the antibacterial activity by bio-autography and Gas Chromatography - Mass Spectrometry (GC-MS) techniques. Methods: The endophytic actinobacteria were isolated from the plant parts and identified by sequencing of the 16S rRNA genes. The secondary metabolites produced by the strains in the ISP-1medium were extracted with ethyl acetate and the crude dry extracts were evaluated for the total phenolic, flavonoid, antioxidant capacities and antibacterial potentials. Results: 14 endophytic actinobacteria belonging to five taxa were identified from three plant species as Corynebacterium, Curtobacterium, Arthrobacter, Streptomyces and Nocardiopsis. The total phenolic content of the actinobacterial extracts ranged from 5.2±0.1 to 24.4±0.08 mg gallic acid equivalent (GAE) /g dry extract. Flavonoid content was detected in three extracts (3.2±0.1 to 5.6±0.3 mg catechin equivalents (CE)/g dry extract). The radical scavenging activity (IC50 values) of actinobacterial extracts varied from 489.4 ± 2.3 µg/mL to 1968.3±112.0 µg/mL. Antibacterial activity was detected in all the extracts against six test bacterial strains viz., Bacillus subtilis (MTCC 121) and Staphylococcus aureus (MTCC 7443), Pseudomonas aeruginosa (MTCC 7093), Escherichia coli (MTCC 729), Enterobacter aerogenes (MTCC 111) and Klebsiella pneumoniae (MTCC 661). Eight extracts among the 14 showed inhibition zones ranging from 7.2±0.1 to 28.8±0.2 mm diameter against four of the six test pathogenic bacteria. S. indiaensis isolated from Z. nimmonii exhibited highest inhibition zones against all test pathogenic strains (14.2±0.1 to 28.8±0.2 mm dia) with the Minimum Inhibitory Concentrations (MIC’s) ranging from 0.02 to 0.16 mg/mL. Characterization of Streptomyces indiaensis extract by thin layer chromatography and GC-MS revealed the presence of eight aliphatic and volatile organic compounds. Conclusions: Three ethnomedicinal plant species from Western Ghats harbor diverse actinobacterial taxa with bioactivities.
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