Antimicrobial activity of bacterial endophytes isolated from Aconitum nagarum Stapf ,a medicinal plant of North East Indo-Burma belt.

The present study aimed to investigate bacterial endophytes from different tissues of Aconitum nagarum, a threatened and medicinally important plant native to North East India, and to evaluate their antimicrobial potential. A total of 47 bacterial isolates were obtained from leaf, stem, and tuberous root samples, with the highest colonization frequency observed in leaf tissues (32.50%). Based on morphological, microscopic, and biochemical characterization, the isolates were categorized into six bacterial genera: Actinomycetes, Bacillus, Pseudomonas, Serratia, Streptococcus, and Streptomyces. All isolates were screened for antimicrobial activity against a panel of clinically significant pathogens: Pseudomonas aeruginosa (ATCC 9027), Klebsiella pneumoniae (ATCC 1705), Bacillus subtilis (ATCC 6051), and Candida albicans (MTCC 227). Among them, two isolates identified as Bacillus amyloliquefaciens and Bacillus siamensis exhibited significant antimicrobial activity, showing the highest zones of inhibition against K. pneumoniae, ranging from 18.04 ± 0.42 mm to 20.03 ± 0.63 mm. Crude metabolites of these isolates were extracted using ethyl acetate and characterized by GC-MS analysis. GC-MS analysis of B. amyloliquefaciens extracts revealed major compounds such as tetradecanoic acid, 4-tert-amylphenol, pyrrolo[1,2-a] pyrazine-1,4-dione, and benzeneacetic. In contrast, the extracts of B. siamensis contained cyclotrisiloxane, hexamethyl, 3-methylpentanoic acid, 2-methylbutanoic acid and 2-ethylheptanoic acid as principal compounds. While many of these compounds are known for their antimicrobial properties, the bioactivity of some remains to be elucidated. This study highlights the potential of endophytic bacteria from A. nagarum as a promising source of novel antimicrobial metabolites. Further in-depth investigations could lead to the discovery of new bioactive compounds with therapeutic applications.