Antimicrobial Activity of Actinobacteria Isolated From Kratom (<i>Mitragyna speciosa</i>) Leaves: Secondary Metabolite Profiling and Genome Analysis of <i>Micromonospora chersina</i> NRAIS18

Actinobacteria associated with medicinal plants are recognized as prolific sources of novel bioactive compounds. Kratom (<i>Mitragyna speciosa</i>) leaves have been reported to contain a diverse array of secondary metabolites with pharmacological activities, including antibacterial, antioxidant, and anti-inflammatory effects. In this study, 16 actinobacterial isolates were successfully obtained from the leaves of <i>M. speciosa</i>. Based on the 16S rRNA gene sequence analysis, the isolates were identified as members of <i>Streptomyces</i> and non-<i>Streptomyces</i> genera, including <i>Micromonospora</i>, <i>Pseudonocardia</i>, <i>Quadrisphaera</i>, <i>Prauserella</i>, and <i>Actinomycetospora</i>. All isolates were screened for antibacterial activity, among which <i>Micromonospora chersina</i> NRAIS18 demonstrated notable inhibitory effects against both Gram-positive and Gram-negative pathogens. The ethyl acetate crude extract of NRAIS18 exhibited the most potent activity against <i>Listeria monocytogenes</i> ATCC 7644, followed by <i>Pseudomonas aeruginosa</i> ATCC 27853. Metabolomic profiling using LC-MS and GC-MS revealed diverse secondary metabolites in the crude extract, including the isoflavone daidzein, harman, norharman, and soyasaponin Bb; siderophores such as ferrioxamine E and desferrioxamine E; and other volatile bioactive compounds such as fatty acids, isoquinolines, and esters. Several of these metabolites have been reported to possess antibacterial properties. These findings suggest that <i>M. chersina</i> NRAIS18 demonstrates significant promise as a source of antibacterial agents for pharmaceutical applications.