Discovery of Antibacterial Compounds from Medicinal Plant Endophytes

Abstract

Antimicrobial resistance is an escalating global health threat that necessitates the discovery of new bioactive compounds. Fungal endophytes, which live asymptomatically within plant tissues, are increasingly recognized as promising sources of novel secondary metabolites with antimicrobial activity. In this study, fungal endophytes were isolated from seven medicinal plant species and screened for antibacterial activity against clinically relevant bacterial pathogens. A total of 264 morphologically distinct fungal endophytes were isolated from surface-sterilized root and leaf tissues. Initial antibacterial screening identified 58 isolates (22%) capable of inhibiting the sensitive Acinetobacter baumannii strain AB258. Antibacterial isolates were disproportionately derived from root tissues (74.1%), suggesting that roots represent a particularly rich source of bioactive endophytes. Of these active isolates, 24 demonstrated broader-spectrum activity against additional bacterial strains. Fifteen isolates producing the strongest inhibition zones were selected for crude extract preparation and secondary screening using disc diffusion assays against a bacterial panel including methicillin-resistant Staphylococcus aureus (MRSA), Pseudomonas aeruginosa, and Acinetobacter spp. Several extracts exhibited multi-strain inhibitory activity, with isolates DL16, GB29, and GB2 producing the largest zones of inhibition. Three candidate isolates (GB2, LV6, and DL16) were prioritized for chemical investigation. Thin-layer chromatography revealed distinct metabolite profiles, with GB2 displaying the greatest compound separation. Consequently, GB2 was selected for further purification using flash chromatography, yielding three pure fractions. Minimum inhibitory concentration (MIC) assays of these fractions revealed relatively high MIC values (128–256 μg mL⁻¹) across tested strains, indicating limited antibacterial potency of individual compounds but suggesting potential synergistic interactions among metabolites. Overall, this study highlights the diversity of fungal endophytes associated with medicinal plants and demonstrates their potential as sources of antibacterial metabolites. These findings support continued exploration of plant-associated fungi for novel antimicrobial compound discovery.