Antibacterial Activity of Equisetin from Endophytic Fusarium incarnatum JMB4 Associated with Jatropha multifida L. Against MRSA: Experimental and In Silico Insights

An antibacterial metabolite was isolated from the endophytic fungus Fusarium incarnatum JMB4 associated with Jatropha multifida L. by bioactivity-guided chromatography and recrystallization. The isolated compound was assigned as equisetin using UV–vis, FTIR, LC–MS/MS, and 1H NMR data. Anti-MRSA activity was evaluated by disk diffusion, with chloramphenicol and amoxicillin as antibiotic comparators. At 1% (w/v; 100 μg/disk), equisetin produced an inhibition zone of 23.94 ± 0.66 mm; the recorded comparator zones were 24.71 ± 0.68 mm for chloramphenicol (30 μg/disk) and 11.76 ± 0.66 mm for amoxicillin (25 μg/disk). The lowest tested equisetin concentration producing a measurable zone was 78.1 μg/mL (7.03 ± 0.25 mm). This diffusion end point does not represent a minimum inhibitory concentration (MIC), and the unequal disk loads preclude a direct comparison of intrinsic antibacterial potency. Exploratory docking against penicillin-binding protein 2a (PBP2a) yielded a score of −5.5041 kcal/mol and a predicted pose outside the canonical catalytic pocket. A 100 ns molecular dynamics simulation described the behavior of the modeled complex but did not establish PBP2a inhibition. These results document anti-MRSA activity of the isolated compound under the tested conditions; standardized MIC determination and further structural validation remain necessary.