Alkaloid-Rich Extracts from Ocimum gratissimum and Zanthoxylum zanthoxyloides as Potential Antibiotic Adjuvants Against Resistant Bacterial Pathogens

Abstract Background Ocimum gratissimum and Zanthoxylum zanthoxyloides are medicinal plants widely used in African traditional medicine for the treatment of infectious diseases. The increasing prevalence of antimicrobial resistance (AMR) has renewed interest in plant-derived bioactive compounds as potential antibacterial agents and antibiotic adjuvants capable of restoring or enhancing the efficacy of existing antibiotics. Aim of the study This study investigated the antibacterial activity of total alkaloid extracts from O. gratissimum (OG-TAE) and Z. zanthoxyloides (ZZ-TAE), and evaluated their ability to modulate the efficacy of tetracycline against selected bacterial pathogens. Materials and methods Total alkaloid extracts from O. gratissimum (OG-TAE, 1.18% w/w) and Z. zanthoxyloides (ZZ-TAE, 1.40% w/w) were evaluated against Gram-positive and Gram-negative bacteria using checkerboard and time–kill kinetic assays. The chemical composition of the extracts was characterized by LC–ESI–MS analysis to identify putative bioactive alkaloids associated with antibacterial activity. Results Both extracts exhibited strain-dependent bacteriostatic activity with greater potency against Gram-positive bacteria. OG-TAE enhanced tetracycline activity against Salmonella poona , producing additive interactions and reducing the antibiotic concentrations required for antibacterial activity. Time–kill studies confirmed bactericidal effects for the combinations, with 5–10-fold improvements in antibacterial activity comparable to ciprofloxacin after 24 h. LC–ESI–MS analysis identified the following bioactive alkaloids; Zanthoamide I, nor-chelerythrine, 8-acetonyldihydro-chelerythrine, skimmianine, and benzimidazol-5-amine. Conclusions These findings demonstrate the potential of alkaloid-rich extracts from O. gratissimum and Z. zanthoxyloides as effective antibiotic adjuvants that enhance tetracycline efficacy, enable antibiotic dose reduction and offer a plant-based combination strategy to combat AMR.