Enhancing tetracycline, ceftazidime, and amoxicillin efficacy with hydroethanolic extracts of Mammea africana (Calophyllaceae) and Baillonella toxisperma (Sapotaceae) against clinically relevant drug-resistant enteric bacteria

Medicinal plants are essential in our battle against drug-resistant bacteria, serving as rich sources of antibacterial agents and modifiers of antibiotic resistance. This study examined the antibacterial efficacy of hydroethanolic extracts from the leaves of Mammea africana (MA) and Baillonella toxisperma (BT) and their interactions with tetracycline, ceftazidime, and amoxicillin on a panel of clinically relevant bacterial strains and clinical isolates of Escherichia coli, Salmonella sp., Shigella sp., Pseudomonas aeruginosa, and Staphylococcus aureus. Broth microdilution was used for antibacterial testing, while the checkerboard assay was employed to study interactions. The qualitative phytochemical analysis adhered to established protocols. Both extracts exhibited noteworthy antibacterial activity, with MICs ranging from 8 to 1024 µg/mL. MA was the most potent, demonstrating MICs < 100 µg/mL against six pathogens, including impressively low MICs (<10 µg/mL) for S. aureus and S. enteritidis, exceeding those of the reference antibiotic (tetracycline). MA also showed bactericidal effects against most drug-resistant strains tested. The combination revealed significant synergistic (fractional inhibitory concentrations, FICi = 0.281–0.5) and additive effects (FICi = 0.625–1), reducing the antibiotic MICs by 2–32-fold. Both extracts worked synergistically with amoxicillin against S. typhi ST-RHB. Additionally, MA demonstrated synergy with all antibiotics on S. aureus SA-RHB. Notably, no antagonistic effects were observed. The primary phytochemicals identified in the extracts included phenols, flavonoids, alkaloids, tannins, anthocyanins, terpenoids, and saponins, all known for their antibacterial properties. The findings support the traditional use of these plants and the practice of complementing herbal preparations with conventional antibiotics. Further research is needed to identify the bioactive components and evaluate their safety and mechanisms of action.