Phytochemical Evaluation of Terminalia catappa L. Extracts with Antibacterial and Antibiotic Potentiation Activities Against β-Lactam Drug-Resistant Bacteria

<i>Terminalia catappa</i> L. (Family: Combretaceae) is used globally to treat various diseases, including bacterial infections. Whilst the antibacterial activity of <i>T. catappa</i> has previously been tested against antibiotic-sensitive bacterial strains, the antimicrobial activity against methicillin and β-lactam-resistant pathogens has been relatively ignored. The antibacterial activity of <i>T. catappa</i> extracts, both alone and combined with selected clinical antibiotics, was evaluated in this study. The inhibition of bacterial growth by the extracts was determined using agar diffusion and broth micro-dilution assays. Combinations of the extracts and several clinical antibiotics were also examined and the ∑FICs were calculated to determine the interaction class. Synergistic combinations were further evaluated by isobologram analysis. The <i>T. catappa</i> leaf extracts were screened for toxicity using <i>Artemia franciscana</i> lethality bioassays (ALA). Orbitrap liquid chromatography-mass spectrometry (LC-MS) profiling analysis was undertaken to identify flavonoid components of the extracts, putatively. The <i>T. catappa</i> methanolic extract inhibited all the tested bacterial strains. It displayed especially good inhibitory activity against <i>E. coli</i> (MIC = 130 µg/mL). Combining the <i>T. catappa</i> extracts with some conventional antibiotics potentiated the inhibitory activity of the combinations compared to the activity of individual components. LC-MS profiling analysis identified multiple flavonoid components, including rutin, quercitin, orientin, the tannin component, and ellagic acid in the extracts. All extracts were non-toxic against <i>Artemia</i> nauplii. The phytochemical constituents present in the <i>T. catappa</i> leaf extracts warrant future investigation as potential antibacterial agents.