Antibacterial and Antibiotic Potentiation Activities of Selected Australian <i>Terminalia</i> spp. Against Gastrointestinal Pathogens

Terminalia spp. (family Combretaceae) are widely used to treat a wide variety of ailments, including bacterial infections. The antibacterial activities of multiple Terminalia spp. have been verified experimentally. Several studies have reported inhibitory activity for Terminalia catappa L. against some bacteria, including a limited number of gastrointestinal pathogens. In contrast, the growth-inhibitory effects of several native Australian species against gastrointestinal pathogens have not yet been evaluated. This study screens the bacterial growth inhibition activity of T. catappa, T. microcarpa, T. muelleri, and T. canescens leaf extracts against a panel of gastrointestinal bacterial pathogens. Solvents with different polarities were used to extract different complements of phytoconstituents from the dried leaves. The T. canescens methanol extract displayed noteworthy activity against A. hydrophila and B. cereus (960 µg/mL). In contrast, none of the ethyl acetate extracts prepared from any species tested displayed bacterial growth inhibitory activity. Notably, combinations of the Terminalia spp. extracts and selected conventional antibiotics yielded 15 synergistic, 59 additive, 79 non-interactive, and 29 antagonistic interactions. All the plant extracts were deemed to be nontoxic in human dermal fibroblast cell line assays, except the T. muelleri and T. microcarpa methanol extracts and the T. canescens water extract.