Antimicrobial activity of selected native Australian Terminalia spp. against gastrointestinal pathogens and potentiation of selected antibiotics

Terminalia species (Family: Combretaceae) have a wide variety of traditional therapeutic, including to treat bacterial infections. However, no previous studies have verified the antibacterial activity of Terminalia petiolaris A. Cunn. Ex Benth. or Terminalia grandiflora Benth. against gastrointestinal pathogens. Similarly, Terminalia ferdinandiana extracts have not yet been evaluated against many gastrointestinal bacteria. Herein, we quantify the minimum inhibitory concentrations (MIC) of T. ferdinandiana leaf and fruit extracts, as well as T. petiolaris and T. grandiflora leaf extracts against several bacterial gastrointestinal pathogens. Aqueous and methanolic T. ferdinandiana leaf extracts exhibited noteworthy inhibitory activity against multiple bacteria, whilst the corresponding fruit extracts displayed good activity against multiple Gram-negative pathogens, but were completely ineffective against B. cereus. The methanolic T. petiolaris extract showed low activity against all pathogens except S. flexneri, against which the activity was moderate. The methanolic T. grandiflora extract was ineffective against all of the bacteria tested. In contrast, all ethyl acetate extracts were effective inhibitors of bacterial growth, except the T. ferdinandiana leaf extract, which potently inhibited A. faecalis growth (MIC = 75 µg/mL). Combining the plant extracts with selected conventional antibiotics substantially enhanced the antibacterial activity of the mixture. Two synergistic combinations, as well as thirty-six additive, fifty-six indifferent, and twenty-four antagonistic combinations were identified. The safety of the extracts was evaluated by screening for toxicity towards human dermal fibroblast cells, with all extracts were classified as non-toxic. The growth inhibitory mechanism(s) of the extracts are discussed, with reference to their phytochemical composition.