Selected Australian Terminalia Species Extracts Inhibit β-Lactam Drug-Resistant Bacteria Growth and Potentiate the Activity of Conventional Antibiotics: Bioactivities and Phytochemistry.

Terminalia ferdinandiana Exell, Terminalia grandiflora Benth., Terminalia microcarpa Decne., and Terminalia muelleri Benth. (family: Combretaceae) belong to the genus Terminalia . Plants of this genus have been extensively used as traditional medicines to treat a variety of illnesses, including pathogen infections. However, we were unable to find any studies that have investigated the antibacterial activity of T. microcarpa . Similarly, whilst some preliminary studies have examined the antimicrobial properties of T. muelleri and T. grandiflora , they did not test the extracts against antibiotic-resistant pathogens. This study screens the antimicrobial activity of T. grandiflora , T. microcarpa , and T. muelleri and compares it to that of T. ferdinandiana extracts prepared from both the fruit and leaves against a range of pathogens, including multi-antibiotic-resistant strains. Solvents with varying polarities were used to extract different phytochemical constituents from the leaves of T. grandiflora , T. microcarpa , and T. muelleri and from the fruit and leaves of T. ferdinandiana . The aqueous and methanolic extracts each displayed significant antimicrobial activity when tested against the bacterial pathogens, including against the multidrug-resistant strains. When these extracts were tested in combination with selected antibiotics, some extracts potentiated the antimicrobial activity. This study identifies twelve synergistic, fifty-eight additive, and sixty non-interactive combinations, as well as thirty antagonistic effects. The extracts were evaluated for toxicity using the Artemia franciscana nauplii lethality assay (ALA) and were each classified as non-toxic, with the exception of the methanolic and aqueous T. ferdinandiana fruit extracts and the aqueous and ethyl acetate T. ferdinandiana leaf extracts. Metabolomic analysis using liquid chromatography-mass spectrometry (LC-MS) highlighted several flavonoids and tannins that may contribute to the antimicrobial activities reported herein. The potential antibacterial mechanism(s) of the T. ferdinandiana extracts are discussed in this study.