Flavonoids Identified in Terminalia spp. Inhibit Gastrointestinal Pathogens and Potentiate Conventional Antibiotics via Efflux Pump Inhibition

The genus <i>Terminalia</i> has a long history of use in traditional medicine to treat various diseases, including bacterial infections. We previously reported a metabolomic analysis using liquid chromatography-mass spectrometry of selected Australian <i>Terminalia</i> spp. and highlighted numerous flavonoids that may contribute to the antimicrobial activities of those plants. This study examines the antibacterial activities of fifteen flavonoids found in <i>Terminalia</i> spp. against a range of gastrointestinal pathogens using broth dilution assays. Flavonoids were also combined with six different classes of conventional antibiotics to investigate interactions. The efflux pump inhibitory activity of the flavonoid was evaluated using ethidium bromide accumulation and efflux assays. Toxicities were assessed via human dermal fibroblast cell line assays. Fisetin, hispidulin, isoorientin, orientin, rutin, and vitexin showed noteworthy growth inhibitory activity (MIC values 62.5-250 µg/mL). Isoorientin and orientin were most potent against <i>Bacillus cereus</i> and <i>Alcaligenes faecalis,</i> displaying MIC values of 62.5 µg/mL against both bacteria. All flavonoids except genistein, isorhamnetin, kaempferol, luteolin, taxifolin, and vitexin were nontoxic in human dermal fibroblast (HDF) cell proliferation assays. When individual flavonoids were combined with selected antibiotics, some potentiated the activity of these antibiotics. Two synergistic, eighteen additive and thirty-one non-interactive interactions were observed. The synergistic interactions were all observed in combination with orientin. Notably, orientin exhibited efflux pump inhibitory effects at concentrations from 15.26 µg/mL to 125 µg/mL. The findings reported herein indicate that the selected flavonoids have the potential for addressing bacterial antibiotic resistance and highlight the need for further study.