# Flavonoids identified in Australian Terminalia inhibit methicillin and β-lactam-resistant pathogens, exhibit efflux pump inhibitory activity, and potentiate conventional antibiotics

## TL;DR

This study finds that flavonoids from Australian Terminalia plants can inhibit antibiotic-resistant bacteria and may help in developing new antibiotic treatments.

## Contribution

The study identifies specific flavonoids that inhibit antibiotic-resistant pathogens and exhibit efflux pump inhibitory activity.

## Key findings

- Orientin and isoorientin showed the strongest inhibition of methicillin- and β-lactam-resistant pathogens.
- Some flavonoids synergistically enhanced the activity of conventional antibiotics.
- Orientin and isoorientin demonstrated efflux pump inhibitory activity at multiple concentrations.

## Abstract

Plants of the genus Terminalia have been widely used in traditional medicine for the treatment of multiple ailments, including infectious diseases. Previously, our group performed metabolomic analyses using liquid chromatography-mass spectrometry of various Terminalia spp. and highlighted several phytochemicals (particularly flavonoids) that may contribute to the antibacterial activities of those species. Herein, we screen 15 flavonoids found in Terminalia spp. against antibiotic-resistant and antibiotic-sensitive strains of Escherichia coli, Klebsiella pneumoniae, and Staphylococcus aureus. Orientin, hispidulin, vitexin, rutin, fisetin, and isoorientin inhibited the growth of both methicillin- and β-lactam-resistant pathogens, producing MIC values ranging from 250 to 62.5 µg/mL. Orientin and isoorientin were the most effective at restricting the growth of methicillin-resistant and β-lactamase pathogens, with MIC values of 125 µg/mL against E. coli and extended-spectrum β-lactamase E. coli and 62.5 µg/mL against S. aureus. In combination with selected conventional antibiotics, some flavonoids potentiated the antimicrobial activity of selected conventional antibiotics. A total of 4 synergistic, 4 additive, and 22 non-interactive interactions were identified. The toxicity of the flavonoids was examined using Artemia franciscana nauplii lethality assays. With the exception of fisetin, genistein, and gossypetin, the flavonoids were non-toxic. Orientin and isoorientin were assessed for their potential to inhibit efflux pumps and demonstrated notable efflux pump inhibitory activity at four different concentrations: 125.0, 62.5, 31.25, and 15.26 µg/mL. The results obtained suggest that these flavonoids could serve as a valuable tool in combating antibiotic resistance.

Bacteria are becoming resistant to many types of antibiotics. This study has identified plant phytochemicals known as flavonoids, which were found to be capable of inhibiting the growth of numerous bacterial pathogens. Evidence is shown which reveals that the compounds are capable of blocking bacterial efflux pumps, which demonstrate that the flavonoids may be valuable compounds in the design of new antibiotic drugs.