Inhibition of Bacillus anthracis growth by Australian native plants used traditionally as antibacterial medicines

Introduction: Anthrax is a zoonotic disease caused by the bacterium Bacillus anthracis. It is often fatal if left untreated. Many Australian plants have documented therapeutic properties as general antiseptics, inhibiting the growth of a wide variety of bacterial species. This study examines the ability of selected Australian plant extracts to inhibit B. anthracis growth. Methods: Solvent extracts were prepared using plants with documented ethnobotanical usage to treat bacterial infections, or published antibacterial activity. The extracts were investigated by disc diffusion assay for the ability to inhibit the growth of an environmental strain of B. anthracis. Their MIC values were determined to quantify and compare their efficacies. Toxicity was determined using the Artemia franciscana nauplii bioassay. Results: Methanolic and aqueous extracts of Eucalyptus baileyana and Eucalyptus major displayed potent antibacterial activity in the disc diffusion assay against B. anthracis. The methanolic extracts were particularly potent with MIC values as low as 290 μg/mL (E. major methanolic extract). Tasmannia insipidia and Tasmannia stipitata extracts also inhibited B. anthracis growth, albeit with low efficacy. The E. baileyana and E. major methanolic leaf extracts as well as the E. baileyana aqueous leaf extract induced significant mortality in the Artemia fransiscana bioassay, with LC₅₀ values substantially <1000 μg/mL, indicating the toxicity of these extracts. Conclusion: The potent inhibitory bioactivity of the E. baileyana and E. major extracts against B. anthracis demonstrate their potential as medicinal agents in the treatment and prevention of anthrax. However, their toxicity indicates that their use may be limited to the treatment of the cutaneous form of the disease, or for sterilisation of infected sites.