The Inhibitory Activity of Banksia collina R.Br. and Banksia oblongifolia Cav. Methanolic Leaf Extracts against a Panel of Bacterial Pathogens

Introduction: The development of multi-antibiotic resistant strains of bacteria has necessitated the search for new, effective antibacterial therapies. B. collina and B. oblongifolia leaves were used by Australian Aborigines to treat bacterial infections. However, little research has been published on antibacterial activity of these species. Methods: The ability of B. collina and B. oblongifolia leaf extracts to inhibit the growth of gram-negative and gram-positive bacterial species was investigated by disc diffusion and growth time course assays. The growth inhibitory activity was further quantified by MIC determination. Toxicity was determined using the Artemia franciscana nauplii bioassay. Results: The B. collina and B. oblongifolia leaf extracts were good inhibitors of the growth of both gram-positive and gram-negative bacteria. The B. collina and B. oblongifolia leaf extracts were particularly good inhibitors of A. faecalis growth (MICs of 225 and 486μg/ mL respectively) and B. cereus growth (MICs of 515 and 875μg/mL respectively). The B. collina extract was also a good inhibitor of B. subtilis growth, whilst the B. oblongifolia extract was a moderate growth inhibitor (MIC values of 923 and 1250μg/mL respectively). A similar, trend was noted for Y. entercolitica growth inhition (MICs of 518 and 1136μg/mL respectively). Whilst MIC values were also determined against other bacterial species, they generally indicated low-moderate activity. The B. collina and B. oblongifolia leaf extracts were further investigated by growth time course assays against A. faecalis and B. cereus. Interestingly, both extracts showed significant growth inhibition within 1h of exposure against both bacterial species. All extracts were determined to be nontoxic in the Artemia franciscana nauplii bioassay, indicating their safety for the treatment of bacterial infections. Conclusion: The lack of toxicity of the B. collina and B. oblongifolia leaf extracts and their growth inhibitory bioactivity against multiple bacterial species indicate their potential in the development of new antibiotic chemotherapies.