Grevillea juncifolia Hook. and Grevillea robusta A. Cunn. Ex. R. Br. Methanolic Leaf and Flower Extracts Inhibit the Growth of Gram Positive and Gram Negative Bacteria

Introduction: The development of multi-antibiotic resistant strains of bacteria has necessitated the search for new effective antibacterial therapies. Several Grevillea spp. were used traditionally to treat pathogenic illness and are rich in phytocompounds with antibacterial activity. Despite this, the antibacterial activity of Australian Grevillea spp. extracts have not been extensively examined. Methods: The ability of G. juncifolia and G. robusta leaf and flower extracts to inhibit the growth of gram-negative and gram-positive bacterial species and some fungi was investigated by disc diffusion assays. The growth inhibitory activity was further quantified by MIC determination. Toxicity was determined using the Artemia franciscana nauplii bioassay. Results: The G. juncifolia and G. robusta extracts were good inhibitors of the growth of both gram-positive and gram-negative bacteria yet were completely ineffective against all fungal species tested. The leaf extracts generally had better antibacterial activity than the flower extracts. The G. juncifolia leaf extract was a particularly good inhibitor of A. faecalis, P. fluorescens, Y. entercolitica and B. subtilis growth, with MIC values of 62, 533, 736 and 682µg/mL respectively. The G. robusta leaf extract was a potent inhibitor of B. cereus and B. subtilis growth (145 and 83 µg/mL respectively). That extract was also a good inhibitor of A. faecalis, P. fluorescens, S. salford, S. aureus and S. epidermidis growth, albeit with substantially higher MIC values. In contrast, none of the extracts inhibited fungal growth. All extracts were determined to be non-toxic in the Artemia franciscana nauplii bioassay, indicating their safety for the treatment of bacterial infections. Conclusion: The lack of toxicity of the G. juncifolia and G. robusta extracts and their growth inhibitory bioactivity against gram-positive and gram-negative bacteria indicate their potential in the development of new antibiotic chemotherapies.