Callistemon linearis Schrad. and J.C. Wendl. Extracts Inhibit the Growth of Gram-Positive Bacteria but have no Effect on Gram Negative Bacteria

Introduction: The development of multi-antibiotic resistant strains of bacteria has necessitated the search for new effective antibacterial therapies. Many Callistemon spp. were used traditionally to treat pathogenic illness and are rich in terpenoids with reported antibacterial activity. Despite this, the antibacterial activity of C. linearis leaf extracts has not been extensively examined. Methods: The ability of C. linearis 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 methanolic and aqueous C. linearis leaf extracts were good inhibitors of the growth of gram-positive bacteria yet were completely ineffective against gram-negative bacteria. The methanolic extract was a particularly good inhibitor of B. cereus and S. pyogenes growth, with MIC values of 610 and 354μg/mL respectively. The aqueous extract was also a good inhibitor of these bacteria (MICs of 927 and 660μg/mL respectively). Whilst the extracts also inhibited the growth of S. aureus and S. epidermidis, the MIC values (in the range 1200-1500μg/mL) were indicative of moderate inhibitory activity. The methanolic extracts were further investigated by growth time course assays that showed significant growth inhibition within 1h of exposure. All extracts were determined to be nontoxic in the Artemia franciscana nauplii bioassay, indicating their safety for the treatment of gram-positive bacterial infections. Conclusion: The lack of toxicity of the C. linearis leaf extracts and their growth inhibitory bioactivity against the gram-positive bacteria indicate their potential in the development of new antibiotic chemotherapies.