Bacillus anthracis growth Inhibitory Properties of Australian Terminalia spp.: Putative Identification of low Polarity Volatile Components by GC-MS Headspace Analysis

Introduction: Anthrax is a severe acute disease caused by Bacillus anthracis infections. If untreated, it often results in mortality. Many Terminalia spp. 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 Terminalia spp. extracts to inhibit B. anthracis growth. Methods: Solvent extracts were prepared from Terminalia carpentariae and Terminalia grandiflora plant material and 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. The most potent extracts were analysed by GC-MS headspace analysis. Results: T. carpentariae and T. grandiflora leaf, fruit and nut solvent extractions displayed good growth inhibitory activity against B. anthracis. Methanolic T. Carpentariae leaf and T. grandiflora nut extracts were particularly potent growth inhibitors, with MIC values of 74 and 155 µg/mL respectively. The T. carpentariae leaf ethyl acetate extract was also a good inhibitor of B. anthracis growth (MIC 340 µg/mL). All other extracts were substantially less potent growth inhibitors. Interestingly, the T. Carpentariae leaf extracts with growth inhibitory activity were nontoxic in the Artemia fransiscana bioassay, with LC₅₀ values >1000 µg/mL. In contrast, the LC₅₀ value 740 µg/mL reported for the methanolic T. grandiflora nut extract indicates low-moderate toxicity. Non-biased GC-MS phytochemical analysis of the most active extracts (methanolic T. carpentariae leaf and T. grandiflora nut) putatively identified and highlighted several compounds that may contribute to the ability of these extracts to inhibit the growth of B. anthracis. Conclusions: The growth inhibitory activity of the methanolic T. Carpentariae leaf and T. grandiflora nutextracts against B. anthracis indicates their potential for the treatment and prevention of anthrax. Furthermore, thelack toxicity of the T. Carpentariae leaf and the low-moderate toxicity of the T. grandiflora nut extract, indicates that their use may extend to all forms of the disease (cutaneous, inhalation or gastrointestinal).