C. Harta, P. Ilankoa, J. Sirdaartaa,b, P. Rayana,b, P.A. McDonnella and I. E. Cocka,b*
aSchool of Natural Sciences, Nathan Campus, Griffith University, 170 Kessels Rd, Nathan, Queensland 4111, Australia.
bEnvironmental Futures Research Institute, Nathan Campus, Griffith University, 170 Kessels Rd, Nathan, Queensland 4111, Australia.
Introduction: Tasmannia stipitata (Dorrigo pepper) is an endemic Australian plant with a history of use by indigenous Australians as a food. It is taxonomically related to Tasmania lanceolata which has documented therapeutic properties as well as uses for food flavouring. Methods: T. stipitata solvent extracts were investigated by disc diffusion assay against a panel of bacteria and fungi. Their MIC values were determined to quantify and compare their efficacies. The ability to inhibit the proliferation of Giardia duodenalis was determined by direct cell counts and by using an MTS based cell proliferation assay. Toxicity was determined using the Artemia franciscana nauplii bioassay. Results: Methanolic, aqueous and ethyl acetate T. stipitata leaf and berry extracts displayed antibacterial activity in the disc diffusion assay. The berry methanolic extract had the broadest antibacterial range, inhibiting the growth of all 22 of the 23 bacteria tested (95.7%) and 2 of the 4 fungal species (50%) tested. In comparison, 18 of the bacterial species (81.8%) and 2 of the fungal species (50%) were inhibited by at least 1 of the leaf extracts. The methanol, water and ethyl acetate extracts of both berries and leaves all had similar efficacies and ranges of microbes inhibited. Whilst broad spectrum activity was seen for these extracts, they displayed only moderate to low efficacy (as determined by the zones of inhibition and MIC analyses). All extracts were more effective at inhibiting the growth Gram-negative bacteria than Gram-positive bacteria or fungi. Furthermore, the methanol, water and ethyl acetate extracts of both berry and leaf were potent inhibitors of Giardial proliferation. All T. stipitata extracts were non-toxic in the Artemia fransiscana bioassay with LC50 values greatly in excess of 1000 μg/ml. Conclusion: The lack of toxicity of the T. stipitata extracts and their moderate broad spectrum inhibitory bioactivity against bacteria, fungi and Giardia indicates their potential as natural food preservatives and as medicinal agents in the treatment and prevention of microbial diseases.
Keywords: Winteraceae, Tasmannia stipitata, Dorrigo pepper, antibacterial, food spoilage, food poisoning, functional food, natural preservative.