Evaluation of the potential of Macadamia integriflora extracts as antibacterial food agents

Abstract

Background: Macadamia integriflora (family Proteaceae) is an endemic Australian plant traditionally used by Australian Aborigines as a food. Its nuts are known to keep well, raising the possibility that they may contain antimicrobial compounds and therefore may have value as a functional food to retard spoilage and prevent food poisoning, as well as potential medicinal antibiotic uses. Methods: The antimicrobial activity of M. integriflora was investigated by disc diffusion assays against a panel of bacteria and fungi. Toxicity was determined using the Artemia franciscana nauplii bioassay. Results: All M. integriflora extracts displayed antimicrobial activity in the disc diffusion assay. The flower methanol extract had the broadest specificity, inhibiting the growth of 7 of the 14 bacteria tested (50 %) and all 3 (100 %) of the fungi tested. All other extracts inhibited the growth of 6 (42.9 %) of the bacterial species tested and up to 2 (66.6 %) of the fungi tested. All extracts were more effective at inhibiting the growth of Gram-negative bacteria than Gram-positive bacteria. Indeed, only the flower methanol extract was capable of inhibiting the growth of any of the Gram-positive bacteria, inhibiting the growth of only 1 (B. cereus) of the 4 Gram-positive bacteria tested (25%). All M. integriflora extracts were non-toxic in the Artemia franciscana bioassay, with no significant increase in mortality induction above that of the negative control. Conclusions: The lack of toxicity of the M. integriflora extracts and their inhibitory bioactivity against a panel of bacteria and fungi demonstrate their potential as food additives to inhibit bacterial spoilage and food borne illnesses without the need for chemical preservative additives. Furthermore, M. integriflora extracts also have promise as antimicrobial agents for medicinal purposes.