Tannin components and inhibitory activity of Kakadu plum leaf extracts against microbial triggers of autoimmune inflammatory diseases

Abstract

Introduction: Autoimmune inflammatory diseases can be triggered by specific bacteria in susceptible individuals. Terminalia ferdinandiana (Kakadu plum) has documented therapeutic properties as a general antiseptic agent. However, the high ascorbic acid levels in Kakadu plum fruit may interfere with this activity. Methods: T. ferdinandiana leaf solvent extracts were investigated by disc diffusion assay against a panel of bacteria known to trigger autoimmune inflammatory diseases.Their MIC values were determined to quantify and compare their efficacies.Toxicity was determined using the Artemia franciscana nauplii bioassay. Non-targeted HPLC separation of crude extracts coupled to high resolution time-of-flight (TOF) mass spectroscopy with screening against 3 compound databases was used for the identification and characterisation of individual components in crude plant extracts. Results: Methanolic, aqueous and ethyl acetate T. Ferdinandiana leaf extracts displayed potent antibacterial activity in the disc diffusion assay against the bacterial triggers of rheumatoid arthritis, ankylosing spondylitis and multiple sclerosis. The ethyl acetate extract had the most potent inhibitory activity, with MIC values less than 120 µg/ml against P. mirabilis and A. baylyi (both reference and clinical strains). The ethyl acetate extract had similar potency against K. pneumonia (both reference and clinical strains), but had higher MIC values (2733 µg/ml) against P. aeruginosa. The methanolic extract was also a potent inhibitor of bacterial growth, with MIC values generally < 1000 µg/ml. In comparison, the water, chloroform and hexane leaf extracts were all substantially less potent antibacterial agents, with MICs values generally well over 1000 µg/ml. All T. ferdinandiana leaf extracts were either nontoxic or of low toxicity in the Artemia fransiscana bioassay.Non-biased phytochemical analysis of the ethyl acetate extract revealed the presence of high levels of tannins (exifone (4-galloylpyrogallol), ellagic acid dehydrate, trimethylellagic acid, chebulic acid, corilagin, punicalin, castalagin and chebulagic acid). Conclusion: The low toxicity of the T. ferdinandiana leaf extracts and their potent inhibitory bioactivity against the bacterial triggers of autoimmune inflammatory disorders indicates their potential as medicinal agents in the treatment and prevention of these diseases.