Kakadu plum fruit extracts inhibit growth of the bacterial triggers of rheumatoid arthritis: Identification of stilbene and tannin components

Rheumatoid arthritis is an autoimmune disease which can be triggered in genetically susceptible individuals by Proteus spp. infections. Terminalia ferdinandiana (Kakadu plum) fruit extracts were investigated by disc diffusion assay against reference and clinical strains of Proteus mirabilis and Proteus vulgaris and their MIC values were determined. Polar extracts displayed potent antibacterial activity against the bacterial triggers of rheumatoid arthritis, with MIC values as low as 32 µg/ml (methanolic extract against the P. mirabilis reference strain). The aqueous extract was also a potent inhibitor of Proteus growth (MIC values <300 µg/ml against all bacterial species). Whilst substantially less potent, the ethyl acetate and chloroform extracts also displayed moderate to good inhibition (as determined by MIC) against both P. mirabilis strains. All T. ferdinandiana fruit extracts were nontoxic in the Artemia franciscana bioassay. The most potent extract (methanolic extract) was analysed by HPLC-QTOF mass spectroscopy (with screening against 3 compound databases). Five stilbenes and 7 tannins were identified in the methanolic extract. The low toxicity of the T. ferdinandiana fruit extracts and their potent inhibitory bioactivity against some bacterial triggers of rheumatoid arthritis indicates their potential as medicinal agents in the treatment and prevention of this disease.