Rheum palmatum L. Root Extracts Inhibit the Growth of Bacterial Triggers of Selected Autoimmune Inflammatory Diseases and Potentiate the Activity of Conventional Antibiotics

Introduction: An increase in antibiotic resistance and a corresponding decrease in antimicrobial discovery have directed researchers towards alternative therapies, including plant-based medicines. However, synergistic combinations of plant extracts with conventional antibiotics may be a more effective approach in overcoming resistance and potentiating the activity of antibiotics that are otherwise ineffective against resistant bacterial strains. Materials and Methods: The antibacterial activity of Rheum palmatum L. (Turkey rhubarb) root extracts was investigated by disc diffusion and quantified by liquid dilution and solid phase MIC assays. The extracts were also combined with a range of conventional antibiotics and tested against various microbial triggers of autoimmune diseases. The ΣFIC values obtained from these assays were used to determine the class of combinational effects and isobologram analysis was used to determine the ideal synergistic ratio(s). Toxicity was evaluated by Artemia nauplii mortality and HDF cell viability assays. Results: The methanolic, aqueous and ethyl acetate R. palmatum root extracts showed noteworthy inhibitory activity against several microbial triggers of autoimmune inflammatory diseases, whilst the chloroform and hexane extracts were also good inhibitors of P. aeruginosa growth. Some combinations of the R. palmatum root extract and conventional antibiotics were substantially more effective in inhibiting bacterial growth. Three synergistic and nine additive interactions were noted. Notably, the methanolic extract restored significant growth inhibitory activity to penicillin and ampicillin when tested in combination against Proteus spp. In contrast, four antagonistic interactions were noted for combinations containing gentamicin (against P. aeruginosa and S. pyogenes), indicating that those combinations should be avoided when treating infections caused by those bacteria. Conclusion: Rheum palmatum extracts have potential as inhibitors of bacterial triggers of selected autoimmune inflammatory diseases. Furthermore, extract components may also potentiate the activity of two antibiotics that are relatively ineffective alone. Isolation of these agents may be beneficial in drug design against several bacteria, including the microbial triggers of rheumatoid arthritis, ankylosing spondylitis, multiple sclerosis and rheumatic fever.