Combinational Inhibitory Effects of Terminalia arjuna (Roxb.) Wight and Arn. Extracts and Conventional Antibiotics against Bacterial Triggers of Selected Inflammatory Diseases

Introduction: Terminalia arjuna (Roxb.) Wight and Arn. is a southern Asian plant commonly used in Ayurveda to treat multiple conditions, including for the treatment of inflammation, and as a general antimicrobial agent. Despite this, few studies have tested this species for the ability to block the growth of bacterial triggers of some autoimmune inflammatory diseases. Furthermore, the ability of T. arjuna to potentiate the activity of conventional antibiotics is yet to be investigated. Materials and Methods: The minimum inhibitory concentration of the extracts and extract-antibiotic combinations was determined by disc diffusion and liquid dilution MIC methods. Fractional inhibitory concentration values were calculated to evaluate the combinational effect of combinations of extracts and conventional antibiotics. When synergistic interactions were detected, isobologram analysis was used to determine the ideal ratios for synergy. Toxicity was evaluated by Artemia nauplii mortality and HDF cell viability assays. Results: Methanolc, aqueous and ethyl acetate T. arjuna extracts were good inhibitors of P. mirabilis and K. pneumoniae growth when tested alone. The methanolic and aqueous extracts also synergised the inhibitory activity of ciprofloxacin against P. mirabilis and K. pneumoniae when used in combination. Additionally, numerous additive combinational effects were noted. T. arjuna chloroform extract was also a moderate inhibitor of a multi-drug resistant P. aeruginosa strain when tested alone, but did not potentiate the activity of any of the conventional antibiotics tested. All extracts and extract-antibiotic combinations were nontoxic in the Artemia nauplii mortality and HDF proliferation assays, indicating their suitability for therapeutic use. Conclusion: T. arjuna extracts have potential as inhibitors of bacterial triggers of selected autoimmune inflammatory diseases. Furthermore, T. arjuna extracts potentiate the activity of ciprofloxacin against P. mirabilis and K. pneumoniae and therefore may be beneficial in drug design against these bacteria.