The Interactive Antimicrobial Activities of Selected South African Terminalia spp. Extracts in Combination with Conventional Antibiotics against Gastrointestinal Pathogens

Background: Recent declines in new antibiotic discovery and the increase in antibiotic resistance have resulted in failing against bacterial pathogens. To develop novel antibiotic therapies, medical researchers have begun to focus on traditional therapies. Combinational therapies consisting of medicinal plants and conventional antibiotics may reactivate current drugs that are otherwise ineffective against antibioticresistant bacteria. Terminalia sericea Burch. Ex DC, Terminalia prunioides Laws. and Terminalia gazensis Bak. f. are native South African plants with antimicrobial properties. However, combinations of Terminalia sericea, Terminalia prunioides and Terminalia gazensis with conventional antibiotics are yet to be evaluated for growth inhibitory activity against gastrointestinal pathogens. Methods: Terminalia spp. leaves were extracted with solvents of varying polarity. Antimicrobial screening was performed using disc diffusion and broth microdilution assays. Toxicity was measured using Artemia franciscana nauplii lethality assays. Results: All extracts (except the T. sericea extracts) showed low to moderate inhibitory activity against B. cereus and E. faecalis, A. faecalis, A. hydrophilia, S. sonnei and S. newport in both the disc diffusion and liquid dilution assay. Twenty-three extract/antibiotic combinations produced synergy, 26 were additive, 24 were non-interactive and seven were antagonistic. Most of the antagonist interactions occurred in combinations containing gentamicin. All extracts were either non-toxic or of low toxicity in the Artemia bioassay. Conclusion: Terminalia spp. extracts may mimic the actions of a resistance modifying agents, enhancing the activity of several antibiotics that are relatively ineffective alone. Further studies are required to identify the bioactive and potentiating components and their mechanisms of action.