Interactive Antimicrobial Profiles of Astragalus membranaceus (Fisch.) Bunge Extracts and Conventional Antibiotics against Pathogenic and Non-pathogenic Gastrointestinal Bacteria

Background: The aim of this project was to investigate the efficacy of Astragalus membranaceus (Astragalus) extracts, alone and in combination with conventional antibiotics, against diarrhoea- and dysentery-causing pathogens, as well as against non-pathogenic gastrointestinal bacterial strains. The study sought to validate the chemotherapeutic potential of a traditional Chinese medicinal plant and to identify combinational thera- pies with increased efficacy compared to either the extracts or conven- tional antibiotics alone. Methods: Astragalus root powder was extracted with solvents of varying polarity and screened for inhibition of bacterial growth. Susceptibility was assessed by disc diffusion techniques, whilst the minimum inhibitory concentrations (MICs) were quantified by liquid dilution assays. To screen for combinatorial effects, the Astragalus root ex- tracts were combined with a range of conventional antibiotics and tested against each bacterial strain using liquid dilution assays. ΣFIC values were determined and used to determine the class of interaction. Results: Aque- ous Astragalus root extracts did not significantly inhibit the growth of the non-pathogenic or beneficial gut microflora bacteria E. cloacae, E. coli or E. faecalis, but possessed mild inhibitory activity against pathogenic A. fae- calis, A. hydrophila, B. cereus, S. newport and S. sonnei bacteria. Combi- nations of the Astragalus extracts and conventional antibiotics generally produced additive or indifferent interactions, indicating that they are safe to use concomitantly without compromising the efficacy of either compo- nent. Two cases of antagonistic combinations were detected against B. cereus and S. sonnei. Conclusion: Mild inhibition of 5 pathogenic bacteria occurred with aqueous Astragalus extracts, with a number of additive and antagonistic interactions arising when tested in combination with conven- tional antibiotics. Astragalus may be used safely in the presence of nor- mal gut bacteria and in most combinations with conventional antibiotics.