Combination effects of modified Gingyo-san extract and antimicrobial agents

Abstract

IntroductionModified Gingyo-san (MGS) has long been empirically used for the treatment of respiratory infections in traditional Chinese medicine (TCM). Therefore, in this study, the combination effects of MGS and antimicrobial agents against several pathogens were analysed. MethodsA total of six species of bacteria were employed in this study. The combination effects were tested with a checkerboard assay. To evaluate the involvement of bacterial efflux pumps in the combination effects, eight efflux pump-deletion mutants from KEIO collection were used. ResultsMost of the tested antimicrobial agents did not show any significant combination effects with MGS. However, when mixed with levofloxacin, MGS showed synergistic effects against highly quinolone-resistant Staphylococcus aureus strains. By contrast, MGS showed antagonistic effects with levofloxacin against gram-negative bacteria such as Escherichia coli. To clarify the mechanism of these antagonistic effects, the combination effects against a series of efflux pump-deleted mutants were tested, demonstrating that the major efflux pump AcrAB (acriflavine resistance protein AB)-TolC (outer membrane protein) contributed to the antagonistic effects. ConclusionOur results indicate that MGS interacts with levofloxacin to exert combination effects, although the direction of the effect (antagonistic or synergistic) depends on the bacterial species. In particular, these results suggest that MGS could attenuate the antimicrobial effects of levofloxacin by interaction with the efflux pump.