Synergistic effects of the combination of 20-hydroxyecdysone with ampicillin and gentamicin against methicillin-resistant Staphylococcus aureus.

The emergence of methicillin-resistant of Staphylococcus aureus (MRSA) has led to an urgent need for the discovery and development of new antibacterial agents. As part of an ongoing investigation into the antibacterial properties of the natural products, 20-Hydroxyecdysone (20E), isolated from the roots of Achyranthes japonica Nakai, was found to be active methicillin-resistant Staphylococcus aureus (MRSA), either alone or in combination with ampicillin (AM) or gentamicin (GM), vis checkerboard assay. This study investigated the antibacterial activity of 20E, which exhibited poor antibacterial activity (MIC=250-500 microg/mL) against all the bacterial strains tested. The combined activity of ampicillin (AM), gentamicin (GE) plus 20E against MRSA resulted in fractional inhibitory concentrations (FICs) ranging from 4.00 to 0.031 microg/mL, respectively. Meanwhile, the FIC index ranged from 0.16-4.50, indicating a marked synergistic relationship between AM, GE and 20E against MRSA with enterotoxin gene. Time-kill assays also showed a decrease remarkably between the combination and the more active compound. Therefore, this study demonstrated that AM, GE, and 20E can act synergistically in inhibiting MRSA in vitro.