Isovalerylshikonin, a new resistance-modifying agent from Arnebia euchroma, supresses antibiotic resistance of drug-resistant Staphylococcus aureus.

Antimicrobial resistance is the greatest threat to the treatment of bacterial infectious diseases. The development of resistance-modifying agents represents a promising strategy to mitigate the spread of bacterial antibiotic resistance. A natural product, isovalerylshikonin (IVS), was isolated from traditional Chinese medicine Arnebia euchroma in this study, which exhibited marginal antibacterial activity against drug-resistant Staphylococcus aureus RN4220, with an MIC of 16 mg/L. In addition, a synergistic effect between IVS and streptomycin was detected by the microdilution antimicrobial checkerboard assay, with a reduction in the MIC of IVS by up to 32-fold against RN4220. A bacterial EtBr efflux assay and RT-PCR were performed to investigate the mechanism of the synergy. IVS significantly inhibited the bacterial efflux and expression of msrA mRNA in the in vitro experiment. A peritonitis/sepsis model was employed to test the in vivo synergistic activity of IVS and STM. IVS synergistically decreased the bacteria count with STM in peritoneal, spleen and liver, and increased mouse survival with STM in 7 days. The acute toxicity of IVS was tested, and the LD50 of IVS with a single exposure was 2.584 g/kg in mice. Overall, isovalerylshikonin, a low toxicity resistance-modifying agent, exhibited synergistic antibacterial activities in vitro and in vivo against drug-resistant S. aureus. The effects were mediated by suppression of msrA mRNA expression and bacterial efflux. In addition, these data support that IVS is a potential resistance-modifying agent against microbial resistance caused by msrA efflux pump.