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

•Isovalerylshikonin (IVS) exhibited a synergistic effect with streptomycin (STM) in vitro.•IVS decreased bacterial counts in mice and increased their survival with STM.•IVS inhibited antimicrobial resistance by suppression of msrA mRNA expression.•With an LD50 of 2.584 g/kg in mice, IVS is a low-toxicity agent.