An effective antimicrobial strategy of colistin combined with the Chinese herbal medicine shikonin against colistin-resistant Escherichia coli

ABSTRACT The treatment of bacterial infections has become more challenging due to the increasing prevalence of colistin-resistant (Col-R) pathogens. Shikonin is a Chinese herbal medicine that has been proven to have antibacterial properties. In recent years, many studies have shown that the combination of traditional antibiotics and non-antibacterial drugs can effectively treat infections caused by multidrug-resistant bacteria. This study evaluated the potential of shikonin in combination with colistin to inhibit colistin-resistant Escherichia coli (Col-R E. coli), and explored potential interaction. In checkerboard analysis, shikonin could significantly reduce the minimum inhibitory concentration (MIC) of colistin by two to eight times, enhancing the susceptibility of Col-R E. coli to colistin. Shikonin also increased the bactericidal effect of colistin against Col-R E. coli in 24 h time-kill assays, resulting in a >3 log-fold decrease in colony-forming units. In vivo tests showed that the combination of colistin and shikonin could significantly improve the survival rate of Galleria mellonella. In addition, confocal laser scanning microscopy and quantitative real-time polymerase chain reaction showed that colistin/shikonin could inhibit the formation of biofilms, and shikonin could down-regulate the transcription level of biofilm-regulated genes (csgA, csgD, flhC, flhD, fliC, fliM, lsrK, and lsrR). The synergistic mechanism of colistin and shikonin was further revealed by detecting membrane integrity, reactive oxygen species production, and mcr-1 gene expression. Our findings indicate that the colistin/shikonin combination may be a promising alternative approach to treat Col-R E. coli infections. IMPORTANCE Infections caused by multidrug-resistant Escherichia coli (MDR E. coli) have become a major global healthcare problem due to the lack of effective antibiotics today. The emergence of colistin-resistant E. coli strains makes the situation even worse. Therefore, new antimicrobial strategies are urgently needed to combat colistin-resistant E. coli. Combining traditional antibiotics with non-antibacterial drugs has proved to be an effective approach of combating MDR bacteria. This study investigated the combination of colistin and shikonin, a Chinese herbal medicine, against colistin-resistant E. coli. This combination showed good synergistic antibacterial both in vivo and in vitro experiments. Under the background of daily increasing colistin resistance in E. coli, this research points to an effective antimicrobial strategy of using colistin and shikonin in combination against colistin-resistant E. coli.