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 .