Honokiol Restores Polymyxin Susceptibility to MCR-1-Positive Pathogens both In Vitro and In Vivo.

The emergence of the plasmid-mediated colistin resistance gene mcr-1 has led to serious multidrug-resistant (MDR) Enterobacteriaceae infections, which are a great threat to the clinic. This study aims to find an inhibitor of MCR-1 to reestablish the use of polymyxins against MDR Enterobacteriaceae infections. Here, we determined that the natural compound honokiol could enhance the efficacy of polymyxins against MDR Enterobacteriaceae infections by a checkerboard MIC assay, a time-kill assay, a combined disk test, Western blotting, molecular simulation dynamics, and mouse infection models. The MIC results indicated that honokiol can recover the sensitivity of polymyxins against MCR-1-positive Klebsiella pneumoniae and Escherichia coli (with a fractional inhibitory concentration index ranging from 0.09 ± 0.00 to 0.27 ± 0.06). Based on time-kill curve analysis, all of the tested bacteria were killed within 1 h following the combined therapy with honokiol and polymyxins. Molecular simulation dynamics results suggested that honokiol directly binds to the MCR-1 active region, reducing the biological activity of MCR-1. The combination of honokiol and polymyxins could increase the 40% protection rate and reduce the bacterial load on the thigh muscles of mice. Our study indicates that honokiol is a predominant natural compound whose combination therapy with polymyxins is very promising in future treatment options for MCR-1-positive Enterobacteriaceae infections.IMPORTANCE In the present study, honokiol could effectively inhibit the activity of MCR-1 and showed almost no cytotoxicity to MH-S cells. According to our results, the combination of honokiol and polymyxin had a clear synergistic effect against MCR-1-positive Enterobacteriaceae in vitro Combination therapy also showed a powerful therapeutic effect in vivo, which can significantly improve mouse livability, reduced the load of bacteria, and reduced pathological change. This combined therapy of small molecule compounds and antibiotics may not continue to induce new bacterial resistance, due to the fact that MCR-1 targeted by honokiol is not indispensable for the bacterial viability; on the other hand, it can reduce the dosage of combined antibiotics, and it is also a promising alternative therapy for the treatment of drug-resistant infections in the future.