Combining Colistin with Furanone C-30 Rescues Colistin Resistance of Gram-Negative Bacteria in Vitro and in Vivo.

The spread of multidrug-resistant (MDR) Gram-negative bacteria (GNB) has led to serious public health problems worldwide. Colistin, as a "last resort" for the treatment of MDR bacterial infections, has been used significantly in recent years and has led to the continuous emergence of colistin-resistant strains. In this study, we aimed to investigate the synergistic effect on the antimicrobial and antibiofilm activities of a colistin/furanone C-30 combination against colistin-resistant GNB in vitro and in vivo. According to antimicrobial resistance profiles, most of the colistin-resistant strains we collected showed MDR phenotypes. The checkerboard method and time-kill curve showed that the combination with furanone C-30 increases the antibacterial activity of colistin significantly. In addition, the furanone C-30/colistin combination can not only inhibit the formation of bacterial biofilm but also has a better eradication effect on preformed mature biofilms. The result of scanning electron microscopy (SEM) demonstrated that the furanone C-30/colistin combination led to a significant reduction in the number of cells in biofilms. Furthermore, furanone C-30 at 50 μg/ml did not cause any additional toxicity to RAW264.7 cells according to a cytotoxicity assay. In in vivo infection experiments, the furanone C-30/colistin combination increased the survival rate of infected Galleria mellonella larvae as well as decreased the microbial load in a mouse thigh infection model. The synergistic effect of the furanone C-30/colistin combination against colistin-resistant GNB is encouraging, and this work may shed light on a new therapeutic approach to combat colistin-resistant pathogens. IMPORTANCE Colistin is among the few antibiotics effective against multidrug-resistant Gram-negative bacteria (GNB) clinical isolates. However, colistin-resistant GNB strains have emerged in recent years. Therefore, the combination of colistin and nonantibacterial drugs has attracted much attention. In this study, the furanone C-30/colistin combination showed good antibacterial and antibiofilm activity in vitro and in vivo. In addition, increased membrane permeability leads to the synergistic effect of the furanone C-30/colistin combination. Because of the low cytotoxicity of furanone C-30, this combination has good application prospects in clinical anti-infective therapy. This finding might shed light on the discovery of combination therapy for infections caused by colistin-resistant GNB pathogens.