Repurposing fusidic acid through colistin-mediated membrane permeabilization against Escherichia coli co-harboring mcr-1 and blaNDM-5.

AIMS: Carbapenem- and colistin-resistant Escherichia coli co-carrying blaNDM-5 and mcr-1 represent a growing therapeutic challenge with limited treatment options. Fusidic acid (FA), a protein synthesis inhibitor, is inactive against Gram-negative bacteria because of its limited ability to permeate their outer membrane. This study aimed to evaluate whether colistin (CST) can restore FA activity against mcr-1-positive E. coli through membrane permeabilization-mediated synergy.

METHODS AND RESULTS: Three E. coli isolates carrying the blaNDM-5 and mcr-1 were subjected to antimicrobial susceptibility testing, checkerboard and time-kill assays, NPN uptake, intracellular FA accumulation analyses, and a murine thigh infection model. All isolates exhibited multidrug resistance but only low-level CST resistance (MIC 4-8 mg l-1). Checkerboard assays revealed strong synergy, with CST reducing FA minimum inhibitory concentrations (MICs) by >1000-fold and restoring apparent susceptibility. Time-kill studies confirmed concentration-dependent synergistic bactericidal activity, achieving complete bacterial eradication when sub-MIC of FA was combined with CST. The tests for NPN uptake evolution and FA accumulation revealed that CST markedly increased membrane permeability and intracellular FA levels (5- to 8-fold). In the mouse thigh infection model, combination therapy with CST and FA at clinically equivalent doses produced a ≥3.0-log10 cfu g-1 reduction relative to the control and a >2.0-log10 cfu g-1 reduction compared with either monotherapy.

CONCLUSIONS: CST markedly enhances FA activity against mcr-1-positive E. coli by compromising outer membrane integrity and facilitating intracellular antibiotic accumulation. This synergy highlights a rational combination approach to restore the efficacy of obsolete antibiotics for multidrug-resistant Gram-negative bacteria.