Discovery of efficient approaches to tackle clinical challenges due to emergence of colistin-resistant Enterobacteriaceae

Abstract

The clinical value of the last-line antibiotic colistin is limited by its toxicity and the increasing prevalence of drug resistance in recent years. These two issues are tackled by searching for adjuvant compounds that enhance colistin activity and facilitate reduction in treatment dosage. This study identifies several FDA-approved drugs, which can act synergistically with colistin to effectively eradicate colistin-resistant bacteria both in vitro and in mouse infection model, and allow a lower dose of colistin to be used in treatment of infections caused by colistin-susceptible bacteria. In the first part of my studies, screening of an FDA-approved drug library resulted in identification of two structurally similar compounds, namely cetylpyridinium chloride and domiphen bromide, which potentiated colistin activity in both colistin-resistant and susceptible strains of Enterobacteriaceae. These compounds were found to insert their long carbon chain into a hydrophobic pocket of bacterial phosphoethanolamine transferases including the mobile element-encoded colistin resistance determinant MCR-1, thereby competitively blocking the binding of lipid A tail for substrate recognition and modification, with the effect of significantly enhancing bacterial sensitivity to colistin. In addition, these compounds were also found to dissipate bacterial membrane potential and further enhance bacterial sensitivity to colistin. Importantly, combined usage of domiphen bromide with colistin exhibited remarkable protection against infections by colistin-resistant bacteria in both mouse thigh infection and sepsis models. For mice infected by colistin-susceptible bacteria, a domiphen bromide and colistin drug combination enables us to use a lower dose of colistin for effective treatment. These properties render domiphen bromide an excellent adjuvant candidate that helps transform colistin into a highly potent antimicrobial agent for treatment of colistin-resistant Gram negative bacterial infections and allowed us to use a much lower dosage of colistin so that the toxicity of the drug regimen used to treat colistin-susceptible bacterial infections, such as those caused by carbapenem-resistant Enterobacteriaceae strains, can be reduced. In the second part of my studies, Econazole, an FDA-approved antifungal drug that acts synergistically with colistin to exert strong antibacterial effect on colistin-resistant gram-negative bacteria was identified. Structural analysis shows that econazole exhibits high lipid affinity and acts as an ionophore. Functional assays and microscopy analysis confirm that econazole causes dissipation of transmembrane proton motive force (PMF) and damages to bacterial cell membrane. Its synergistic effect with colistin might be due to the fact that a combination of these two compounds caused rapid and complete collapse of PMF, thereby arresting various cellular functions and eventually resulting in bacterial cell death. These findings suggest that the econazole and colistin drug combination is highly effective in eradicating colistin-resistant Gram-negative bacterial pathogens regardless of their mechanism of colistin resistance.