Combining Colistin and Fluconazole Synergistically Increases Fungal Membrane Permeability and Antifungal Cidality

Abstract

The increasing emergence of drug-resistant fungal pathogens, together with the limited number of available antifungal drugs, presents serious clinical challenges to treating systemic, life-threatening infections. Repurposing existing drugs to augment the antifungal activity of well-tolerated antifungals is a promising antifungal strategy with the potential to be implemented rapidly. Here, we explored the mechanism by which colistin, a positively charged lipopeptide antibiotic, enhances the antifungal activity of fluconazole, the most widely used orally available antifungal. In a range of susceptible and drug-resistant isolates and species, colistin was primarily effective at reducing fluconazole tolerance, a property of subpopulations of cells that grow slowly in the presence of a drug and may promote the emergence of persistent infections and resistance. Clinically relevant concentrations of colistin synergized with fluconazole, reducing fluconazole minimum inhibitory concentration 4-fold. Combining fluconazole and colistin also increased survival in a C. albicans Galleria mellonella infection, especially for a highly fluconazole-tolerant isolate. Mechanistically, colistin increased permeability to fluorescent antifungal azole probes and to intracellular dyes, accompanied by an increase in cell death that was dependent upon pharmacological or genetic inhibition of the ergosterol biosynthesis pathway. The positive charge of colistin is critical to its antifungal, and antibacterial, activity: colistin directly binds to several eukaryotic membrane lipids ( i.e. , l -α-phosphatidylinositol, l -α-phosphatidyl- l -serine, and l -α-phosphatidylethanolamine) that are enriched in the membranes of ergosterol-depleted cells. These results support the idea that colistin binds to fungal membrane lipids and permeabilizes fungal cells in a manner that depends upon the degree of ergosterol depletion.

Physiological and transcriptional profiling of surfactin exerted antifungal effect against Candida albicans

“…Several ergosterol (ERG1, ERG3, ERG9, ERG10, ERG11) and fatty acid metabolism-related genes (ACC1, FAS1, FAS2) were down-regulated, suggesting possible changes in the structure and fluidity of the cell membrane. The observed synergistic interaction between surfactin and fluconazole supports these presumed membrane changes (41). Surprisingly, a remarkable increase in ethanol content was detected in surfactin-exposed cultures both at 2 and 4 hours compared to the untreated control.…”

Drug Repurposing in Medical Mycology: Identification of Compounds as Potential Antifungals to Overcome the Emergence of Multidrug-Resistant Fungi

“…The colistin-azoles combination has also been reported more recently in strains showing low susceptibilities to fluconazole. Bibi M. et al confirm that colistin binds to the lipids of fungal membranes and works in relation to an ergosterol depletion level due to the previous action of azoles [42]. Other non-antifungal agents with bacterial activity have demonstrated a synergy with antifungals, as described by Rossato et al [65].…”

Mining RNA-Seq Data to Depict How Penicillium digitatum Shapes Its Transcriptome in Response to Nanoemulsion

“…Its primary function is to maintain the fluidity and permeability of the cell membrane. The decrease in ergosterol content usually leads to the disorder of cell permeability and the interruption of cell growth and proliferation ( 58 ). Some nanoemulsions encapsulated with antifungal agents showed a promising antifungal activity by inhibiting the ergosterol biosynthesis, leading to cell death ( 59 , 60 ).…”