Selection of Ethanol Tolerant Strains ofCandida albicansby Repeated Ethanol Exposure Results in Strains with Reduced Susceptibility to Fluconazole

Abstract

Candida albicans is a commensal yeast that has important impacts on host metabolism and immune function, and can establish life-threatening infections in immunocompromised individuals. Previously, C. albicans colonization has been shown to contribute to the progression and severity of alcoholic liver disease. However, relatively little is known about how C. albicans responds to changing environmental conditions in the GI tract of individuals with alcohol use disorder, namely repeated exposure to ethanol. In this study, we repeatedly exposed C. albicans to high concentrations (10% vol/vol) of ethanol—a concentration that can be observed in the upper GI tract of humans following consumption of alcohol. Following this repeated exposure protocol, ethanol small colony (Esc) variants of C. albicans isolated from these populations exhibited increased ethanol tolerance, altered transcriptional responses to ethanol, and cross-resistance/tolerance to the frontline antifungal fluconazole. These Esc strains exhibited chromosomal copy number variations and carried polymorphisms in genes previously associated with the acquisition of fluconazole resistance during human infection. This study identifies a selective pressure that can result in evolution of fluconazole tolerance and resistance without previous exposure to the drug. Author Summary: Previous work has shown that Candida albicans, a common fungal component of the gastrointestinal (GI) microbiome, is found in high abundance in the alcoholic GI tract. However, it was unknown how repeated ethanol exposure, similar to what C. albicans would be exposed to in the upper GI tract of individuals with alcohol use disorder, would affect the yeast. In this work, we show that repeated ethanol exposure selects for C. albicans strains with altered transcriptional responses to ethanol, increased ethanol tolerance, and cross-resistance/tolerance to the common frontline antifungal fluconazole. This work shows that ethanol exposure can promote evolution of C. albicans to antifungal resistance without previous exposure to the antifungal drug. These findings could be relevant to C. albicans in the upper GI tract in individuals with alcohol use disorder and further studies could reveal optimized antifungal regimens for C. albicans infections in these patients.