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Evolution of drug resistance in experimental populations of Candida albicans
L E Cowen1, D Sanglard, D Calabrese
1Department of Botany, University of Toronto, Mississauga, Ontario, Canada L5L 1C6. lcowen@credit.erin.utoronto.ca
Abstract:
Adaptation to inhibitory concentrations of the antifungal agent fluconazole was monitored in replicated experimental populations founded from a single, drug-sensitive cell of the yeast Candida albicans and reared over 330 generations. The concentration of fluconazole was maintained at twice the MIC in six populations; no fluconazole was added to another six populations. All six replicate populations grown with fluconazole adapted to the presence of drug as indicated by an increase in MIC; none of the six populations grown without fluconazole showed any change in MIC. In all populations evolved with drug, increased fluconazole resistance was accompanied by increased resistance to ketoconazole and itraconazole; these populations contained ergosterol in their cell membranes and were amphotericin sensitive. The increase in fluconazole MIC in the six populations evolved with drug followed different trajectories, and these populations achieved different levels of resistance, with distinct overexpression patterns of four genes involved in azole resistance: the ATP-binding cassette transporter genes, CDR1 and CDR2; the gene encoding the target enzyme of the azoles in the ergosterol biosynthetic pathway, ERG11; and the major facilitator gene, MDR1. Selective sweeps in these populations were accompanied by additional genomic changes with no known relationship to drug resistance: loss of heterozygosity in two of the five marker genes assayed and alterations in DNA fingerprints and electrophoretic karyotypes. These results show that chance, in the form of mutations that confer an adaptive advantage, is a determinant in the evolution of azole drug resistance in experimental populations of C. albicans.
Insights
Experimental evolution of Candida albicans demonstrates that adaptation to fluconazole, an antifungal drug, occurs through random mutations. These genetic changes lead to increased drug resistance and altered genomic profiles over time.
Area of Science:
- Microbiology
- Evolutionary Biology
- Genetics
Background:
- Antifungal resistance is a growing public health concern.
- Candida albicans is a common opportunistic fungal pathogen.
- Azole antifungals like fluconazole are widely used to treat Candida infections.
Purpose of the Study:
- To investigate the evolutionary pathways of fluconazole resistance in Candida albicans.
- To identify genetic mechanisms underlying adaptation to antifungal drugs.
- To understand the role of chance in the development of drug resistance.
Main Methods:
- Experimental evolution of Candida albicans populations over 330 generations.
- Monitoring of fluconazole minimum inhibitory concentration (MIC).
- Analysis of gene expression (CDR1, CDR2, ERG11, MDR1), ergosterol content, and genomic changes (loss of heterozygosity, DNA fingerprinting, karyotyping).
Main Results:
- Populations exposed to fluconazole showed increased MIC, indicating adaptation and resistance.
- Acquired fluconazole resistance correlated with cross-resistance to other azoles (ketoconazole, itraconazole).
- Distinct gene overexpression patterns and genomic alterations were observed in resistant populations, highlighting the role of chance mutations.
Conclusions:
- Adaptation to fluconazole in Candida albicans is driven by random mutations conferring a selective advantage.
- The evolution of azole drug resistance involves complex genetic changes, including gene overexpression and broader genomic instability.
- Understanding these evolutionary dynamics is crucial for developing effective antifungal therapies.