Evolution of drug resistance in experimental populations of Candida albicans

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 gen erations. The concentration of fluconazole was maintained at twice the MIC in six populations; no fluconazole was added to another six populations. Al l six replicate populations grown with fluconazole adapted to the presence of drug as indicated by an increase in MIG; none of the six populations gro wn without fluconazole showed any change in MIC. In all populations evolved with drug, increased fluconazole resistance was accompanied by increased r esistance to ketoconazole and itraconazole; these populations contained erg osterol in their cell membranes and were amphotericin sensitive. The increa se in fluconazole MIC in the six populations evolved with drug followed dif ferent trajectories, and these populations achieved different levels of res istance, with distinct overexpression patterns of four genes involved in az ole resistance: the ATP-binding cassette transporter genes, CDR1 and CDR2; the gene encoding the target enzyme of the azoles in the ergosterol biosynt hetic pathway, ERG11; and the major facilitator gene, MDR1. Selective sweep s 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 elect rophoretic karyotypes. These results show that chance, in the form of mutat ions that confer an adaptive advantage, is a determinant in the evolution o f azole drug resistance in experimental populations of C. albicans.