Abstract
We followed adaptation in experimental microbial populations to inhibitory concentrations of an antimicrobial drug. The evolution of drug resistance was accompanied in all cases by changes in gene expression that persisted in the absence of the drug; the new patterns of gene expression were constitutive. The changes in gene expression occurred in four replicate populations of the pathogenic fungus Candida albicans during 330 generations of evolution in the presence of the antifungal drug fluconazole. Genome-wide expression profiling of over 5,000 ORFs identified 301 whose expression was significantly modulated. Cluster analysis identified three distinct patterns of gene expression underlying adaptation to the drug. One pattern was unique to one population and included up-regulation of the multidrug ATP-binding cassette transporter gene, CDR2. A second pattern occurred at a late stage of adaptation in three populations; for two of these populations profiled earlier in their evolution, a different pattern was observed at an early stage of adaptation. The succession of early- and late-stage patterns of gene expression, both of which include up-regulation of the multidrug major facilitator transporter gene, MDR1, must represent a common program of adaptation to this antifungal drug. The three patterns of gene expression were also identified in fluconazole-resistant clinical isolates, providing further evidence that these patterns represent common programs of adaptation to fluconazole.