Increase of virulence and its phenotypic traits in drug-resistant strains of Candida albicans.

There is concern about the rise of antifungal drug resistance, but little is known about comparative biological properties and pathogenicity of drug-resistant strains. We generated fluconazole (FLC; CO23 RFLC)- or micafungin (FK; CO23 RFK)-resistant strains of Candida albicans by treating a FLC- and FK-susceptible strain of this fungus (CO23 S) with stepwise-increasing concentrations of either drug. Molecular analyses showed that CO23 RFLC had acquired markedly increased expression of the drug-resistance efflux pump encoded by the MDR1 gene, whereas CO23 RFK had a homozygous mutation in the FSK1 gene. These genetic modifications did not alter to any extent the growth capacity of the drug-resistant strains in vitro, either at 28 degrees C or at 37 degrees C, but markedly increased their experimental pathogenicity in a systemic mouse infection model, as assessed by the overall mortality and target organ invasion. Interestingly, no apparent increase in the vaginopathic potential of the strains was observed with an estrogen-dependent rat vaginal infection. The increased pathogenicity of drug-resistant strains for systemic infection was associated with a number of biochemical and physiological changes, including (i) marked cellular alterations associated with a different expression and content of major cell wall polysaccharides, (ii) more rapid and extensive hypha formation in both liquid and solid media, and (iii) increased adherence to plastic and a propensity for biofilm formation. Overall, our data demonstrate that experimentally induced resistance to antifungal drugs, irrespective of drug family, can substantially divert C. albicans biology, affecting in particular biological properties of potential relevance for deep-seated candidiasis.