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Antimicrobial Agents and Chemotherapy, June 2000, p. 1578-1584, Vol. 44, No. 6
0066-4804/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Experimental Induction of Fluconazole Resistance in Candida tropicalis ATCC 750

Francesco Barchiesi,1,* David Calabrese,2 Dominique Sanglard,2 Luigi Falconi Di Francesco,1 Francesca Caselli,1 Daniele Giannini,1 Andrea Giacometti,1 Stefano Gavaudan,3 and Giorgio Scalise1

Istituto di Malattie Infettive e Medicina Pubblica, Università degli Studi di Ancona, Ancona, Italy1; Institut de Microbiologie, Centre Hospitalier Universitaire Vaudois, Lausanne, Switzerland2; and Istituto Zooprofilattico Sperimentale Umbria-Marche, Perugia, Italy3

Received 15 November 1999/Returned for modification 22 February 2000/Accepted 19 March 2000

Candida tropicalis is less commonly isolated from clinical specimens than Candida albicans. Unlike C. albicans, which can be occasionally found as a commensal, C. tropicalis is almost always associated with the development of fungal infections. In addition, C. tropicalis has been reported to be resistant to fluconazole (FLC). To analyze the development of FLC resistance in C. tropicalis, an FLC-susceptible strain (ATCC 750) (MIC = 1.0 µg/ml) was cultured in liquid medium containing increasing FLC concentrations from 8.0 to 128 µg/ml. The strain developed variable degrees of FLC resistance which paralleled the concentrations of FLC used in the medium. The highest MICs of FLC were 16, 256, and 512 µg/ml for strains grown in medium with 8.0, 32, and 128 µg of FLC per ml, respectively. Development of resistance was rapid and could be observed already after a single subculture in azole-containing medium. The resistant strains were cross-resistant to itraconazole (MIC > 1.0 µg/ml) and terbinafine (MIC > 512 µg/ml) but not to amphotericin B. Isolates grown in FLC at concentrations of 8.0 and 32 µg/ml reverted to low MICs (1.0 µg/ml) after 12 and 11 passages in FLC-free medium, respectively. The MIC for one isolate grown in FLC (128 µg/ml) (128 R) reverted to 16 µg/ml but remained stable over 60 passages in FLC-free medium. Azole-resistant isolates revealed upregulation of two different multidrug efflux transporter genes: the major facilitators gene MDR1 and the ATP-binding cassette transporter CDR1. The development of FLC resistance in vitro correlated well with the results obtained in an experimental model of disseminated candidiasis. While FLC given at 10 mg/kg of body weight/day was effective in reducing the fungal burden of mice infected with the parent strain, the same dosing regimen was ineffective in mice infected with strain 128 R. Finally, the acquisition of in vitro FLC resistance in strain 128 R was related to a loss of virulence. The results of our study elucidate important characteristics and potential mechanisms of FLC resistance in C. tropicalis.

* Corresponding author. Mailing address: Istituto di Malattie Infettive e Medicina Pubblica, Università degli Studi di Ancona, Azienda Ospedaliera, Umberto I°, Largo Cappelli 1, 60121 Ancona, Italy. Phone: 39. 71. 5963467. Fax: 39. 71. 5963468. E-mail: cmalinf{at}popcsi.unian.it.

Antimicrobial Agents and Chemotherapy, June 2000, p. 1578-1584, Vol. 44, No. 6
0066-4804/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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