Characteristics of biofilm formation by Candida tropicalis and antifungal resistance.

Candida tropicalis is a common species related to nosocomial candidemia and candiduria. Most Candida spp. infections are associated with biofilm formation on implanted medical devices or on host epithelial cell surfaces. Sessile cells display phenotypic traits dramatically different from those of their free-living, planktonic counterparts, such as increased resistance to antimicrobial agents and to host defenses. The characteristics of C. tropicalis biofilm formation in vitro are described. By an XTT-reduction assay, an increase in metabolic activity was observed up to 24 h of biofilm formation, and this activity showed a linear relationship with sessile cell density. Scanning electron microscopy was used to further characterize C. tropicalis biofilms. The initial adherence of yeast cells was followed by germination, microcolony formation, filamentation and maturation at 24-48 h. Mature biofilms consisted of a dense network of yeast cells and filamentous forms of C. tropicalis. Increased resistance of sessile cells against fluconazole and amphotericin B was also demonstrated. Real-time reverse transcription-PCR quantification showed that sessile cells overexpressed ERG11 (coding for lanosterol 14 alpha-demethylase) and MDR1 (coding for an efflux protein belonging to the major facilitator superfamily). These mechanisms may contribute to the fluconazole resistance of the C. tropicalis biofilm.