We assessed fluconazole susceptibility in 52 Candida tropicalis
clinical strains using seven antifungal susceptibility methods, including broth microdilution (BMD) [standard M27 A3 (with neutral and acid pH), ATB Fungus 3, Vitek 2 system and flow cytometric analysis] and agar-based methods (disk diffusion and E-test). Trailing growth, detection of cell-associated secreted aspartic proteases (Saps) and morphological and ultrastructural traits of these clinical strains were also examined. The ranges of fluconazole 24 h-minimum inhibitory concentration (MIC) values were similar among all methods. The essential agreement among the methods used for MIC determinations was excellent and all methods categorised all strains as susceptible, except for one strain that showed a minor error. The presence
of the trailing effect was assessed by six methods. Trailing positivity was observed for 86.5-100% of the strains. The exception was the BMD-Ac method where trailing growth was not observed. Morphological and ultrastructural alterations were detected in C. tropicalis
trailing cells, including mitochondrial swelling and cell walls with irregular shapes. We tested the production of Saps in 13 C. tropicalis
strains expressing trailing growth through flow cytometry. Our results showed that all of the C. tropicalis
strains up-regulated surface Sap expression after 24 h or 48 h of exposure to fluconazole, which was not observed in untreated yeast strains. We concluded that C. tropicalis
trailing growth presented some particular features on both biological and ultrastructural levels.