In vitro susceptibility of fluconazole-susceptible and -resistant isolates of Malassezia pachydermatis against azoles.

OBJECTIVES: The first aim of this study was to evaluate the in vitro efficacies of fluconazole, ketoconazole, itraconazole and voriconazole on M. pachydermatis growth inhibition. This study also evaluated M. pachydermatis azole cross-resistance, comparing wild clinical isolates and the same isolates with in vitro-induced fluconazole resistance. METHODS: Two techniques were used: (1) a broth microdilution method based on protocol M27-A3 from the Clinical and Laboratory Standards Institute to determine the minimum inhibitory concentration (MIC) and (2) the Fekete-Forgács method to induce fluconazole resistance in vitro. The isolates were divided into two groups: group 1 included fluconazole-susceptible clinical isolates (n=30) and group 2 contained the same isolates with in vitro-induced fluconazole resistance (n=30). RESULTS: The two groups exhibited differences in susceptibility (p<0.001). Group 1 isolates were susceptible to azoles: ketoconazole (MIC 0.01-1.0 µg/mL), itraconazole (MIC 0.01-1.0 µg/mL), voriconazole (MIC 0.01-4.0 µg/mL), and fluconazole (MIC 0.01-4.0 µg/mL). Group 2 isolates demonstrated a wider range of MICs to azoles: ITZ (MIC 0.06-64.0 µg/mL), KTZ (MIC 0.25-32.0 µg/mL), VRZ (MIC 2.0-128.0 µg/mL), and FLZ (MIC 64.0-128.0 µg/mL). CONCLUSIONS: It was shown that FLZ-resistant M. pachydermatis isolates exhibit cross-resistance to other azoles, reinforcing the importance of susceptibility tests as a guide for the therapeutic prescription of antifungals in medical and veterinary mycology.