A novel formulation of RPMI 1640 medium for susceptibility testing of Malassezia yeasts by broth microdilution (BMD) and Etest is proposed. A modification of the NCCLS M27-A2 BMD method was used to test 53 isolates of Malassezia furfur (12 isolates), M. sympodialis (8 isolates), M. slooffiae (4 isolates), M. globosa (22 isolates), M. obtusa (2 isolates), M. restricta (2 isolates), M. pachydermatis (1 isolates), and M. dermatis (2 isolates) against amphotericin B, ketoconazole, itraconazole, fluconazole, voriconazole, terbinafine, and posaconazole by BMD and Etest. RPMI and antibiotic medium 3 (AM3) were supplemented with glucose, bile salts, a mixture of fatty acids, and noctadecanoate fatty acids and Tween 20. M. furfur ATCC 14521 and M. globosa ATCC 96807 were used as quality control strains. Depending on the species, MICs were read after 48 or 72 h of incubation at 32°C. Low azole and terbinafine MICs were recorded for all Malassezia species, whereas amphotericin B displayed higher MICs (>16 g/ml) against M. furfur, M. restricta, M. globosa, and M. slooffiae strains, which were AM3 confirmed. Agreement of the two methods was 84 to 97%, and intraclass correlation coefficients were statistically significant (P < 0.001). Because of higher amphotericin B MICs provided by Etest for strains also displaying high BMD MICs (>1 g/ml), agreement was poorer. The proposed media are used for the first time and can support optimum growth of eight Malassezia species for recording concordant BMD and Etest MICs.Malassezia yeasts are members of the normal human skin flora and agents of skin disorders and systemic infections in subgroups of severely immunocompromised patients. The obligatory lipophilic nature of human pathogenic Malassezia species has delayed developments in susceptibility testing of azoles, as varying results have been reported (7,13,20). The alternative approach (20), involving indirect assessment of susceptibility using the metabolic activity of the yeast as a viability marker, was also found unsatisfactory for testing azoles. By contrast, reproducible MIC results have been obtained in urea broth, although the method had not been assessed with solid media (13). Broth microdilution (BMD) methods using Leeming-Notman (LN) medium with Alamar Blue (20) and LN medium (7) have also been employed, but interlaboratory agreement protocols and comparative MIC data using the solid-medium tests have not been determined since.As the incidence of yeasts in deep-seated infections continues to increase in proportion to the growing numbers of immunocompromised, cancer, and postoperative patients, standardized MIC testing for more yeast genera can become pertinent to clinical practice. Regarding bloodstream Malassezia infections, poor response to amphotericin B has led to discontinuation of parenteral therapy as an additional therapeutic measure (18) or modification of therapy in order to increase the amphotericin B concentration in the catheter lumen, aiming at eradicating Malasssezia yeasts (1). However, the observed poor cl...
