Influence of different susceptibility testing methods and media on determination of the relevant fluconazole minimum inhibitory concentrations for heavy trailing Candida isolates with low-high phenotype.

We investigated the incidence of trailing growth with fluconazole in 101 clinical Candida isolates (49 C. albicans and 52 C. tropicalis) and tried to establish the convenient susceptibility testing method and medium for fluconazole minimum inhibitory concentration (MIC) determination. MICs were determined by CLSI M27-A2 broth microdilution (BMD) and Etest methods on RPMI-1640 agar supplemented with 2% glucose (RPG) and on Mueller-Hinton agar supplemented with 2% glucose and 0.5 μg ml(-1) methylene blue (GMB). BMD and Etest MICs were read at 24 and 48 h, and susceptibility categories were compared. All isolates were determined as susceptible with BMD, Etest-RPG and Etest-GMB at 24 h. While all isolates were interpreted as susceptible at 48 h on Etest-RPG and Etest-GMB, one C. albicans isolate was interpreted as susceptible-dose dependent (S-DD) and two C. tropicalis isolates were interpreted as resistant with BMD. On Etest-RPG, trailing growth caused widespread microcolonies within the inhibition zone and resulted in confusion in MIC determination. On Etest-GMB, because of the nearly absence of microcolonies within the zone of inhibition, MICs were evaluated more easily. We conclude that, for the determination of fluconazole MICs of trailing Candida isolates, the Etest method has an advantage over BMD and can be used along with this reference method. Moreover, GMB appears more beneficial than RPG for the fluconazole Etest.