In Vitro Susceptibility Testing of Aspergillus fumigatus against Posaconazole: Comparison of NCCLS M38-P and E-Test Methods

Posaconazole is a second-generation triazole and structural analogue of itraconazole. This drug has fungicidal activity against yeasts and filamentous fungi. The aim of our study was to evaluate E-test method for in vitro susceptibility testing of Aspergillus fumigatus isolates against posaconazole. METHODS: A total of 121 isolates of A. fumigatus were selected as follows: 106 clinical strains from colonized patients, 18 from patients with invasive aspergillosis and 7 environmental isolates. Their in vitro susceptibility was evaluated by E-test (Abbiodisk, Sweden) and compared with NCCLS microdilution reference method (M38-P). Both tests were performed with RPMI 1640 medium at 35 degrees C. MIC values were read after 24h (MIC-24h) and 48h (MIC-48h) incubation time by E-test method. Two MIC endpoints were determined by NCCLS method: 1.no visible reduction of growth (MIC-0); 2. 50% reduction (or more) of growth (MIC-2). Three A. fumigatus reference strains (IHEM 5734, 6149 and 13935) were included as control. RESULTS: Geometric mean MICs (microg/ml) were respectively 0.02 for E-test at 24h and 0.029 at 48h. MIC-0 and MIC-2 values were respectively 0.19 and 0.018 microg/ml. One correlation between both methods was observed for MICs-24h and MICs-2s (p<.05). However, there was no significant difference according to origin of isolates (p<.05). CONCLUSIONS: 1. This study assessed the potent role of posaconazole against A. fumigatus isolates with very low MICs. 2. MIC values were not predictive of pathogenicity. 3. E-test method by reading after 24h-incubation time could easily replace the time-consuming NCCLS M38-P reference method.