English

Abstract

The existing standardized micro-broth dilution methods for in vitro testing of moulds (CLSI, EUCAST) are referred to as reference methods and therefore not intended for routine testing. They are time-consuming and dependent on sporulating hyphomycetes. In this study a new, time saving and easy-to-perform method for inoculum preparation for routine susceptibility testing has been applied. It is independent of spore production and proofed to produce comparable results to the conidia based methods, indicating that it can be used for all types of hyphae-and/or conidia-forming fungi. The MICs of amphotericin B, flucytosine, fluconazole, posaconazole, voriconazole, anidulafungin, caspofungin and micafungin of 198 moulds were determined with two different culture media (YST and RPMI 1640) according to the DIN microdilution assay, and compared to appropri- ate international studies. The inocula with YST (DIN) and RPMI 1640 (EUCAST) medium showed similar MIC distributions for all moulds tested to the conidia method, with more than 92% of the MICs read at 24 h and 48 h within ± 1log2-dilution. Although azoles, flucyto- sine and amphotericin B showed comparable results, differences in echinocandin endpoints between different multicentre studies were de- termined. According to the literature, the minimum effective concentration (MEC) should be equivalent to the minimum inhibitory concentra- tion (MIC). However, due to the encountered bias in echinocandin-endpoint determinations this has to be questioned. Evaluation of cross- resistance demonstrated that no individual strain out of 198 was in parallel susceptible to all eight antifungal agents tested. Cross-resistance between azoles, echinocandins, amphotericin B, and flucytosine could be detected quantitatively with a new method for fungi. It ranges from two to sevenfold, and demonstrates quantitatively different and species-specific susceptibility/resistance patterns.