Comparative Evaluation of Micronaut-AM and CLSI broth micro-dilution method for antifungal susceptibility testing of Aspergillus species against four commonly used antifungals

Abstract

The aim of this study was to evaluate a colorimetric method, Micronaut-AM, for determining susceptibility testing of anidulafungin, amphotericin, voriconazole and itraconazole by comparing the Minimum Inhibitory (Effective) Concentrations (MICs/MECs) obtained by this method to those generated by the reference Clinical Laboratory Standard Investigation (CLSI) method. 78 clinical isolates of Aspergillus species, nine of them azole-resistant, were tested against above antifungals. A fumigatus ATCC 204305 was used as a reference strain and test was performed in accordance with slightly modified yeast susceptibility testing instruction of the manufacture; conidia suspension inoculum and alamarBlue concentration were optimised. These same isolates were referred to Bristol Mycology reference laboratory and tested by CLSI method. The MICs and MECs generated by the two methods were compared using concordance analysis.\n\nMicronaut-AM (MN) showed significant concordance (P< 0.0001) with CLSI method and overall agreement was high ([≥] 90%). In addition, Micronaut-AM produced echinocandin MECs results within 18-24h incubation time and reliably detected azole resistant isolates. Essential agreement (within 2 log2 dilution of median) between MN and CLSI reference laboratory method was 99 % for anidulafungin, 100 % for amphotericin; 90% for voriconazole and 87 % for itraconazole. Categorical agreement for anidulafungin, amphotericin B, voriconazole and itraconazole were 100%, 96%, 97% and 99% respectively.\n\nMicronaut-AM showed very good agreement with the reference broth micro-dilution method results for all antifungal agents tested and was able to detect azole resistance. This colorimetric method is very promising and appears to be a suitable alternative susceptibility testing method to labour intensive broth microdilution method for Aspergillus species.