Development and multicentre validation of an agar-based screening method for echinocandin susceptibility testing of...

Reference antifungal susceptibility testing of echinocandins against Aspergillus spp. relies on the determination of the minimal effective concentration, which is difficult to perform, time-consuming and subjective. We developed and evaluated in a multicentre study an agar-based screening method for echinocandin susceptibility testing of Aspergillus spp.

Forty WT isolates [10 Aspergillus fumigatus species complex (SC), 10 Aspergillus flavus SC, 10 Aspergillus terreus SC and 10 Aspergillus niger SC] and 4 non-WT A. fumigatus isolates with or without known fks alterations were used. The optimal test conditions and stability over time were evaluated in preliminary studies monitoring colony growth. Twenty-microlitre aliquots of 1-2 McFarland inocula in 0.1% Tween 20 aqueous solution were added to each well and plates were incubated for 24/48 h at 35 ± 2°C. Subsequently, all isolates were tested blindly at three centres using four-well screening plates, containing anidulafungin, caspofungin, micafungin or no antifungal in each of the four wells, respectively.

WT isolates produced fluffy colonies on drug-free agar wells only. The non-WT isolates produced fluffy colonies on echinocandin-containing and control agar wells. Using the echinocandin concentrations of 0.25 mg/L anidulafungin, 1 mg/L caspofungin and 0.125 mg/L micafungin, and the compact (non-fluffy) versus fluffy colony morphology endpoint, all centres successfully discriminated non-WT and WT strains even after 24 h. Among the three echinocandins, anidulafungin produced the clearest endpoints.

The four-well plate agar method is suitable for echinocandin susceptibility screening of Aspergillus spp. and can be used to detect echinocandin non-WT isolates.