New methods to assess susceptibilities of Aspergillus isolates to caspofungin

Echinocandins are a group of antifungal agents that target 1,3-beta-glucan synthase, causing disruption of mold growth at cells compromising tips and branch points. In part because echinocandins do not induce clear growth inhibition end points using broth dilution techniques, methods to test susceptibility have not yet been standardized. We developed a novel susceptibility assay that measures growth of Aspergillus species on solid agar media that contain serial dilutions of caspofungin (agar dilution). Results of agar dilution testing of multiple isolates were compared to results obtained by broth microdilution (MIC), microscopic evaluation (minimal effective concentration [MEC]), and a new method to measure fungal burden, quantification of secreted hyphal antigen. MICs obtained by the agar dilution method were within 1 dilution of MECs for 85% of the Aspergillus isolates; the highest agreement was observed for isolates of Aspergillus niger (95%), which were particularly susceptible to caspofungin. Agar dilution MICs were also consistent with those obtained by quantifying antigen secretion. MICs obtained by broth microdilution were different than MICs by any other method. Several Aspergillus isolates with decreased susceptibility to caspofungin were identified. Agar dilution is simple and reproducible, and results were consistent with the results of more technically demanding techniques. This method may be appropriate for use in the clinical laboratory.