Breakthrough Aspergillus fumigatusand Candida albicansDouble Infection during Caspofungin Treatment: Laboratory Characteristics and Implication for Susceptibility Testing

ABSTRACTCaspofungin is used for the treatment of acute invasive candidiasis and as salvage treatment for invasive aspergillosis. We report characteristics of isolates of Candida albicansand Aspergillus fumigatusdetected in a patient with breakthrough infection complicating severe gastrointestinal surgery and evaluate the capability of susceptibility methods to identify candin resistance. The susceptibility of C. albicansto caspofungin and anidulafungin was investigated by Etest, microdilution (European Committee on Antibiotic Susceptibility Testing [EUCAST] and CLSI), disk diffusion, agar dilution, and FKS1sequencing and in a mouse model. Tissue was examined by immunohistochemistry, PCR, and sequencing for the presence of A. fumigatusand resistance mutations. The MICs for the C. albicansisolate were as follows: >32 μg/ml caspofungin and 0.5 μg/ml anidulafungin by Etest, 2 μg/ml caspofungin and 0.125 μg/ml anidulafungin by EUCAST methods, and 1 μg/ml caspofungin and 0.5 μg/ml anidulafungin by CLSI methods. Sequencing of the FKS1gene revealed a mutation leading to an S645P substitution. Caspofungin and anidulafungin failed to reduce kidney CFU counts in animals inoculated with this isolate (P> 0.05 compared to untreated control animals), while both candins completely sterilized the kidneys in animals infected with a control isolate. Disk diffusion and agar dilution methods clearly separated the two isolates. Immunohistochemistry and sequencing confirmed the presence of A. fumigatuswithout FSK1resistance mutations in liver and lung tissues. Breakthrough disseminated aspergillosis and candidiasis developed despite an absence of characteristic FKS1resistance mutations in the Aspergillusisolates. EUCAST and CLSI methodology did not separate the candin-resistant clinical isolate from the sensitive control isolate as well as did the Etest and agar methods.