Antifungal Susceptibility of Yeasts and Filamentous Fungi by CLSI Broth Microdilution Testing

Antifungal susceptibility testing is performed against yeasts and filamentous fungi (molds) for various purposes. In clinical settings, the results of these in vitro assays may be used to help guide therapy for individual patients. Surveillance studies use susceptibility testing to monitor for the development of resistance to different agents. These assays are also frequently used in preclinical drug development to determine the spectrum of activity and assess the in vitro potency of investigational agents. Broth microdilution is a form of antifungal susceptibility testing that is frequently used for each of these purposes. The Clinical and Laboratory Standards Institute (CLSI) has developed standardized methods for broth microdilution antifungal susceptibility testing of both yeasts and molds. These methods are useful in the clinical diagnostics arena for determining if a particular fungal isolate may be resistant to clinically available antifungals, and they are also amendable for research purposes, as the microtiter trays can be prepared in different formats with different antifungal and investigational agent concentration ranges and with different growth media if necessary. This chapter describes the CLSI methods for antifungal susceptibility testing by broth microdilution for both yeasts and molds, how endpoints are read for different classes of antifungal agents, and how the results may be interpreted.