Susceptibility Testing of Fungi to Antifungal Drugs

Abstract

Susceptibility testing of fungi against antifungal drugs commonly used for therapy is a key component of the care of patients with invasive fungal infections. Antifungal susceptibility testing (AFST) has progressed in recent decades to finally become standardized and available as both Clinical and Laboratory Standards Institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) reference methods and in commercial manual/automated phenotypic methods. In clinical practice, the Sensititre YeastOne and Etest methods are widely used for AFST, particularly for sterile site isolates of Candida. Nevertheless, AFST is moving toward new phenotypic methods, such as matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS), that are capable of providing rapid, and potentially more actionable, results for the treating clinician. Our objective is to summarize updated data on phenotypic methods for AFST of Candida and Aspergillus species and to assess their significance in view of opposing, but emerging, molecular genotypic methods.

Keywords: Aspergillus; CLSI; Candida; EUCAST; MALDI-TOF MS; antifungal susceptibility.

Figure 1

Potential treatment of invasive C. glabrata infection based on mass spectrometry– antifungal susceptibility testing (MS-AFST) results. In a clinical context of prior echinocandin exposure, results of susceptibility or resistance to anidulafungin and/or fluconazole within 3 h or, in cases of isolates with FKS2 HS1 mutations, 6–12 h after testing may guide the appropriate administration of antifungal therapy [69].

Figure 2

Potential screening of azole-resistant A. fumigatus isolates. Shown is a stepwise workflow that eliminates false-positive results (indicated by a red X) and can be helpful to build up local epidemiological data on the prevalence and phylogenetic linkage of resistant isolates. In this figure, true positives are derived from the growth of isolates on selective agar plates containing drug concentrations that range from 0.5 μg/mL for posaconazole (PSZ) to 1 to 4 μg/mL for itraconazole (ITZ) or voriconazole (VRZ). Then, isolates are identified at the species level by multilocus DNA sequencing [46] (got permission to use).