Use of a colorimetric system for yeast susceptibility testing

Area of Science:

• Clinical Microbiology
• Antifungal Drug Discovery
• Diagnostic Assays

Background:

• Accurate antifungal susceptibility testing is crucial for guiding treatment of Candida albicans infections.
• Established methods like broth macrodilution require optimization for improved efficiency and endpoint clarity.
• Colorimetric assays present a potential alternative for simplifying susceptibility testing.

Purpose of the Study:

• To evaluate the reliability and accuracy of the Alamar Blue colorimetric assay for Candida albicans antifungal susceptibility testing.
• To compare Alamar Blue-modified broth macrodilution and microdilution methods against the National Committee for Clinical Laboratory Standards (NCCLS) reference method.
• To assess the performance of Alamar Blue with fluconazole and itraconazole.

Main Methods:

• Compared NCCLS broth macrodilution with Alamar Blue-modified broth macrodilution and microdilution methods.
• Tested 97 Candida albicans isolates and 3 control strains against fluconazole and itraconazole.
• Determined Minimum Inhibitory Concentrations (MICs) and calculated concordance rates.

Main Results:

• Alamar Blue-modified broth macrodilution showed 94% concordance for fluconazole and 97% for itraconazole compared to NCCLS.
• Alamar Blue-modified microdilution showed 95% concordance for fluconazole but lower (77% within 2 dilutions) for itraconazole.
• All methods demonstrated excellent reproducibility with quality control strains, and Alamar Blue provided clear endpoints.

Conclusions:

• The Alamar Blue assay is a reliable and accurate method for Candida albicans antifungal susceptibility testing.
• Modified broth macrodilution with Alamar Blue demonstrates high concordance with NCCLS standards for both fluconazole and itraconazole.
• While reproducible, the Alamar Blue-modified microdilution method yielded higher MICs for itraconazole compared to the NCCLS method.