Yeasts-Colorimetric Antifungal Susceptibility Testing: Introduction, Principle, Clinical Significance, and Keynotes

Introduction

Yeasts, especially Candida and Cryptococcus species, are frequent causes of opportunistic infections in immunocompromised patients. Determining their antifungal susceptibility is crucial for guiding effective therapy. While broth microdilution methods (CLSI/EUCAST) serve as reference standards, they are labor-intensive and time-consuming. Colorimetric antifungal susceptibility testing systems (such as Sensititre YeastOne and similar assays) provide a simplified, rapid, and reproducible method for testing yeasts against a wide panel of antifungal drugs.

Principle

• Inoculum Preparation: Standardized yeast suspensions are prepared from clinical isolates.
• Microdilution Format: Each well of a 96-well plate contains a dried antifungal agent at specific concentrations, along with a colorimetric growth indicator (commonly resazurin or alamarBlue).
• Inoculation & Incubation: The inoculum is added to the wells and incubated (usually at 35°C for 24–48 hours).
• Colorimetric Reaction: The growth of yeast reduces the indicator, resulting in a visible color change (e.g., blue → pink). Wells without growth remain the original color.
• MIC Determination: The lowest concentration of antifungal that prevents the color change is recorded as the minimum inhibitory concentration (MIC).

Clinical Significance

1. Rapid Results: Provides MIC values within 24–48 hours, faster and easier than manual broth microdilution.
2. Broad Antifungal Coverage: Tests include azoles, echinocandins, amphotericin B, and flucytosine.
3. High Reproducibility: Colorimetric endpoints are easier to read and reduce subjectivity compared to visual turbidity readings.
4. Resistance Detection: Identifies resistant isolates (e.g., azole-resistant Candida glabrata or echinocandin-resistant Candida auris).
5. Clinical Application: Supports therapeutic decision-making and antifungal stewardship programs.
6. Limitations: Some discrepancies with CLSI/EUCAST standards. Rare species may not have validated breakpoints.

Keynotes

1. Colorimetric antifungal susceptibility testing is a reliable, practical alternative to reference microdilution methods.
2. Based on the visible color change, making MIC determination is straightforward.
3. Plays a crucial role in monitoring antifungal resistance trends.
4. Used extensively in clinical microbiology labs for routine yeast susceptibility testing.
5. While highly practical, confirmation with reference methods may be needed for rare or resistant isolates.
6. Helps optimize patient management, antifungal therapy, and infection control.

Further Readings

• https://pmc.ncbi.nlm.nih.gov/articles/PMC7194854/
• https://academic.oup.com/ofid/article/8/11/ofab444/6367646
• https://pmc.ncbi.nlm.nih.gov/articles/PMC8579947/
• https://pubs.acs.org/doi/10.1021/acs.analchem.3c02243
• https://www.sciencedirect.com/science/article/pii/S1046202307000436
• https://www.sciencedirect.com/topics/medicine-and-dentistry/antifungal-susceptibility
• https://www.researchgate.net/publication/375864266_Development_and_Validation_of_a_Colorimetric_Antifungal_Susceptibility_Testing_Method_for_the_Dimorphic_Fungus_Talaromyces_marneffei
• https://www.elitechgroup.com/microbiology/product/fungifast-afg/
• https://pmc.ncbi.nlm.nih.gov/articles/PMC6162686/
• https://www.elitechgroup.com/microbiology/product/fungifast-afg/
• https://pmc.ncbi.nlm.nih.gov/articles/PMC6958406/
• https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1153544/epub