Variability of agar dilution-replicator method of yeast susceptibility testing

Area of Science:

• Mycology
• Antimicrobial Susceptibility Testing

Background:

• In vitro antifungal susceptibility testing is crucial for guiding treatment decisions.
• Standardization of methods is essential for reproducible results.

Purpose of the Study:

• To analyze the variability of the agar dilution method for antifungal susceptibility testing.
• To evaluate the impact of inoculum size and reading time on minimal inhibitory concentrations (MICs).

Main Methods:

• Utilized a Steers-type inoculum replicator for consistent fungal strain delivery.
• Tested ten fungal strains against three antifungal drugs: ketoconazole, 5-fluorocytosine, and nystatin.
• Compared agar dilution results with broth dilution methods.

Main Results:

• Ketoconazole and 5-fluorocytosine MICs were inoculum-dependent, unlike nystatin.
• MICs for ketoconazole and 5-fluorocytosine varied significantly with reading time.
• Discrepancies were observed between agar and broth dilution methods.
• Variations in reading time, especially with fast-growing strains, led to large MIC fluctuations.

Conclusions:

• The agar dilution method exhibits significant variability, particularly for ketoconazole and 5-fluorocytosine.
• Inoculum size and reading time are critical factors influencing MICs.
• Methodological variations can lead to inter-laboratory and inter-observer discrepancies in antifungal susceptibility testing.