Detection of fluconazole-resistant Candida strains by a disc diffusion screening test

Area of Science:

• Medical Mycology
• Antimicrobial Susceptibility Testing

Background:

• Candida species are common causes of opportunistic infections.
• Fluconazole resistance in Candida is a growing clinical concern.
• Accurate susceptibility testing is crucial for effective antifungal therapy.

Purpose of the Study:

• To evaluate a commercial disc diffusion test as a screening method for detecting Candida species with decreased susceptibility to fluconazole.
• To compare disc diffusion results with Minimum Inhibitory Concentration (MIC) results.
• To assess the reliability of the disc test in classifying Candida isolates based on fluconazole susceptibility.

Main Methods:

• A total of 1,407 Candida strains were tested using a commercial disc diffusion method.
• Results were compared against established National Committee for Clinical Laboratory Standards (NCCLS) breakpoint criteria.
• Isolates were categorized as susceptible (MIC ≤8 µg/ml), susceptible-dose dependent (S-DD; MIC 16-32 µg/ml), or resistant (MIC ≥64 µg/ml).

Main Results:

• The disc test showed high concordance for resistant and S-DD strains, with most resistant/S-DD isolates exhibiting zone diameters ≤21 mm.
• It was not possible to differentiate between resistant and S-DD strains using the disc test alone.
• A small percentage (4.1%) of susceptible strains were misclassified as resistant or S-DD by the disc test.

Conclusions:

• The fluconazole disc diffusion test is a simple and reliable screening tool for identifying Candida strains with reduced fluconazole susceptibility.
• Confirmatory MIC testing is necessary to distinguish between resistant and S-DD isolates and to identify false-resistant susceptible strains.
• This screening method aids in guiding appropriate antifungal treatment strategies.