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Published on: July 18, 2012
Disk Diffusion Susceptibility Testing for the Rapid Detection of Fluconazole Resistance in Candida Isolates
Suhak Jeon1, Jong Hee Shin1, Ha Jin Lim1
1Department of Laboratory Medicine, Chonnam National University Medical School and Chonnam National University Hospital, Gwangju, Korea.
Background:
Given the increased fluconazole resistance (FR) among Candida isolates, we assessed the suitability of disk diffusion susceptibility testing (DDT) for the early detection of FR using well-characterized Candida isolates.
Methods:
In total, 188 Candida isolates, including 66 C. albicans (seven Erg11 mutants), 69 C. glabrata (33 Pdr1 mutants), 29 C. parapsilosis (15 Erg11 mutants), and 24 C. tropicalis (eight Erg11 mutants) isolates, were tested in this study. FR was assessed using DDT according to the standard CLSI M44-ED3 method, except that two cell suspensions, McFarland 0.5 (standard inoculum) and 2.5 (large inoculum), were used, and the inhibition zones were read at 2-hour intervals from 10 hours to 24 hours.
Results:
DDT results for the standard inoculum were readable after 14 hours (C. albicans, C. glabrata, and C. tropicalis) and 20 hours (C. parapsilosis) for >95% of the isolates, whereas the results for the large inoculum were readable after 12 hours (C. glabrata and C. tropicalis), 14 hours (C. albicans), and 16 hours (C. parapsilosis) for >95% of the isolates. Compared with the results produced using the CLSI M27-ED4 broth microdilution method, the first readable results from the DDT method for each isolate exhibited an agreement of 97.0%, 98.6%, 72.4%, and 91.7% for the standard inoculum and 100%, 98.6%, 96.6%, and 95.8% for the large inoculum for C. albicans, C. glabrata, C. parapsilosis, and C. tropicalis, respectively.
Conclusions:
DDT using large inoculum may detect FR rapidly and reliably in the four most common Candida species.
Insights
Disk diffusion testing (DDT) can rapidly detect fluconazole resistance (FR) in common Candida species. Using a large inoculum with DDT offers a reliable and quick method for identifying FR, aiding early clinical decisions.
Area of Science:
- Medical Mycology
- Antimicrobial Susceptibility Testing
- Clinical Microbiology
Background:
- Increasing fluconazole resistance (FR) in Candida species necessitates rapid detection methods.
- Disk diffusion testing (DDT) is a potential tool for early identification of FR.
- This study evaluates DDT's suitability for detecting FR in well-characterized Candida isolates.
Purpose of the Study:
- To assess the efficacy of disk diffusion testing (DDT) for the rapid and reliable detection of fluconazole resistance (FR).
- To compare DDT results with standard broth microdilution methods for common Candida species.
- To determine the optimal inoculum size and reading time for DDT-based FR detection.
Main Methods:
- 188 Candida isolates (C. albicans, C. glabrata, C. parapsilosis, C. tropicalis) were tested.
- Fluconazole resistance (FR) was assessed using DDT with standard (McFarland 0.5) and large (McFarland 2.5) inoculums.
- Inhibition zones were read at 2-hour intervals from 10 to 24 hours.
Main Results:
- DDT results were readable for >95% of isolates within 12-20 hours, depending on inoculum size and species.
- The large inoculum method showed high agreement with broth microdilution: 100% for C. albicans, 98.6% for C. glabrata, 96.6% for C. parapsilosis, and 95.8% for C. tropicalis.
- The standard inoculum method also demonstrated good agreement, ranging from 72.4% to 98.6%.
Conclusions:
- Disk diffusion testing (DDT) with a large inoculum provides rapid and reliable detection of fluconazole resistance (FR).
- This method is suitable for the four most common Candida species.
- DDT offers a valuable alternative for early FR identification in clinical settings.
