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Comparative Study
. 2001 Jul;39(7):2708-12.
doi: 10.1128/JCM.39.7.2708-2712.2001.

Novel fluorescent broth microdilution method for fluconazole susceptibility testing of Candida albicans

R S Liao  1 , R P Rennie, J A Talbot
Affiliations
Comparative Study

Novel fluorescent broth microdilution method for fluconazole susceptibility testing of Candida albicans

R S Liao et al. J Clin Microbiol. 2001 Jul.

Abstract

A comparative evaluation of the reference National Committee for Clinical Laboratory Standards (NCCLS) broth microdilution method with a novel fluorescent carboxyfluorescein diacetate (CFDA)-modified microdilution method for the susceptibility testing of fluconazole was conducted with 68 Candida strains, including 53 Candida albicans, 5 Candida tropicalis, 5 Candida glabrata, and 5 Candida parapsilosis strains. We found trailing endpoints and discordant fluconazole MICs of < 8 microg/ml at 24 h and of > or =64 microg/ml at 48 h for 12 of the C. albicans strains. These strains satisfy the definition of the low-high MIC phenotype. All 12 low-high phenotype strains were correctly shown to be susceptible at 48 h with the CFDA-modified microdilution method. For the 41 non-low-high phenotype C. albicans strains, the CFDA-modified microdilution method yielded 97.6% (40 of 41 strains) agreement within +/-1 dilution at 24 h compared with the reference method and 92.7% (38 of 41 strains) agreement within +/-1 dilution at 48 h compared with the reference method. The five strains each from C. tropicalis, C. glabrata, and C. parapsilosis that were tested showed 100% agreement within +/-2 dilutions for the two methods being evaluated.

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Figures

FIG. 1
FIG. 1
Range of fluoconazole MICs for 53 isolates of C. albicans at 24 and 48 h determined with the reference M27-A broth microdilution method. The fluconazole breakpoints for Candida species are as follows: susceptible, ≤8 μg/ml; susceptible-dose dependent, 16 to 32 μg/ml; and resistant, ≥64 μg/ml (12).
FIG. 2
FIG. 2
Effect of fluconazole on representative strains of C. albicans, which are low-high phenotype (707-15), susceptible (Y91), susceptible-dose dependent (Y98), and resistant (965). Each isolate was tested for susceptibility with the M27-A microdilution method (MICM27-A) and the CFDA-modified microdilution method (MICCFDA) at both 24 and 48 h. Results for the CFDA-modified method are shown graphically as relative fluorescence units. The fluorescence of the growth in the drug-free control was defined as 100%, and the fluconazole-exposed wells were scaled to this value. Error bars indicate standard error. The results of the reference M27-A method are shown below as a digital image of the unagitated growth in the 96-well plate at 24 and 48 h.
See this image and copyright information in PMC

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