Investigation of different methods for susceptibility testing of Candida strains to fluconazole

Kustimur, S. , A. Kalkanci, H. Mansuroglu, K. Senel (Ankara, TR)


Over the past decade, significant progress has been made with standardization of the testing method in the field of antifungal susceptibility testing. We compared the results of standard macrotube dilution reference method; two different microdilution methods and disc diffusion method for testing the susceptibility of 150 Candida strains including two reference strains to fluconazole. Candida parapsilosis ATCC 22019 (Kukens, Istanbul) and Candida krusei ATCC 6258 (Groupe Hospitalier Pitie Salpetriere, Unite de Mycologie, Paris) were used as control strains. 148 clinical isolates were 50 C.albicans, 31 C.krusei, 19 C.parapsilosis, 16 C.kefyr, 15 C.tropicalis, 10 C.glabrata, 3 C.rugosa, 2 C.lusitaniae, 1 C.famata, 1 C.dubliniensis. Macrodilution method was performed as described in NCCLS document M27-A with RPMI 1640. In the procedure of microdilution method, Yeast Nitrogen Base Broth (YNBB) and RPMI 1640 mediums were used and the Minimal Inhibitory Concentration (MIC) results were compared. Disc diffusion testing was performed on the plates of Yeast Nitrogen Base Agar (YNBA) and Sabouraud Dextrose Agar (SDA) by using 25mg(microgram) and 50mg fluconazole containing discs. All statistical analysis were calculated by the help of the computer program, SPSS 9:01. Inhibition zone diameters and MICs were determined after 24h incubation. 81% of the 150 strains were susceptible to fluconazole with macrodilution method. 85% of the 150 strains were susceptible with microdilution using RPMI 1640 medium. 79% of them were susceptible with microdilution using YNBB. When we compared these three methods in terms of their MIC results, overall correlation was 86% between them (p>0.05). All isolates grew but the quality of growth after 24h incubation was variable on the two different solid media for the disc diffusion procedure. Majority of the isolates showed no inhibition zone on the SDA. Fluconazole inhibition zone diameters were measured easily on the YNBA. 21 of the 150 isolates (14%) were found to be resistant to fluconazole with disc diffusion on YNBA. There was no significant difference between 25mg and 50mg fluconazole containing discs. Due to the fact that there was no difference between the susceptibility results of Candida strains with disc diffusion and macrodilution tests, we tend to believe that; disc diffusion test was low-cost, reproducible as a screening procedure for the determination of susceptibility.

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11th European Congress of Clinical Microbiology and Infectious Diseases
    • ECCMID 11th (2001)