Evaluation of a capacitance method for direct antifungal susceptibility testing of yeasts in positive blood cultures

Abstract

The feasibility of using a capacitance method (CM) for direct antifungal susceptibility testing of yeasts in positive blood cultures was evaluated. The CM used the same test conditions as those recommended by the National Committee for Clinical Laboratory Standards. After direct inoculation of positive culture broths into module wells (Bactometer; bioMérieux, Inc., Hazelwood, Mo.), the end-point determination was made by monitoring the capacitance change in the culture broths with Bactometer. The MIC of amphotericin B was the lowest concentration at which yeast growth was completely inhibited, while the MICs of ketoconazole, flucytosine, and fluconazole were the concentrations at which a >/=80% reduction in capacitance change was observed. The MICs of the four drugs against each blood isolate obtained on subculture plates were also determined by the macrodilution method. For 51 positive blood cultures tested, the percent agreement (+/-2 log(2) dilutions) between the CM and the macrodilution method were as follows: amphotericin B (98%), ketoconazole (92%), flucytosine (84%), and fluconazole (96%). The CM was further used for breakpoint susceptibility testing of fluconazole (8 and 64 microg/ml) and flucytosine (4 and 32 microg/ml) against yeasts in positive blood cultures. After testing of 74 specimens by the CM, flucytosine and fluconazole produced one (1.4%) major error and two (2.8%) minor errors, respectively. All yeasts that displayed resistance to flucytosine or fluconazole were detected within 24 h after direct inoculation of the positive broths into Bactometer. The CM may be useful for the rapid detection of antifungal resistance in positive blood cultures containing yeasts.

FIG. 1

Capacitance growth curves for C. krusei ATCC 6258 in the presence of different concentrations of fluconazole. Curve A, growth control; curves B, C, D, and E, 8, 16, 32, and 64 μg/ml, respectively; curve F, negative control. The MIC was determined to be 64 μg/ml.

FIG. 2

Capacitance growth curves for C. parapsilosis C4-13 in the presence of different concentrations of amphotericin B. Curve A, growth control; curves B, C, D, and E, 0.06, 0.12, 0.25, and 0.5 μg/ml, respectively; curve F, negative control. The MIC was determined to be 0.5 μg/ml.

FIG. 3

Capacitance growth curves for a positive blood culture (C. albicans 5364524) directly inoculated into Bactometer and grown at the two breakpoint concentrations of fluconazole. Curve A, growth control; curves B and C, 8 and 64 μg/ml, respectively; curve D, negative control. The detection time for curve C was determined to be 21 h by Bactometer.