Recent developments in antifungal susceptibility testing

Determination of antifungal MICs by a rapid susceptibility assay

A novel microtiter assay for antifungal susceptibility testing was developed. This method has several potential advantages over the M27-A assay of the National Committee for Clinical Laboratory Standards. These include provision of MIC results within 6 to 19 h, graphical display of data, and the availability of objective quantitative endpoints. We refer to the method as the rapid susceptibility assay (RSA). RSA is based on substrate utilization by fungi in the presence of antifungal drugs. Substrate uptake is determined by a colorimetric method, which can be scored by analysis of data obtained from a microplate reader. Variables evaluated in the development of the RSA included inoculum size, incubation period, and efficacy with different classes of antifungal drugs and different yeast isolates. With the rapidly available and quantitative endpoints of the RSA, correlation of MICs and therapeutic drug doses can be evaluated more successfully than they can be evaluated by existing assays.

[Susceptibility testing of yeasts against fluconazole: proposal for a standardized agar diffusion method with 25 microgram fluconazole paper discs]

This paper gives a proposal for a standardised agar diffusion susceptibility testing method with 25 micrograms fluconazole discs. The methodology compiles the results of several years of work to develop a reliable and reproducible routine-method for the microbiology laboratory. In this proposal, in addition, the critics and experiences of a collaborative study for susceptibility testing of fluconazole with 21 laboratories from Germany and Austria are included.

An evaluation of seven methods of testing in vitro susceptibility of clinical yeast isolates to fluconazole

Four commercially available in vitro test systems (Candifast, E-test, Mycototal and spiral-gradient end point method), agar diffusion with 25-micrograms fluconazole paper test discs and 15-micrograms test tablets and agar dilution were compared with the microbroth dilution method for fluconazole susceptibility testing of 145 clinical isolates. In addition, the culture media provided or recommended by the manufacturers of the test systems were compared with high-resolution (HR) antifungal test medium. With all currently available culture media, growth problems (inhibition or delayed growth of the clinical isolates) occurred with solid or semisolid media. With minor improvements, HR medium demonstrated the most reproducible and comparable results (supplementation with asparagine and deletion of sodium hydrogen carbonate). The best correlation with microdilution was obtained by the agar dilution method (> 95% concordance) followed by the spiral-gradient end point method (85%), Candifast (83%), Mycototal (81%) and the E-test (78%). Regression analysis demonstrated good correlation between agar diffusion and micro-/agar dilution (r > 0.9).

[Comparison of seven methods of in vitro susceptibility testing of clinical yeast isolates against fluconazole]

Four commercially available in vitro test systems (Candifast, E-test, Mycototal, Spiral-Gradient Endpoint Method), agardiffusion with 25 micrograms fluconazole paper test discs and 15 micrograms test tablets, and agardilution were compared to the microbroth dilution method by fluconazole susceptibility testing of 145 clinical isolates. In addition, the culture media provided or recommended by the manufacturers of the test systems were compared to the high resolution (HR) antifungal test medium. With all currently available culture media growth problems (inhibition or delayed growth of the clinical isolates) occurred with solid or semi-solid media. With minor improvements, HR medium demonstrated the most reproducible and comparable results (supplementation with asparagine and deletion of sodium hydrogen carbonate). Best correlation to microdilution was obtained by the agardilution method > (95% concordance) followed by the spiral gradient endpoint method (85%), Candifast (83%), Mycototal (81%) and the E-test (78%). Regression analysis demonstrated good correlation between agardiffusion and micro-/agardilution(r > 0.9).

Comparative efficacy of amphotericin B, clotrimazole and itraconazole against Aspergillus spp. An in vitro study

The susceptibilities of two isolates of Aspergillus flavus, one from a human case of recalcitrant mycotic keratitis, and an environmental isolate of A. fumigatus, to itraconazole, clotrimazole and amphotericin B were measured. Observations of macroscopic growth and microscopic evaluations of conidia germination both indicated that the two isolates of A. flavus were markedly more resistant to amphotericin B than to itraconazole and clotrimazole. Itraconazole was more effective than clotrimazole for all isolates. Our in vitro susceptibility results suggest the use of itraconazole should be a primary consideration in the treatment of Aspergillus keratitis.