Determination of amphotericin B antifungal susceptibility in Candida strains using an inverted microscope

Abstract

Introduction: Invasive Candida infections have recently shown a significant increase in prevalence and are associated with high mortality rates. Initiating early antifungal treatment in patients with candidemia is vital. The aim of our study was to compare the antifungal susceptibility results of a new method called Flat Plate Method modified from reference "Clinical and Laboratory Standards Institute (CLSI) microdilution method by us with Sensitititre Yeast One colorimetric method and the reference CLSI method.

Methodology: We tested 100 Candida isolates from blood cultures. We followed the CLSI M27-A3 (reference method for broth dilution antifungal susceptibility testing of yeasts; third edition) guidelines for testing in vitro susceptibility to amphotericin B. In the Flat Plate method, 96-well plates were used for evaluation with an inverted microscope. Minimum inhibitory concentration (MIC) values in the SYO method were measured following the manufacturer’s instructions. The MIC values obtained by all three methods were considered compatible if they were within ± 2 dilution limits.

Results: The SYO method detected C. albicans and C. glabrata with 100% essential agreement, whereas there was 96.29% essential agreement in the case of C. parapsilosis. In the Flat Plate method, the essential agreement with amphotericin B was 91.42%, for C. albicans isolates and 89.47%.for C. glabrata strains.

Conclusions: When determining early antifungal susceptibility using the Flat Plate method, the results are obtained quickly, with high accuracy, and without incurring additional costs. However, there is a need for comprehensive studies comparing different antifungals.