Comparison of the Broth Microdilution Method with the Modified MTT Method for In Vitro Susceptibility Testing of Dermatophytes.


Author address: 

Canisius-Wilhemina Hospital, Nijmegen, Netherlands.


Background: The systemic treatment of nail and skin infections has increased over recent years. Laboratory guidance of in vitro susceptibility of dermatophytes is still an unexplored area. In order to overcome the shortcomings of visual MIC determination, we evaluated the performance of an automated method with 3-(4,5-Dimethyl-2-Thiazyl)-2,5-Difenyl-2H-Tetrazolium Bromide (MTT) and compared the results with those obtained with visual reading (Fernández-Torres et al. J Clin Microbiol 2002;40:3999). Methods: The susceptibility of 20 clinical isolates belonging to the species Microsporum canis, Microsporum gypseum, Trichophyton mentagrophytes, and Trichophyton rubrum was tested against the antifungal agents amphotericine B (AMB), fluconazole (FLC), itraconazole (ITC), voriconazole (VOR), and terbinafine (TER). Conidia suspensions (104 CFU/ml) were prepared as described by Fernández and MTT was added. The 96 well microtiterplates were inoculated with the MTT/inoculum suspension (final concentration MTT: 0.1 mg/ml), and incubated by 30°C for 7 days. After incubation the optical density was measured with a microtitration plate spectrophotometer at 540 nm. Results: An agreement of: - 100% accured to all strains for AMB and FLC, to M. gypseum for TER, and T.rubrum for VOR - 80% to T. mentagrophytes for VOR and TER - 62.5% to M.canis for TER, and T.rubrum for ITC - 50% to M. canis for VOR, M. gypseum for ITC, and T.rubrum for TER - 40% to T. mentagrophytes for ITC - 12.5% to M.canis for ITC, and - 0% to M.gypseum and VOR. Conclusions: Several problems were encountered in the automated reading of the MICs such as unequal growth, clumping, and adherence thus prohibiting the generation of a clear endpoint. Visual reading appears at the moment the best approach for in vitro susceptibility testing of dermatophytes.

abstract No: 


Full conference title: 

43rd Interscience Conference on Antimicrobial Agents
    • ICAAC 43rd