Dermatophytes growth curve andin vitrosusceptibility test: a broth micro-titration method

Abstract

The introduction of systemic antifungal drugs which act upon different targets is the main issue of the in vivo antifungal resistance control. Different factors, such as growth curve phase, quality of the specimen, quantity of the inoculum, temperature, pH, culture medium composition, incubation duration and solvent, are believed important factors affecting minimum inhibitory concentration (MIC) value to most of the antifungal agents. We assayed an in vitro susceptibility test with 40 isolates of dermatophytes: Microsporum canis, Trichophyton rubrum, Trichophyton mentagrophytes and Epidermophyton floccosum against griseofulvin, fluconazole, itraconazole and terbinafine, using the guidelines of the M38-P document approved by the NCCLS. We determined the growth curves, to estimate the specific growth rate (mu max) and the generation time (G) of each dermatophyte, using dry weight and spectrophotometry methods. We demonstrate that, at 192 h, all fungi tested had a constant growth curve and we considered this as the optimal time for MIC determination. Terbinafine, griseofulvin and itraconazole possessed the highest antifungal activity against the four groups of dermatophytes studied. Fluconazole demonstrated no efficacy. Our MIC results differ from other authors and this difference is due to the timing of the MIC determination based on the growth curve of each fungi tested.

Sertaconazole Antifungal Profile Determined by a Microdilution Method versus Nine Topical Substances against Dermatophyte Fungi

“…Previously published data show the validity of MIC obtained at 3–5 days of incubation [12,13]. The data that we obtained with the microdilution method with a spectrophotometric measurement to determine the MICs made it possible to comparatively measure the activity of the antifungals at the time of the highest inoculum growth index, which was completely different from what was proposed by Alió et al [19]. Using the macromethod, these authors obtained growth indexes with inoculum sizes greater than for 8 days of incubation and using an incubation temperature suboptimal for dermatophyte development.…”

“…Using the macromethod, these authors obtained growth indexes with inoculum sizes greater than for 8 days of incubation and using an incubation temperature suboptimal for dermatophyte development. For a 3-day incubation time, some studies describe the best antifungal activity of substances as the most demonstrational, whereas in other cases this is assigned to 8 days [14,19]. The differences may be attributed to the use of macromethods for which the best growth rates are obtained with longer incubation times, and also to the optical density wavelength, which in our case was 460 nm instead of 570 nm [17].…”

Preanalytical conditions for broth microdilution antifungal susceptibility of Microsporum spp.

“…Some previous studies tried to establish a method for dermathophyte antifungal susceptibility testing 8–10,12 . All of them generalized the three genera of dermatophytes.…”

“…[1] agreed with this conclusion. In contrast, some other authors have defined different endpoints ranging from 50% to 90% growth inhibition 5,7,10,12 . This variability of interpretative endpoints is directly related to the different size and type of inoculum used.…”

“…Although tinea capitis and tinea unguium caused by Microsporum canis and M. gypseum are the most difficult‐to‐treat dermatophytoses, there are few specific studies about their antifungal susceptibility in vitro. 1,5–10 …”