S6.2b In vitro susceptibility testing of dermatophytes: toward standardization

S6.2 RESURGENCE OF DERMATOPHYTIC INFECTIONS, SEPTEMBER 22, 2022, 4:45 PM - 6:15 PM: Antifungal treatment-resistant dermatophytosis has been known for years(1). It has mainly been reported as sporadic cases with clinical failure to a specific antifungal confirmed by in vitro resistance to antifungal compounds determined by antifungal susceptibility testing (AFST). However, in vivo AFST of dermatophytes is not routinely available in most countries, and, therefore, many clinicians solve the problem by changing the antifungal treatment to another drug class hoping that it will result in clinical response. Unfortunately, cross-resistance revealing concomitantly reduced sensitivity to different classes of drugs including terbinafine and azoles have been reported(2,3). Furthermore, an increase of antifungal resistant dermatophytosis has been noted mainly in India and other Asian countries(4) but sporadic cases have also been registered in the Middle East, Europe, and North and South America suggesting that this may be the top of the iceberg(5-8). This stress the need for a standardized AFST, which can be used routinely in order to surveil the disease spread and implement targeted antifungal treatment. : Molecular-based methods are able to detect a genetic mutation known to cause antifungal resistance (e.g., mutation in the squalene epoxidase gene)(2) whereas culture-based AFST methods are able to determine the minimum inhibitory concentration (MIC) of a given drug for a specific clinical isolate. This should ideally enable to classify the isolate as sensitive, intermediate, or resistant to a specific antifungal agent, but unfortunately, it may be difficult to compare results across studies as the interpretations of MIC results are depending on the AFST method used. Following AFST methods have been used to determine the MIC of dermatophytes: E-test, agar dilution, agar disc diffusion, and macro- and microbroth dilution methods(9). They differ in several ways including inoculum concentration, incubation temperature, incubation time, different culture media, and end-point criterion of fungal growth (percentage of growth inhibition)(9,10). Standardization is therefore important, and currently, two standardized guidelines for in vitro AFST of dermatophytes exist. One is from the European Committee on Antimicrobial Susceptibility Testing (EUCAST)(11) and the other is from the Clinical Laboratory Standards Institute (CLSI)(12). Both are using microtiter plates and the EUCAST (E.Def 11.0) guideline has included MIC breakpoints for the classification of whether an isolate is susceptible or resistant to a given drug. Unfortunately, breakpoints are only available for a limited number of dermatophytes and antifungals(11). : Standardized validated AFST methods will enable us to target the antifungal treatment thereby reducing the risk of ineffective and unnecessary exposure to inappropriate antifungals with potential side effects and reducing the risk of disease spreading. SOURCES: 1. Michaelides P, Rosenthal SA, Sulzberger MB, Witten VH. Trichophyton tonsurans infection resistant to griseofulvin. A case demonstrating clinical and in vitro resistance. Arch Dermatol. 1961;83(6):988-90. 2. Martinez-Rossi NM, Peres NTA, Bitencourt TA, Martins MP, Rossi A, Mignon BR et al. State-of-the-Art Dermatophyte Infections: Epidemiology Aspects, Pathophysiology, and Resistance Mechanisms. 2021; Available from: https://doi.org/10.3390/jof7080629. 3. Singh A, Masih A, Khurana A, Singh PK, Gupta M, Hagen F et al. High terbinafine resistance in Trichophyton interdigitale isolates in Delhi, India harbouring mutations in the squalene epoxidase gene. Mycoses. 2018 Jul;61(7):477-84. 4. Tang C, Kong X, Ahmed SA, Thakur R, Chowdhary A, Nenoff P et al. Taxonomy of the Trichophyton mentagrophytes/T. interdigitale Species Complex Harboring the Highly Virulent, Multiresistant Genotype T. indotineae. Mycopathologia. 2021 Jun 1;186(3):315-26. 5. Saunte D, Pereiro-Ferreirós M, Rodríguez-Cerdeira C, Sergeev A, Arabatzis M, et al. Emerging antifungal treatment failure of dermatophytosis in Europe: take care or it may become endemiC. J Eur Acad Dermatol Venereol. 2021 Jul 1;35(7):1582-6. 6. Süß A, Uhrlaß S, Ludes A, Verma SB, Monod M, Krüger C et al. [Extensive tinea corporis due to a terbinafine-resistant Trichophyton mentagrophytes isolate of the Indian genotype in a young infant from Bahrain in Germany]. Hautarzt. 2019 May 16. 7. Lana AJD, Pippi B, Carvalho AR, Moraes RC, Kaiser S, Ortega GG et al. In Vitro additive effect on griseofulvin and terbinafine combinations against multidrug-resistant dermatophytes. Brazilian J Pharm Sci. 2018 Jan 1;54(2). 8. Chen E, Ghannoum M, Elewski BE. Treatment-resistant tinea corporis, a potential public health issue. Br J Dermatol. 2021 Jan 1;184(1):164-5. 9. Dogra S, Shaw D, Rudramurthy S. Antifungal Drug Susceptibility Testing of Dermatophytes: Laboratory Findings to Clinical Implications. Indian Dermatol Online J. 2019;10(3):225. 10. Curatolo R, Juricevic N, Leong C, Bosshard PP. Antifungal susceptibility testing of dermatophytes: Development and evaluation of an optimised broth microdilution method. Mycoses. 2021 Mar 1;64(3):282-91. 11. Arendrup MC, Kahlmeter G, Guinea J, Meletiadis J, Arendrup MC, Meletiadis J, et al. How to: perform antifungal susceptibility testing of microconidia-forming dermatophytes following the new reference EUCAST method E.Def 11.0, exemplified by Trichophyton. Clin Microbiol Infect. 2021 Jan 1;27(1):55–60. 12. Wayne P. CLSI. Reference Method for Broth Dilution Antifungal Susceptibility testing of Filamentous Fungi. 3rd Ed. CLSI standard M38 [Internet]. 2017. Available from: www.clsi.org.