Abstract :
[en] Objectives: The new emerging dermatophyte Trichophyton indotineae closely related to T. interdigitale/mentagrophytes has become a major concern in dermatology because of its high resistance to terbinafine. This species is at epidemic level in India giving rise to extended tinea corporis. Its spread in Europe has been already described. Mutations on the gene coding for squalene epoxidase (SQLE) have abundantly been observed among terbinafine resistant strains. Regarding this context, the National Reference Center for mycoses of the CHU of Liege (NRCML) conducted a study at the Belgian level to collect all T. interdigitale/mentagrophytes/indotineae isolated from skin and scalp. The aim of the study was to evaluate the situation in Belgium and characterize these strains in terms of identification and terbinafine susceptibility. A phylogenomic study based on whole genome sequencing (WGS) data was also performed with a particular focus on SQLE gene.
Methods: The study was conducted from 1 April 2022 and 1 April 2023. In total, 129 strains have been sent to the NRCML by 18 laboratories. The terbinafine susceptibility of these strains was evaluated by a four wells dilution on agar screening method containing 0, 0,05, 0,1 and 0,2 µg/ml of terbinafine. Minimal inhibitory concentration (MIC) of each strain was then determined by EUCAST E. Def.11.0 microdilution method. WGS was performed by Illumina sequencing (GIGA Genomics, Liège, NovaSeq S4 V1.5 300 cycles XP workflow). Assembly of the genome was done using SPAdes integrated in a custom made bioinformatic pipeline “WGS typer” (Hedera22, Liège). Similarity dendrogram was generated by WGS typer using the maximum likelihood method. Reference genomes of T. indotineae/mentagrophytes/interdigitale were included on the tree.
Results: Among the 129 strains, 4 (3,1%) showed a growth in well containing 0,2µg/ml of terbinafine with the dilution on agar method. MICs higher than 0.25µg/ml for terbinafine were confirmed by EUCAST microdilution method for these strains. The other 125 strains showed MICs <0,1µg/ml for terbinafine. At this day, only 85/129 strains have been characterized by WGS. Preliminary results show that among these strains 5 (5,8%) were T. indotineae, 20 (24,3%) were T. mentagrophytes and 60 (73,5%) were T. interdigitale. These three species were separated into three distinct clades on the dendrogram. The analysis of SQLE region permitted to identify the substitutions F397L (2), L393F (1) and L393S (1) among the five T. indotineae, one being a wild type. Our genomic study also highlighted that T. interdigitale even if mainly causing tinea pedis (47/60), was also isolated from tinea corporis (13/60). On the contrary, only 4/20 T. mentagrophytes were isolated from tinea pedis, the majority (16/20) being isolated from tinea corporis. T. indotineae was exclusively isolated from extended tinea corporis/cruris.
Conclusion: Our study demonstrates the presence of T. indotineae resistant to terbinafine on the Belgian territory. The phylogenomic analysis permitted to precisely identify T. indotineae among a distinct clade from T. interdigitale/mentagrophytes and to show that T. interdigitale is not only isolated from foot skin. The substitutions on SQLE gene F397L, L393F and L393S have been described in Belgian T. indotineae strains.
