Objectives:

Development of molecular laboratory tools has led to a complete revision of Arthrodermataceae classification and a consequent reduction of species number within this family. However, the phylogenetic place of Trichophyton soudanense remains uncertain inside the Trichophyton rubrum complex. Trichophyton soudanense is synonym either to T rubrum or T. violaceum according to the authors, while, more recently, De Hoog et al. gave it the status of « borderline species »[1]. T. soudanense remains one of the major etiological agents of tinea capitis in our hospital[2] and its epidemiological, clinical and phenotypic features led us to consider T. soudanense, T. rubrum and T. violaceum as separate entities. All studies that have been interested in this issue have only used collection strains.

The main aim of our study was to determine the phylogenetic place of T. soudanense using clinical samples obtained in dermatology consultation at Saint Louis Hospital, Paris, France. Furthermore, this study aimed at evaluating the quality of identifications by phenotypic methods in our laboratory and to determine the discriminating power of different loci used in molecular identification of dermatophytes.

[1] De Hoog et al., « Toward a Novel Multilocus Phylogenetic Taxonomy for the Dermatophytes », 2017

[2] Gits-Muselli et al., « Continuous Increase of Trichophyton Tonsurans as a Cause of Tinea Capitis in the Urban Area of Paris, France: A 5-Year-Long Study », 2017



Methods:

We sequenced all the isolates obtained between April and September 2016 using four loci: ITS1-ITS4, β2-tubulin gene, α1 elongation factor gene and chitin synthase gene. Molecular identification of isolates was obtained by comparing ITS sequences to the Mycobank® database. The sequences of the 4 loci were concatenated and a phylogenetic analysis was performed. In parallel, we photographed each culture on Sabouraud agar medium. The colors of the photographs were extracted and compared according to the RGB color model.



Results:

We analysed 122 clinical isolates phenotypically identified as T. rubrum (n=48), Trichophyton mentagrophytes/interdigitale (n=34), T. soudanense (n=10), Trichophyton tonsurans (n=22), Trichophyton sp. (n=1), Microsporum audouinii (n=6), and Microsporum canis (n=1). The concordance of phenotypic and molecular identifications was 94% with no error regarding tinea capitis cases. One isolate was identified as Chrysosporium queenslandicum which has not been yet described in human pathology. The color coding of the isolates objectified the pleomorphism of the dermatophytes (Figure 1). The phylogenetic organization of the genus Trichophyton was identical to that obtained by De Hoog et al. and isolates of T. soudanense appear to be separated from those of T. rubrum. The isolates identified as T. soudanense took an intermediate place between T. rubrum and T. violaceum when comparison of ITS sequences alone (Figure 2) but appear closer to T. rubrum with the other loci. The majority of isolates identified as T. interdigitale or T. mentagrophytes present identical ITS sequences, probably corresponding to the species T. interdigitale.



Conclusion:

The sequence differences between T. rubrum and T. soudanense remain tenuous whereas they are easily differentiated on a clinical presentation. The comparison of their transcriptome could help to understand the origin of their differences.

8 / Place of Trichophyton soudanense in the Trichophyton rubrum complex : a clinical isolates analysis