Prevalence of T. rubrum and T. interdigitale Exhibiting High MICs to Terbinafine in Clinical Samples Analyzed in the Portuguese Mycology Reference Laboratory

Abstract

Cutaneous fungal infections represent a significant burden worldwide with a high impact on public health. Accurate identification of dermatophyte species causing these infections is vital for an appropriate treatment. Terbinafine is the primary agent against Trichophyton species due to its clinical efficacy; however, cases of elevated minimum inhibitory concentration (MIC) have been reported, raising clinical and epidemiological concerns. Herein, we aimed to detect Trichophyton rubrum and Trichophyton interdigitale isolates collected from clinical samples with terbinafine-high MICs (TRB-hMIC). A total of 168 isolates, recovered from 2017 to 2023, were identified as T. rubrum complex (140/83.4%) or T. interdigitale (28/16.7%) and further screened regarding their terbinafine susceptibility. Four isolates with capacity to grow in terbinafine media were detected by screening, and these and a further sixteen random isolates were submitted to the broth microdilution method. This methodology confirmed the four (2.4%) isolates as TRB-hMIC. One T. rubrum and three T. interdigitale showed a minimum inhibitory concentration (MIC) higher than 1 mg/L. Partial sequencing of the SQLE gene identified point mutations in T. rubrum (Phe397Iso) and in one T. interdigitale (Phe397Leu) isolate. Notably, in the other two T. interdigitale isolates with TRB-hMIC, no point mutations in the SQLE gene were identified. In conclusion, TRB-hMIC isolates (T. rubrum and T. interdigitale) were identified in clinical samples analyzed in Portugal, as antifungal susceptibility testing is a crucial routine for identifying treatment failures and also for epidemiological purposes aiming to monitor the dynamics of terbinafine resistance.

Keywords: Trichophyton; antifungal resistance; dermatophytes; squalene epoxidase mutations; terbinafine.

Figure 1

Growth of the T. rubrum isolate #126 (MIC = 8 mg/L), resistant to terbinafine in the terbinafine screening agar. (A) = growth control; (B) = SDA with terbinafine 0.06 mg/L; (C) = SDA with terbinafine 0.125 mg/L.

Figure 2

Growth of T. interdigitale isolate #94 (MIC > 8 mg/L), resistant to terbinafine in the terbinafine screening agar. (A) = growth control; (B) = SDA with terbinafine 0.125 mg/L; (C) = SDA with terbinafine 0.06 mg/L.