In vitro Azole Susceptibility Testing of Japanese Isolates of Itraconazole-Resistant Dermatophytes.

We conducted antifungal susceptibility testing on itraconazole (ITCZ)-resistant isolates of Trichophyton interdigitale and Trichophyton rubrum collected from Japanese patients in 2021 and 2022. The aim of the present study was to determine the most effective drug against ITCZ-resistant strains of dermatophytes. In all isolates, the minimum inhibitory concentrations (MICs) were > 32 mg/l for ITCZ, < 0.03 to 0.5 mg/l for ravuconazole (RVCZ), and < 0.03 mg/l for efinaconazole (EFCZ), luliconazole (LUCZ), and terbinafine (TRBF). Thus, in tinea unguium cases with ITCZ-resistant strains, treatment should be switched to TRBF or other azoles with a stronger antifungal efficacy, such as EFCZ, LUCZ, or RVCZ, and treatment must continue until the infectious organisms are completely eliminated.

Genomic Analysis of Antifungal Drug Resistance Induced in Trichophyton rubrum After Prolonged Culture with Terbinafine.

In this study, we induced terbinafine (TRF) resistance in a T. rubrum strain in vitro for 18 months then compared the genomes of the TRF-resistant (N42-3) and TRF-susceptible wild-type (N42WT) strains to identify mutations. In the SQLE gene, N42WT had no mutation while N42-3 had a F397L mutation. We sequenced approximately 22.53 Mb of the genomes of the N43WT and N42-3 strains. Other than the F397L mutation in SQLE, there were three other genetic mutations in three different genes that were found in N42-3, but not in N43WT; however, these three mutations were not detected in other TRF-resistant T. rubrum strains. From this genome sequencing analysis, the only variation that was confirmed to be associated with drug resistance in the genome of the TRF-resistant T. rubrum was a hotspot mutation in SQLE.

Long Amplification PCR (LA-PCR) Detection of Azole Resistant Trichophyton indotineae.

Trichphyton indotineae, a species newly designated in 2020 independent of T. interdigitale, comprises highly terbinafine (TRF)-resistant dermatophytosis that is epidemic in North India and spreding to worldwide. Some clinical isolates of T. indotineae have been resistance both TRF and azoles that might be caused the treatment failure. To detect the azole resistance strains, we developed a long amplification PCR (LA-PCR) detection method for the tandem repeat of the CYP51B (encoding sterol 14a-demethylase gene) in T. indotineae. Contrasting the drug susceptibility test results with the LA-PCR results confirmed a trend toward low susceptibility to azole antifungal agents in strains with amplifications of 9.5 kbp or greater (3 or more copies of CYP51B). Our results suggest that the method could be detected rapidly of low-susceptibility strains to azole antifungal agents.

Internal Transcribed Spacer Region Typing of Trichophyton interdigitale Isolated from Japanese Patients.

Trichophyton interdigitale, an anthropophilic species, is one of the main causative agents of tinea unguium and tinea pedis. T. interdigitale and the zoophilic species T. mentagrophytes are morphologically and physiologically very similar. Isolates of the T. interdigitale/T. mentagrophytes complex from around the world have been classified into more than 10 internal transcribed spacer (ITS) genotypes. In this study, we isolated T. interdigitale from Japanese patients and investigated which ITS type was more common. The ITS regions of 29 clinical isolates of T. interdigitale and one clinical isolate of T. mentagrophytes were sequenced. The phylogenetic analysis of the ITS region sequences revealed that the 29 isolates of T. interdigitale belong to ITS type II of T. interdigitale. The one clinical isolate of T. mentagrophytes was in the same cluster with ITS type II* of T. mentagrophytes. One terbinafine-resistant strain of T. interdigitale also belonged to ITS type II of T. interdigitale.