Long-term viability of dermatophytes: a comparative evaluation of cryopreservation media

Reliable long-term preservation of dermatophytes is essential for diagnostics, taxonomy, and experimental reproducibility, but simple and comparative protocols remain scarce. We evaluated viability and colony morphology of 59 clinical isolates of Epidermophyton floccosum, Microsporum canis, M. gypseum, Trichophyton mentagrophytes, T. tonsurans and T. rubrum after about 15 years of storage under five conditions: Sauton broth either with 3 % or 10 % (v/v) glycerol at -80 °C; Sabouraud dextrose broth (SDB) with 0 % or 10 % (v/v) glycerol at -80 °C; and water storage at room-temperature (Castellani method, 25 °C). Overall recovery was highest with Sauton broth at low glycerol (79.7 %), followed by Castellani (42.4 %), Sauton broth at high glycerol (35.6 %), Sabouraud broth at low glycerol (27.1 %) and Sabouraud broth at high glycerol (5.1 %). Species-specific patterns were evident: all T. tonsurans and T. mentagrophytes isolates remained viable in Sauton broth at low glycerol; T. rubrum showed good recovery in both Sauton (73.3 %) broth at low glycerol and with the Castellani approach (86.7 %); M. gypseum performed best with Sauton broth at low glycerol (75 %) and Sabouraud broth at low glycerol (62.5 %); and M. canis survived only with the Castellani method (80 %). M. canis cultures showed an altered phenotype characterized by macroconidia loss after long-term maintenance in water. These findings support storage in Sauton broth at low glycerol at -80 °C as a broadly effective option for long-term dermatophyte preservation. This study highlights the need for screening collections before storage so that more difficult species, such as M. canis, can be identified and stored by other techniques.