Evaluation of the Viabilities and Stabilities of Pathogenic Mold and Yeast Species Using Three Different Preservation Methods Over a 12-Year Period Along with a Review of Published Reports.

Serious mycological work requires a reliable source of cultures that are maintained under safe long-term storage. In this study, 1186 clinical fungal isolates consisting of molds (20 species in 11 genera) and yeasts (21 species in seven genera) maintained in water, under mineral oil at room temperature and cryopreserved at -80 °C for periods ranging from 1 to 12 years, were evaluated for their viabilities and stabilities. The strains were subcultured onto either Sabouraud dextrose agar or potato dextrose agar to determine the viabilities and purities. The stabilities of the dermatophytes were investigated using urease test medium, the Trichophyton agar test and morphological examination. The stabilities of yeasts were evaluated by microscopic morphology and by determining the antifungal susceptibilities of random samples of yeasts (n = 120). Additionally, 365 strains (dermatophytes, n = 115; yeasts, n = 250) were further characterized by "matrix-assisted laser desorption/ionization time-of-flight mass spectrometry." After 12 years of preservation, the survival rates with the three different preservation techniques, i.e., in water, under mineral oil and by freezing, were assessed as 94.7, 82.0 and 97.4 %, respectively. Viability was generally unrelated to the duration of storage. More stable and consistent growth was achieved after storage in water and freezing compared with mineral oil preservation. Our results demonstrate that the procedure for maintaining fungal cultures in water is a simple and inexpensive method, next to cryopreservation, and that both can be reliably used for the long-term preservation of most fungal isolates.