Evaluation of a Method for Long-Term Cryopreservation of Fungal Strains

The conservation of microorganisms is essential for their in-depth study. However, today's most widely used conservation methods, based on the use of distilled water, soil, oils, or silica, do not guarantee the stability of fungal cells, especially dermatophytes. This problem led us to evaluate the conservation capacity of a cryogenic vials system containing glass beads covered in a cryopreservant hypertonic solution as an alternative method of storage of fungal cells at -80°C. Up to 570 strains of fungi belonging to 27 different species, isolated from clinical samples, were inoculated into cryotubes containing 25 glass beads covered in a cryopreserving hypertonic solution. Suspensions were mixed vigorously and the cryopreserving solution was discarded. The tubes were frozen at -80°C for a period of 42 months and periodically, a glass bead was removed from each cryotube and inoculated onto Sabouraud dextrose agar, and incubated at 30°C for 7-14 days to evaluate the number of colonies recovered, their purity, and phenotypic characteristics. All yeast isolates were recovered, unlike 2 isolates (4.4%) of the mold group and 21 (10.7%) of the dermatophytes. Survival rates were close to 100% for yeasts and molds, with expiration times being estimated for almost indefinite stocks, and 62% for dermatophytes, with an average expiration date of 25.5 years. The phenotypic characteristics remained comparable to those of the strains before storage. Conservation at -80°C using cryogenic vials is a reliable and efficient system for the conservation of fungal collections, and although the behavior differs by groups, stratified survival data are obtained to avoid extinction.