Viability of pathogenic Acanthamoeba and Naegleria and virulence of N. fowleri during long-term cryopreservation.

This is a followup report on the viability of pathogenic Acanthamoeba castellanii, Naegleria australiensis, and N. fowleri during 5 years of cryopreservation and the virulence of N. fowleri during 30 months of cryostorage, all at -70 degrees C. The greatest decrease in viability occurred during the first year of freezing and was 10-fold greater than the average yearly decrease during years 2-5. At 5 years of cryostorage viability was 33% for A. castellanii, 38% for N. fowleri and 51% for N. australiensis. Virulence of N. fowleri did not decrease during 30 months of freezing and what appeared to be an increase in virulence during cryopreservation may be the result of reduced viability of the less virulent amebae in a culture.