Giardia sp.: physical factors of excystation in vitro, and excystation vs eosin exclusion as determinants of viability.

The effects of several factors on Giardia sp. excystation in vitro were investigated. Temperature, pH, time, and incubation medium were shown to affect the levels of excystation achieved. In general, those conditions most closely approximating the organism's in vivo environment induced the highest levels of excystation. The viability of Giardia sp. cyst suspensions was compared by eosin exclusion and excystation. Eosin exclusion consistently indicated higher cyst viability than could be demonstrated by in vitro excystation. Using excystation as the criterion of viability, the effect of storage at ?13, 8, 21, and 37 C and of exposure to boiling water on Giardia sp. cyst survival was studied. Storage at 8 C permitted longest cyst survival, 77 days, at which time the cyst suspension was exhausted. Cysts stored at 21 C retained their viability for 5 to 24 days, while those at 37 C never survived longer than 4 days. Freezing and thawing cysts resulted in an almost complete loss of viability although a low level of viability (< 1%) persisted for at least 14 days. Cysts exposed to boiling water were immediately incapable of excystation.