Factors promoting in vitro excystation of Giardia muris cysts

Abstract
Giardia muris cysts, isolated from mouse faeces, excysted routinely at levels greater than 90%, when induced in 1X Hanks' supplemented with 17 mM glutathione, 29 mM L-cysteine-HCl, and 50 mM NaHCO3 for 30 minutes at 35 °C, followed by washing and suspension in trypsin-Tyrode's solution at pH 8·0. Although trypsin was not required in this final step, it enhanced the escape of the trophozoites from their cysts. G. muris excystation was dependent upon the length of the induction period, pH, oxidation-reduction potential and temperature. Optimal induction conditions for excystation were: an induction period of 5 to 30 min; pH of 2; 120 mV oxidation-reduction potential; and a temperature around 35 °C. A gradual decline in excystation occurred as pH and oxidation-reduction potential were changed to 7 and 57 mV, respectively. There was a pronounced increase in excystation percentages with increasing temperatures between 0 and 37 °C. At 40 °C and above, the G. muris cysts showed signs of inactivation. The thermal death point of G. muris cysts was determined to be about 54 °C. G. muris cysts showed no polarity; however, the tail or posterior trophozoite portion always emerged through one end of the cyst first. Cytokinensis began within the first hour after excystation. This method always produced extremely active, normal-looking G. muris trophozoites.