In Vitro Excystation of Spironucleus muris

ABSTRACT. In vitro excystation of Spironucleus muris cysts, purified by sequential sucrose and Percoll gradients from mouse feces, was studied. Three in vitro excystation procedures, used for Giardia , were assessed to determine the most useful method. Excystation was monitored by light microscopy and subsequently characterized by transmission and scanning electron microscopy. Spironucleus muris excysted routinely at a level greater than 90% when induced in Hanks' balanced salt solution containing sodium bicarbonate at pH 2.0 and transferred to Tyrodes' salt solution as an excystation medium. Similarly, high rates of excystation were recorded after induction of S. muris cysts in 0.1 M potassium phosphate buffer (pH 7.0) with sodium bicarbonate and excystation in trypticase-yeast extract-iron medium (TYI medium) or phosphate-buffered saline. A lower rate and percentage of excystation were observed after induction of S. muris cysts in an aqueous hydrochloric acid solution (pH 2.0) followed by excystation in TYI medium. All excystation methods produced extremely active S. muris trophozoites with normal morphology. Nonexcysting S. muris cysts have a wall composed of an outer fibrous and an inner membranous portion. Following induction, numerous vesicles appeared in the peritrophic space. Excystation began by the cyst wall opening at one pole, and the anterior part of the trophozoite protruding from the cyst wall. The trophozoite emerged progressively from the cyst wall and the empty cyst wall appeared to collapse. Excysted trophozoites exhibited normal morphological features of S. muris trophozoites isolated from the mouse intestine.