Tolerance of the resting cysts of protozoa against gastric acid and proteases is a strategy for surviving in the digestive tract of animals. The present study aimed to examine the protection mechanism against HCl in resting cysts of Colpoda cucullus, which were surrounded by a cyst wall, which is composed of an outermost layer (ectocyst) and several inner layers (endocyst). In addition to water, paraformaldehyde, hydrogen peroxide(H2O2) and ethanol may diffuse across the cyst wall, because the cysts hardly showed tolerance against these molecules. However, the cysts showed tolerance against HCl (0.01 to 1 M) and pepsin. The tolerance against HCl disappeared soon after the onset of excystment induction. Electron micrographs of the excysting cells showed that digestion of the endocyst began within 30 min after the onset of excystment induction. The encysting cells surrounded by the ectocyst (3 h after encystment induction) did not show tolerance against HCl, but acquired it just after a first layer of the endocyst was formed beneath the ectocyst (6 h after encystment induction). These results suggest that the tolerance of the resting cyst of C. cucullus against HCl may be acquired by preventing its diffusion across the endocyst, although the possibility that cytoplasmic molecules and the plasma membrane may acquire tolerance has not been ruled out.
Key words: Colpoda cucullus, resting cysts, tolerance, HCl, excystment.
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