Ultrastructural Study of Encystation and Excystation in Acanthamoeba castellanii

Abstract

Encystation and excystation of Acanthamoeba castellanii were studied by transmission electron microscopy. The differentiation process was induced in asynchronous cultures grown axenically. Cytoplasmic vesicles containing a dense fibrous material very similar in appearance to the cyst wall were observed in trophozoites induced to encyst. When these trophozoites were incubated with calcofluor white m2r, fluorescence was observed in cytoplasmic vesicles, suggesting that the material contained in these vesicles corresponded to cyst wall precursors. Semithin cryosections of mature cysts with the same treatment showed fluorescence in the ectocyst and a less intense fluorescence in the endocyst, suggesting the presence of cellulose in both structures of the cyst wall. In mature cysts induced to excystation, small structures very similar to electron-dense granules (EDG) previously described in other amoebae were frequently observed. The EDGs were either sparsely distributed in the cytoplasm or associated with the cytoplasmic face of the plasma membrane. Many of them were located near the ostiole. In advanced phases of excystation, endocytic activity was suggested by the formation of endocytic structures and the presence of vacuoles with fibrous content similar to that of the cyst wall. Electron-dense granules in the process of dissolution were also observed in these vacuoles. Furthermore, the formation of a pseudopod suggests a displacement of the amoeba toward the ostiole.

Resistance of Acanthamoeba Cysts to Disinfection Treatments Used in Health Care Settings

“…The observed differences in resistance may partly be due to the ectocyst structure, since it was significantly (P Ͻ 0.001) thicker in the more resistant strain than in the others. This cyst wall structure has been demonstrated to contain large amounts of cellulose (15), and an increase in cyst wall cellulose content during encystment has been demonstrated to coincide with increasing resistance to biocides (60). Other mechanisms may also play a role, since the cysts of strain 4 presented a thickness similar to those of strain 1 but were significantly (P Ͻ 0.001) more resistant to treatments.…”

A retinoblastoma ortholog controls stalk/spore preference inDictyostelium

“…Dictyostelium is almost certainly derived from ancestors that were solitary in both proliferative and developmental stages. Solitary Amebozoans generally encyst when starved, and these cysts may resemble Dictyostelium spores in function (Mazur et al, 1995) and in aspects of morphology (Chavez-Munguia et al, 2005). It thus seems likely that encystment in the non-social ancestors of Dictyostelium involved an Rb-family gene.…”

Cellular, Biochemical, and Molecular Changes during Encystment of Free-Living Amoebae

“…Chávez-Munguía et al suggest that cellulose is present in both layers of the cyst wall (9), while previous studies proposed that cellulose is present only in the endocyst (30,66). Recently, it has been shown that the ectocyst presented an irregular surface and that vesicles were found within its wall.…”

“…These layers are composed of at least acid-insoluble proteins (45) and cellulose (62), but the exact composition is not well known. The structure of Acanthamoeba cysts has been well described by electron microscopy (5,9). One to several holes named ostioles are present at the surface of the cyst wall.…”

“…One to several holes named ostioles are present at the surface of the cyst wall. It has been proposed that excystment occurs through these ostioles after digestion of the opercula covering the ostioles (9).…”