Blastocystis hominis ultrastructure and encystment in polixenicculture.

Blastocystis hominis (Bh) is an organism, whose taxonomy, biological cycle and mode of transmission is controversial. A variety of forms have been described using ligth and transmission electron microscopy (TEM) and there is not uniformity in reporting cysts characteristics in TEM. The knowledge of the morphology of Blastocystis hominis is indispensable for identification and biological studies. Morphologic characterization was made in B. hominis, isolated Bh69:UCV, obtained from the original culture from a patient's faeces with intestinal symptomatology and maintained in the Amibiasis laboratory in polixenic culture in Boeck-Drbohlav (modified) media (BDM). The vegetative forms of the parasite, central body form (CBF), granular form (GF), amoeboid form (AF), and cysts which spontaneously appeared on the cultures, are described, using light microscope (400x and 1000x) and TEM, according to conventional techniques. All the vegetative forms are characteristically surrounded by a mucoid cover, and have a central body that occupies most of cell and delimits a thin cytoplasmic ring, in which the cellular organelles are observed like refractile inclusions. There are two types of cysts, naked cyst are spherical to ovoid, refractile, 5-6 μm of diameter and cyst with external cover, separated by a space of 2-3 μm. By TEM, in the cytoplasm of all the forms, a nucleus with chromatin in crescent-shaped form, spherical mitochondria with few crests, digestive vacuoles, Golgi, RER and great amount of ribosomes and glicocalix on the plasmatic membrane were observed. Cyst wall showed three defined zones, with the external cover made of fibrogranular and vesicular material. Some cysts presented spherical structures whose morphology does not correspond with mitochondrias or nuclei and they could be daughters cells. The morphologic characteristics agree with previously reported for the vegetative forms and cysts, and we suggest a possible sequence in vegetative forms transformation to resistance forms