Protozoan Characteristics ofBlastocysts Hominis

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Abstract
Blastocystis hominis, an organism long regarded as an intestinal yeast, was studied by means of light and electron microscopy; its cultural characteristics also were investigated. In culture B. hominis occurs in 3 major forms: vacuolated (the cell, 8-10 [mu] in diameter, has a large central vacuole and multiplies by binary fission); ameba-like (a polymorphous cell with a large size range, manifesting pseudopodal and phagocytic activity and multiplying by asymmetrical fission); granule-filled (a spherical cell, found predominantly in older cultures, containing reproductive granules, probably a cyst form of B. hominis). B. hominis grows only at 37[degree]C in the presence of a mixed bacterial flora, fails to grow on fungal media, and shows a preference for neutral or slightly alkaline pH. In culture it manifests pseudopod extension and retraction and phagocytic activity. It is rapidly destroyed by environmental changes in tonicity, shows no evidence of mycelial formation, and fails to stain with periodic acid-Schiff reagent. The ultrastructure shows absence of a cell wall, presence of a well-defined nucleus and nucleolus, mitochondria, ribosomes, Golgi apparatus, and smooth and rough endoplasmic reticulum. The majority of these characteristics seem to ally the organism more with the protozoa than with the fungi.

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