Morphology of Human and Animal Blastocystis Isolates with Special Reference to Reproductive Modes

Abstract

Blastocystis, an anaerobic unicellular eukaryotic protozoon, is one of the most encountered microorganisms in fecal samples of various host animals including humans. The parasite is generally spherical in shape but shows a great variation in size from 5 to 50 μm. When the isolates from the feces were cultured in vitro, the size of the cell increases and some cells reach up more than 200 μm in diameter. Since a large central vacuole occupies the central part of the organisms, the cytoplasm containing nucleus, mitochondria-like organelle, and Golgi apparatus is pushed to the peripheral rim. The central vacuole possesses storage function and contains various substances such as carbohydrates, lipids, and basic proteins identified under the light and electron microscopy. These substances are accumulated in the vacuole via Golgi apparatus or crathrin-based endocytosis. There are various naming for morphologically different organisms, such as vacuolar (vacuolated), multivacuolar, avacuolar, granular, amoeboid, and cyst forms, which appear in fecal and in vitro samples. The vacuolar and granular forms are predominantly detected in the fresh feces or in vitro cultures, while some other forms, multivacuolar and avacuolar forms are rare and small in size and are mainly characterized by electron microscopy. The amoeboid (ameba) forms are also small in size and are generally more frequently observed in the samples of in vitro culture. Irregular shape of the amoeboid form is difficult to be distinguished from the amoebic forms of other intestinal protozoa, and locomotion of the amoeboid form of Blastocystis has not been recognized. The cyst forms are mainly seen in the fecal samples and rarely in in vitro cultures. The nuclei in vacuolar and granular forms can be clearly seen when the cells are stained with DAPI or Giemsa. In general, small cells show one or two nuclei, while bigger cells show many nuclei. When there were two nuclei in a cell, both the nuclei were usually located at the opposite pole of the cell. The life cycle of Blastocystis is still not clear. The most conflicting mode is the reproductive stages in the proposed life cycle. Binary fission is frequently observed in fresh fecal and cultured samples. However, other reproductive stages such as schizogony, plasmotomy (budding), endodyogeny, and sac-like pouches are not accepted generally for the true modes of Blastocystis. Schizogony-like and budding-like organisms, which possess many nuclei in the peripheral cytoplasm or have many daughter cell-like structures in the central vacuole, are observed especially in in vitro cultures. Many reproductive processes have been proposed for Blastocystis, however, to date, only binary fission, budding, or plasmotomy have been proven. Pseudopodal activity, on a few occasions, is seen in the amoeboid forms, and this form may be proposed as another alternative reproduction process except three identified modes of reproduction, binary fission, plasmotomy, and budding (or schizogony) under ultrastructural insights while locomotion of the amoeboid form has not still been confirmed.