Six form of Blastocystis hominis has been reported, consist of vacuolar, avacuolar, multivacuolar, ameboid, granular and cystic forms. Reports of the morphology of B. hominis from culture samples commonly have noted only three major forms; vacuolar, granular, and ameboid, but vacuolar form is most commonly found. Physical factors, such as osmotic changes, the presence of certain drugs, and metabolic status, may influence the morphology of the organism both in vivo and in vitro.
This variation in morphology has important implications for diagnosis, because B. hominis in fecal samples usually is identified by the presence of forms approximately 10 to 15 mm in diameter with a large central vacuole. However, recent studies have indicated that this form is not the most predominant form of B. hominis present in fresh fecal samples. The presence of smaller forms, including multivacuolar and cyst
forms (approximately 5 mm in diameter) in fecal samples, suggests that many B. hominis infections may be missed during laboratory examinations. The lack of information on forms of B. hominis other than the vacuolar form, combined with the small size of other forms and the disparate morphology of the organism in some samples, complicates identification, even for experienced laboratory personnel.
Vacuolar Form and Granular Form
Vacuolar form is also referred to as the vacuolated or “central-body” form of B. hominis, has been considered to be the typical Blastocystis cell form. This is the form generally used for diagnosis of B. hominis. It is the predominant form of the organism in culture and is also found in fecal samples.
Granular form has ultrastructure similar to that of the vacuolar form, apart from having morphologically and cytochemically different central vacuole contents. It does not appear necessary to evoke the concept of a distinct form for the granular cell but, rather, to consider it to be a vacuolar form with granules in the central vacuole.
Both forms usually are spherical cells with considerable heterogeneity in the ultrastructure from different isolates, but the ultrastructural appearance is consistent and stable for each isolate, even after prolonged in vitro culture.
Vacuolar forms vary greatly in size, ranging from 2 mm (204) to more than 200 mm in diameter, with the average diameter of cells usually being between 4 and 15 mm. Granular forms often are slightly larger
The central vacuole has been reported to function in endodyogeny and schizogony via the development of reproductive granules and granules which are seen in the central vacuole are the result of metabolism or storage within the cell.
A number of culture conditions are known to induce the production of the granular form from the vacuolar form. These include increased serum concentrations in the culture medium, transfer of the cells to a different culture medium, axenization of the culture
Binary fission is the only demonstrated method of reproductionof these forms of B. hominis
Multivacuolar and Avacuolar Forms
Multivacuolar-form frequently are noted within B. hominis cells from fecal material Rather than a large single vacuole, as is present in cultured cells, multiple small vacuoles of different sizes and morphologies. This form is smaller (approximately 5 to 8 mm in diameter) than the “typical” vacuolar or granular forms. Therefore, this form of B.hominis is usually missed by examiners.
One nucleus (or occasionally two nuclei) has been found. Morphological heterogeneity was seen in B. hominis isolates from fresh fecal samples. This may indicate the presence of different demes or even different species of Blastocystis in the host. However, it is more likely to represent a continuum of developmental stages or changes due to differing environmental conditions.
After short-term laboratory culture, only vacuolar or granular forms were found and only vacuolar forms were found after longer periods of culture.
Acacuolar form is smaller (approximately 5 mm in diameter) than B. hominis forms from culture and did not contain a central vacuole. The cells contained one or two nuclei, which often were slightly larger than the nuclei reported in the culture forms of B. hominis.
This form has been termed the ameba-like form, the ameba form, the amebiform and the ameboid . The cells were 2.6 to 7.8 mm in diameter, were irregular in shape, and often had extended pseudopodia. Engulfed bacteria were seen in lysosome-like bodies within the cytoplasm and appeared to be digested by the cell. A large central vacuole was not seen.
The mode of division of ameboid cells is unproven
Cystic form is found in fecal material, particularly material that had been stored for several days before being fixed, and occasionally in laboratory cultures. It is smaller than vacuolar and granular forms found in cultures and generally are smaller than the multivacuolar forms found in fresh fecal material.
This form of B. hominis probably confers resistance in the external environment. It doesn’t lyse in distilled water and its presence in culture medium containing antiprotozoal drugs has been reported.
The life cycle and modes of reproduction of B. hominis have not been conclusively demonstrated. The life cycle presented in most recent texts is that proposed by Zierdt on the basis of his light microscopy observations. This life cycle indicates that the vacuolar form differentiates either into the granular form, subsequently producing daughter vacuolar cells within the central vacuole, or into the ameboid form, subsequently producing vacuolar cells by budding. Binary fission is the only plausible method of reproduction.
A cycle of “autoinfection” also is proposed. In this cycle, the vacuolar form is indicated to differentiate into the thin-walled cyst form via the multivacuolar and precyst forms. Schizogony is suggested to occur within the thin-walled cyst, which then ruptures to release daughter vacuolar forms.
The life cycle shown in the picture has been proposed for B. hominis on the basis of current data. The form present in the intestines of humans appears to be a small avacuolar cell without a surface coat. As the avacuolar form passes through the intestine, the small vesicles present in the cytoplasm probably coalesce, and subsequently the cell appears as the multivacuolar form. The multivacuolar form, found as the predominant form in fecal material, is surrounded by a thick surface coat. The cyst wall appears to form beneath the surface coat, which subsequently appears to slough off. The resultant cyst form is likely to be the infective form of B. hominis. Ingestion by a new host and excystation of the cell would complete the cycle. Excystation may occur as a result of exposure of the cyst form to gastric acid and intestinal enzymes
It is possible that the ameboid form arises from the avacuolar form
The vacuolar form has been demonstrated to form after culture of the multivacuolar form, apparently by the coalescence and enlargement of the smaller, multiple vacuoles to make up the large central vacuole. It appears likely that this morphological alteration also occurs when B. hominis is placed in various preservative, diluent, and staining solutions, such as are used in clinical laboratories. It is assumed that the granular form differentiates from the vacuolar form by the accumulation of granules in the central vacuole.
Summarized by Thirayost Nimmanon
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