Blastocystis is a highly prevalent single-celled parasite that infects the gastrointestinal tract of humans and animals. Many different types of Blastocystis exist, and they can infect humans, farm animals, birds, rodents, amphibians, reptiles, fish, and even cockroaches.
Blastocystis has presented a challenge to the medical and scientific community due to the diversity of hosts the organism can infect, the diversity of Blastocystis species which exist, and the fact that most species of Blastocystis found in mammals and birds are able to cause infection in humans. The organism has been called controversial, cryptic, and enigmatic. Even its classification has proved challenging. Blastocystis was originally classified as a yeast, then as a protozoan. An analysis of gene sequences was finally performed in 1996, which placed it into the Stramenopile kingdom.
For many years, scientists believed one species of Blastocystis infected humans, while different species of Blastocystis infected other animals. So they called Blastocystis from humans Blastocystis hominis and gave different species names to Blastocystis from other animals, for example Blastocystis ratti from rats. Various genetic analysis showed Blastocystis hominis as a unique entity does not really exist -- there is no single species of Blastocystis that infects humans. In fact, nine distinct 'species' of Blastocystis (as defined by genetic differences) can infect humans, including those previously called Blastocystis ratti. Because of this, in 2007 scientists proposed discontinuing the use of the term Blastocystis hominis. Their proposal is to refer to Blastocystis from humans and animals as Blastocystis sp. subtype nn where nn is a number from 1 to 9 assigned to each species group. 
The appropriate classification of Blastocystis has only recently been resolved. The original description of Blastocystis was as a yeast due to its yeast-like glistening appearance in fresh wet mounts and the absence of pseudopodia and locomotion.  This was then contradicted by Zierdt who reclassified it under subphylum Sporozoa based on some distinctive protistan features that the Blastocystis cell has, such as the presence of nuclei, smooth and rough endoplasmic reticulum, Golgi complex, and mitochondrion-like organelles. Its sensitivity to antiprotozoal drugs and its inability to grow on fungal media further indicated that it was a protozoan. However, major revisions were made to its classification more recently based on modern molecular approaches to classification, and these studies have shown that Blastocystis is neither yeast nor a protozoan. It is placed in a new Kingdom known as the Stramenopiles. Other Stramenopiles include brown algae, mildew, diatoms, the organism that caused the Irish potato famine, and the organism responsible for Sudden oak death disease.
The great diversity of morphological forms in which Blastocystis exists in poses identification and diagnostic problems. Four commonly described forms are the vacuolar (otherwise known as central body), granular, amoeboid, and cyst forms. The appearance of the organism is largely dependent upon environmental conditions as it is extremely sensitive to oxygen. Whether all of these forms exist in the host intestine is unclear.
The vacuolar form is the typical cell form of Blastocystis seen in culture and is often used for the identification of the organism. These vacuolar forms vary greatly in size, with diameters ranging between 2 µm and 200µm. The vacuolar form is otherwise known as central body form because it has a large central vacuole surrounded by a thin band of peripheral cytoplasm which contains other organelles. Flocculent material has been described as being scattered unevenly throughout the vacuole. The function of the vacuole is still unclear, however, it has been suggested that, like for many eukaryotic cells, it is for storage purposes. Other functions, such as cell division during reproduction and the deposition of apoptotic bodies, have been proposed, although more tests need to be done to validate these roles.
The granular form is somewhat morphologically similar to the vacuolar forms except that distinct granules are observed in the central vacuole and / or cytoplasm. Within the central vacuole, these granules appear in different forms too. Three types were suggested – metabolic, lipid, and reproductive granules. Metabolic granules play a role in chemical processes that are necessary for the maintenance of life in the organism. It was also put forward that reproductive granules were involved in the development of progeny cells. These hypotheses were made based on microscopy alone, which may be deemed misleading, hence more need to be done before making a definite conclusion. It has also been suggested that the granules may be an indication that the cell is dying!
The other form that exists is the amoeboid form. The amoeboid form of Blastocystis is non-motile and strongly adhesive. A research study has reported that amoeboid forms are produced only in cultures taken from symptomatic individuals, with asymptomatic individuals producing exclusively vacuolar forms. The study suggested this method could be used for diagnosing symptomatic infection. Additionally, it suggested the symptoms could be due to the accumulation of the strongly adhesive amoeboid forms on the host's intestinal wall. A detailed ultra-structural study of amoeboid forms was published in 2007. 
The Blastocystis cyst form is a more recent discovery and has helped in the advancement of understanding the way the infection is transmitted. As compared to the other forms, it is generally smaller in size and has a thick multilayered cyst wall. It lacks a central vacuole and few nuclei, multiple vacuoles and food storage deposits were observed. The cyst form is the most resistant form of this parasite and is able to survive in harsh conditions because of its thick multilayered cyst wall. Experiments have been carried out to show its ability to withstand acidic gastric juices. Besides, the cysts did not lyse when placed in distilled water and could survive well at room temperature for up to 19 days, indicating its strong resistance.  In another experiment, the cyst form was even able to survive in culture medium containing antiprotozoal drugs! This further supports the idea that the cyst form is the most resistant of the four forms.
The proposed life cycle begins with ingestion of the cyst form. After ingestion, the cyst develops into other forms which may in turn re-develop into cyst forms. Through human feces, the cyst forms enter the external environment and are transmitted to humans and other animals via the fecal-oral route, repeating the entire cycle.
Obtaining and culturing Blastocystis
The ATCC maintains a collection of Blastocystis isolates. Some records show whether the isolates were obtained from symptomatic or asymptomatic carriers. As yet, no publication has identified the subtypes of most of the ATCC isolates, which are mostly axenic. Researchers have reported that patients with Irritable bowel syndrome may provide a reliable source for xenic Blastocystis isolates. Some researchers have reported being able to culture Blastocystis from 46% of IBS patients.  Researchers have described different culture mechanisms for growing Blastocystis. Colony growth on solid medium colonies on solid culture medium using a synthetic medium with added supplements have both been described.  However, most cultivation is performed in liquid media of various types.
^ Stensvold CR, Suresh GK, Tan KS, et al (2007). "Terminology for Blastocystis subtypes--a consensus". Trends Parasitol.23 (3): 93-6. doi:10.1016/j.pt.2007.01.004. PMID 17241816.
^ Brumpt E (1912). "Blastocystis Hominis N. sp et formes voisines.". Bull. Soc. Pathol. Exot.5: 725-730.
^ Tan TC, Suresh KG (2006). "Amoeboid form of Blastocystis hominis - a detailed ultrastructural insight". Parasitol. Res.99 (6): 737-42. doi:10.1007/s00436-006-0214-z. PMID 16816959.
^ Zaman V, Howe J, Ng M (1995). "Ultrastructure of Blastocystis hominis cysts". Parasitol. Res.81 (6): 465-9. PMID 7567903.
^ Moe KT, Singh M, Howe J, et al (1996). "Observations on the ultrastructure and viability of the cystic stage of Blastocystis hominis from human feces". Parasitol. Res.82 (5): 439-44. PMID 8738284.
^ Tan KS (2004). "Blastocystis in humans and animals: new insights using modern methodologies". Vet. Parasitol.126 (1-2): 121-44. doi:10.1016/j.vetpar.2004.09.017. PMID 15567582.
^ Yakoob J, Jafri W, Jafri N, et al (2004). "Irritable bowel syndrome: in search of an etiology: role of Blastocystis hominis". Am. J. Trop. Med. Hyg.70 (4): 383-5. PMID 15100450.
^ Tan SW, Singh M, Yap EH, et al (1996). "Colony formation of Blastocystis hominis in soft agar". Parasitol. Res.82 (4): 375-7. PMID 8740557.
^ Tan SW, Singh M, Thong KT, et al (1996). "Clonal growth of Blastocystis hominis in soft agar with sodium thioglycollate". Parasitol. Res.82 (8): 737-9. PMID 8897510.