Blastocystis is a genus of single-celled protozoan parasites belonging to a group of organisms known as the Stramenopiles (also called Heterokonts) that includes algae, diatoms, and water molds. Blastocystis comprises several species, living in the gastrointestinal tracts of species as diverse as humans, farm animals, birds, rodents, reptiles, amphibians, fish, and cockroaches.[1] The species residing in humans is known as Blastocystis hominis and is the focus of this article.
| Blastocystis | |
|---|---|
| |
| Blastocystis sp. | |
| Scientific classification | |
| Domain: | |
| Kingdom: | |
| Phylum: | |
| Class: | Blastocystae
|
| Order: | Blastocystida
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| Family: | Blastocystidae
|
| Genus: | Blastocystis (Alexieff 1911) Brumpt 1912
|
Blastocystis hominis has a widespread geographic distribution and is common in countries of all income levels. Its status as a true pathogen is controversial– while it has been found in patients with gastrointestinal symptoms it is not proven to be the cause, and many carriers are asymptomatic. Research on Blastocystis hominis is limited, with large gaps remaining in our understanding of its life cycle, transmission mechanisms, incubation period, epidemiology, and treatment options.
Classification
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.[2] This was then contradicted by Zierdt, who reclassified it under subphylum Sporozoa, based on some distinctive protistan features of the Blastocystis cell, 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. An analysis of gene sequences was performed in 1996, which placed it into the group Stramenopiles.[3][4] Other Stramenopiles include brown algae, mildew, diatoms, the organism that caused the Irish potato famine, and the organism responsible for Sudden oak death disease. However, the position of Blastocystis within the stramenopiles remains enigmatic.[5]
Different species
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. [6]
In fact, nine distinct species of Blastocystis (as defined by genetic differences) can infect humans, including those previously called Blastocystis ratti.[7]
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, according to the genetic identify of the Blastocystis organism, rather than the host that was infected by it.[8]
A tenth group was reported in China in 2007,[9] but a full analysis of its relationships has not yet been performed and it is not yet clear whether it is a group within a described subtype or a new subtype. A definite tenth subtype has been found in a variety of other mammals, including primates, but not yet in humans.[10]
Clinical Presentation
In so much as it exists, the clinical presentation of Blastocystis infection is known as blastocystosis. Again, there is controversy over whether or not Blastocystis is actually pathogenic or just part of the gut flora. Blastocystis has been found in people who report no symptoms, and it is estimated that most cases may be asymptomatic. Whether this is due to Blastocystis’ lack of pathogenicity or immunity of the host is unknown. A third possibility is that Blastocystis is split in several demes, some of which are disease-causing and some of which are not. Since most of the research on Blastocystis infection has been based on case reports, no conclusive evidence has emerged to answer these questions.[11] Where symptoms have been attributed to the parasite, they have included diarrhea, nausea, abdominal cramps, bloating, excessive gas, and anal itching.[12] The timescale of infection with the parasite can range from weeks to years.[13]
Transmission
The precise transmission mechanisms of Blastocystis remain murky. Fecal-oral tansmission is the most accepted pathway, and recent studies have shown that transmission involves only the cyst form of the parasite.[14] The extent to which human-human, human-animal, and animal-human transmission occurs is still unknown. Genomic studies provide evidence for all three routes, though experimental studies have yet to provide conclusive proof for the existence of either.[15]
Reservoir
Conclusively stating that Blastocystis has an animal reservoir depends upon unraveling the true nature of its transmission. If, as Noël et al. deem likely based upon their own molecular work and a review of the literature, animal-to-human transmission is possible, then animals such as pigs and dogs could in fact be acting as a large reservoir capable of human infection.[16] Epidemiological studies finding that infection is more common in people living in proximity to farm animals or pets[13] further supports this notion.
Morphology
Blastocystis has various morphological forms.
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.
- Vacuolar form
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 200 µm. 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.
- Granular form
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.
- Amoeboid form
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.[17]
- Cyst form
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.[18][19] 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.
Life cycle
The supposed 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.[21] 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.[22][23] However, most cultivation is performed in liquid media of various types.
Diagnostic Tests
Blastocystis is diagnosed using microscopy on a stool sample. The CDC recommends that samples be concentrated and at least three separate samples should be taken before a negative result is confirmed. The two most common methods are a wet mount and a trichrome stain. In the wet mount Blastocystis is stained with iodine and appears as a large vacuole in the middle of many small nuclei, though it may be difficult to see. In the trichrome stain, the parasite is stained with trichrome giving the large central body a gray or green appearance and the cytoplasm elements a dark red color.[24]
Management and Therapy
Even though the pathogenicity of Blastocystis remains controversial, anti-protozoan drugs have been used to treat individuals in whom the parasite is found. The most widely used treatment option is the antibiotic drug Metronidazole which, while effective in some individuals, shows signs of resistance by Blastocystis or limited effectiveness in others. At least one group of researchers has also used the drug combination Trimethoprim/Sulfamethoxazole (TMP/SMX) with similar effects.[25] More recently, Nitazoxanide has been used with much more positive effects.[26]
Epidemiology
Blastocystis has not been as reliably tracked as diseases such as malaria. Case reports indicate that while nearly worldwide in distribution, it is more common in less developed nations and may prefer tropical and sub-tropical climates. In addition, researchers have found that the ratio of urban-to-rural infection rates is 3:1. Proximity to pets and farm animals also appear to be linked with increased infection rates,[13] but this link remains circumstantial. A study in Salamanca, Spain showed prevalence rates to be 2-3 times higher in day care centers and 4-6 times higher in primary schools than in the general population.[27]
Prevention and Public Health
Due to the uncertain infective nature and transmission pathways of the parasite, there are no widespread public health or prevention strategies directly aimed at Blastocystis. The CDC does list the following, however, as potentially useful preventative and control measures:[12]
- Handwash with soap and water before handling food and after using the toilet. If employed in a child-care center, also wash after each diaper change even if gloves were used.
- Avoid potentially infected water and food
- Wash and peel all raw fruits and vegetables
- avoid untreated water in countries with less established water-safety standards
It does not appear as if there are any vaccines for Blastocystis on the horizon.[original research?]
Mitochondrion-like organelles
The organelles in Blastocystis that resemble mitochondria are an enigma as the organism is a strict anaerobe. Recent sequence analyses of the organelle genome and over 12,000 expressed sequence tags (ESTs) has given us many insights into the role these organelles play in the metabolism of the cell. The genome encodes several subunits of NADH dehydrogenase (complex I) but lacks all trace of genes for cytochrome and ATPase subunits (Complexes III-V). ESTs confirm the presence of complexes I and II, and indicate that this partial electron transport chain may lead to an alternative oxidase. The ESTs also suggest that many other metabolic pathways characteristic of mitochondria are still present in the Blastocystis organelles. However, other findings show that the organelle also has characteristics in common with hydrogenosomes, as a gene encoding [FeFe] hydrogenase is present and the protein has been localised to the organelles.[28]
See also
References
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