Blastocystis
Encyclopedia
Blastocystis is a genus of single-celled protozoa
n parasites belonging to a group of organisms known as the Stramenopiles (also called Heterokonts) that includes algae
, diatom
s, and water molds. Blastocystis comprises several species, living in the gastrointestinal tract
s of species as diverse as humans, farm animals, birds, rodents, reptiles, amphibians, fish, and cockroaches.
Blastocystis exhibits low host specificity, and many different species of Blastocystis can infect humans and by current convention, any of these species would be identified as Blastocystis hominis if they were identified in a human. These have a widespread geographic distribution and are found at a rate of 5-10% in most developed countries, and a rate of up to 50% in less developed
areas. High rates of infection are also found in individuals in developed countries who work with animals. Although the role of Blastocystis hominis in human disease is often referred to as controversial, a systematic survey of research studies conducted by 11 infectious disease specialists from nine countries, found that over 95% of papers published in the last 10 years identified it as causing illness in immunocompetent individuals. The paper attributed confusion over pathogenicity to the existence of asymptomatic carrier
s, a phenomenon the study noted is common to all gastrointestinal protozoa.
due to its yeast-like glistening appearance in fresh wet mounts
and the absence of pseudopod
ia and locomotion
. This was then contradicted by Zierdt, who reclassified it under subphylum
Sporozoa
, based on some distinctive protist
an features of the Blastocystis cell, such as the presence of nuclei
, smooth and rough endoplasmic reticulum
, Golgi complex
, and mitochondrion
-like organelle
s. Its sensitivity to antiprotozoal drugs and its inability to grow on fungal
media further indicated that it was a protozoa
n.
However, major revisions were made to its classification. An analysis of gene sequences was performed in 1996, which placed it into the group 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. However, the position of Blastocystis within the stramenopiles remains enigmatic.
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, according to the genetic identify of the Blastocystis organism, rather than the host that was infected by it.
A tenth group was reported in China in 2007, 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.
There are at least 13 genetically distinct small subunit ribosomal RNA lineages.
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 diameter
s 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.
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. 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.
, 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.
s 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 (EST
s) 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 hydrogenosome
s, as a gene encoding [FeFe] hydrogenase is present and the protein has been localised to the organelles.
Protozoa
Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...
n parasites belonging to a group of organisms known as the Stramenopiles (also called Heterokonts) that includes algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
, diatom
Diatom
Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...
s, and water molds. Blastocystis comprises several species, living in the gastrointestinal tract
Gastrointestinal tract
The human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus. ....
s of species as diverse as humans, farm animals, birds, rodents, reptiles, amphibians, fish, and cockroaches.
Blastocystis exhibits low host specificity, and many different species of Blastocystis can infect humans and by current convention, any of these species would be identified as Blastocystis hominis if they were identified in a human. These have a widespread geographic distribution and are found at a rate of 5-10% in most developed countries, and a rate of up to 50% in less developed
Developing country
A developing country, also known as a less-developed country, is a nation with a low level of material well-being. Since no single definition of the term developing country is recognized internationally, the levels of development may vary widely within so-called developing countries...
areas. High rates of infection are also found in individuals in developed countries who work with animals. Although the role of Blastocystis hominis in human disease is often referred to as controversial, a systematic survey of research studies conducted by 11 infectious disease specialists from nine countries, found that over 95% of papers published in the last 10 years identified it as causing illness in immunocompetent individuals. The paper attributed confusion over pathogenicity to the existence of asymptomatic carrier
Asymptomatic carrier
An asymptomatic carrier is a person or other organism that has contracted an infectious disease, but who displays no symptoms. Although unaffected by the disease themselves, carriers can transmit it to others...
s, a phenomenon the study noted is common to all gastrointestinal protozoa.
Classification
The appropriate classification of Blastocystis has only recently been resolved. The original description of Blastocystis was as a yeastYeast
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...
due to its yeast-like glistening appearance in fresh wet mounts
Microscope slide
A microscope slide is a thin flat piece of glass, typically 75 by 25 mm and about 1 mm thick, used to hold objects for examination under a microscope. Typically the object is placed or secured on the slide, and then both are inserted together in the microscope for viewing...
and the absence of pseudopod
Pseudopod
Pseudopods or pseudopodia are temporary projections of eukaryotic cells. Cells that possess this faculty are generally referred to as amoeboids. Pseudopodia extend and contract by the reversible assembly of actin subunits into microfilaments...
ia and locomotion
Animal locomotion
Animal locomotion, which is the act of self-propulsion by an animal, has many manifestations, including running, swimming, jumping and flying. Animals move for a variety of reasons, such as to find food, a mate, or a suitable microhabitat, and to escape predators...
. This was then contradicted by Zierdt, who reclassified it under subphylum
Subphylum
In life, a subphylum is a taxonomic rank intermediate between phylum and superclass. The rank of subdivision in plants and fungi is equivalent to subphylum.Not all phyla are divided into subphyla...
Sporozoa
Apicomplexa
The Apicomplexa are a large group of protists, most of which possess a unique organelle called apicoplast and an apical complex structure involved in penetrating a host's cell. They are unicellular, spore-forming, and exclusively parasites of animals. Motile structures such as flagella or...
, based on some distinctive protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
an features of the Blastocystis cell, such as the presence of nuclei
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
, smooth and rough endoplasmic reticulum
Endoplasmic reticulum
The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...
, Golgi complex
Golgi apparatus
The Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....
, and mitochondrion
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
-like organelle
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....
s. Its sensitivity to antiprotozoal drugs and its inability to grow on fungal
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
media further indicated that it was a protozoa
Protozoa
Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...
n.
However, major revisions were made to its classification. An analysis of gene sequences was performed in 1996, which placed it into the group Stramenopiles.
Other Stramenopiles include brown algae, mildew, diatoms, the organism that caused the Irish potato famine, and the organism responsible for Sudden oak death
Sudden oak death
Sudden Oak Death is the common name of a disease caused by the oomycete plant pathogen Phytophthora ramorum. The disease kills oak and other species of tree and has had devastating effects on the oak populations in California and Oregon as well as also being present in Europe...
disease. However, the position of Blastocystis within the stramenopiles remains enigmatic.
Signs and symptoms
Most published studies have reported that between 50% and 80% of individuals mono-infected with Blastocystis will show symptoms. Factors influencing presentation of symptoms have been listed as the patient's age, with younger patients less likely to show symptoms, as well genetic changes that influence the production of cytokines. Some studies have suggested that pathogenicity may be linked to specific subtypes of Blastocystis and experimental infection of animals has reported varying degrees of illness depending on the subtype used. While some subtypes appear to be less likely to result in symptomatic infection, those subtypes are also found in symptomatic individuals who have no other infection found. Symptoms associated with the infection are diarrhea, nausea, abdominal cramps, bloating, excessive gas, and anal itching. Most cases of the infection appear to become diagnosed as irritable bowel syndrome, according to studies from Denmark, Pakistan, the United Kingdom, and Italy. The timescale of infection with the parasite can range from weeks to years.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.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, according to the genetic identify of the Blastocystis organism, rather than the host that was infected by it.
A tenth group was reported in China in 2007, 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.
There are at least 13 genetically distinct small subunit ribosomal RNA lineages.
Transmission
Fecal-oral transmission is the most accepted pathway, and recent studies have shown that transmission involves only the cyst form of the parasite. 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.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. Epidemiological studies finding that infection is more common in people living in proximity to farm animals or pets 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 diameter
Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...
s ranging between 2 µm and 200 µm. The vacuolar form is otherwise known as central body form because it has a large central vacuole
Vacuole
A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...
surrounded by a thin band of peripheral cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the 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
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
, it is for storage purposes. Other functions, such as cell division during reproduction
Biological reproduction
Reproduction is the biological process by which new "offspring" individual organisms are produced from their "parents". Reproduction is a fundamental feature of all known life; each individual organism exists as the result of reproduction...
and the deposition of apoptotic
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...
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
Granule (cell biology)
In cell biology, a granule is a small particle. It can be any structure barely visible by light microscopy. The term is most often used to describe a secretory vesicle.-Leukocytes:...
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
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
, 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
Offspring
In biology, offspring is the product of reproduction, of a new organism produced by one or more parents.Collective offspring may be known as a brood or progeny in a more general way...
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.
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
Lysis
Lysis refers to the breaking down of a cell, often by viral, enzymic, or osmotic mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a "lysate"....
when placed in distilled water
Distilled water
Distilled water is water that has many of its impurities removed through distillation. Distillation involves boiling the water and then condensing the steam into a clean container.-History:...
and could survive well at room temperature
Room temperature
-Comfort levels:The American Society of Heating, Refrigerating and Air-Conditioning Engineers has listings for suggested temperatures and air flow rates in different types of buildings and different environmental circumstances. For example, a single office in a building has an occupancy ratio per...
for up to 19 days, indicating its strong resistance. In another experiment, the cyst form was even able to survive in culture medium
Growth medium
A growth medium or culture medium is a liquid or gel designed to support the growth of microorganisms or cells, or small plants like the moss Physcomitrella patens.There are different types of media for growing different types of cells....
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 routeFecal-oral route
The fecal-oral route, or alternatively, the oral-fecal route or orofecal route is a route of transmission of diseases, in which they are passed when pathogens in fecal particles from one host are introduced into the oral cavity of another potential host.There are usually intermediate steps,...
, repeating the entire cycle.
The ATCC
ATCC
ATCC may refer to:* American Touring Car Championship* American Type Culture Collection* Asociación de Trabajadores Campesinos del Carare* Asian Touring Car Championship* Air Traffic Control Center* Atona Case Competition...
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
Axenic
In biology, axenic describes a culture of an organism that is entirely free of all other "contaminating" organisms. The earliest axenic cultures were of bacteria or unicellular eukaryotes, but axenic cultures of many multicellular organisms are also possible...
. 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
Colony (biology)
In biology, a colony reference to several individual organisms of the same species living closely together, usually for mutual benefit, such as stronger defense or the ability to attack bigger prey. Some insects live only in 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.
Mitochondrion-like organelles
The organelleOrganelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....
s in Blastocystis that resemble mitochondria are an enigma as the organism is a strict anaerobe. Recent sequence analyses of the organelle genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
and over 12,000 expressed sequence tags (EST
Expressed sequence tag
An expressed sequence tag or EST is a short sub-sequence of a cDNA sequence. They may be used to identify gene transcripts, and are instrumental in gene discovery and gene sequence determination. The identification of ESTs has proceeded rapidly, with approximately 65.9 million ESTs now available in...
s) 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
NADH dehydrogenase
NADH dehydrogenase is an enzyme located in the inner mitochondrial membrane that catalyzes the transfer of electrons from NADH to coenzyme Q...
(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 hydrogenosome
Hydrogenosome
A hydrogenosome is a membrane-enclosed organelle of some anaerobic ciliates, trichomonads and fungi. The hydrogenosomes of trichomonads produce molecular hydrogen, acetate, carbon dioxide and ATP by the combined actions of pyruvate:ferredoxin oxido-reductase, hydrogenase, acetate:succinate CoA...
s, as a gene encoding [FeFe] hydrogenase is present and the protein has been localised to the organelles.