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The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a different undulating pattern than flagella. Cilia occur in all members of the group (although the peculiar
Suctoria Suctoria are ciliates that become sessile in their developed stage and then lose their redundant cilia. They feed by extracellular digestion. They were originally thought to feed by suction – hence their name. In fact, they use specialize ...
only have them for part of their
life cycle Life cycle, life-cycle, or lifecycle may refer to: Science and academia *Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring * Life-cycle hypothesis ...
) and are variously used in swimming, crawling, attachment, feeding, and sensation. Ciliates are an important group of protists, common almost anywhere there is water—in lakes, ponds, oceans, rivers, and soils. About 4,500 unique free-living species have been described, and the potential number of extant species is estimated at 27,000–40,000. Included in this number are many ectosymbiotic and
endosymbiotic An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within ...
species, as well as some obligate and
opportunistic Opportunism is the practice of taking advantage of circumstances – with little regard for principles or with what the consequences are for others. Opportunist actions are expedient actions guided primarily by self-interested motives. The term ...
parasites. Ciliate species range in size from as little as 10 µm in some
colpodea The Colpodea are a class of ciliate The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present ...
ns to as much as 4 mm in length in some geleiids, and include some of the most morphologically complex protozoans. In most systems of taxonomy, "Ciliophora" is ranked as a phylum under any of several kingdoms, including
Chromista Chromista is a biological kingdom consisting of single-celled and multicellular eukaryotic species that share similar features in their photosynthetic organelles ( plastids). It includes all protists whose plastids contain chlorophyll ''c'', ...
, Protista or Protozoa. In some older systems of classification, such as the influential taxonomic works of Alfred Kahl, ciliated protozoa are placed within the class "Ciliata" (a term which can also refer to a genus of fish). In the taxonomic scheme endorsed by the International Society of Protistologists, which eliminates formal
rank Rank is the relative position, value, worth, complexity, power, importance, authority, level, etc. of a person or object within a ranking, such as: Level or position in a hierarchical organization * Academic rank * Diplomatic rank * Hierarchy * ...
designations such as "phylum" and "class", "Ciliophora" is an unranked taxon within Alveolata.


Cell structure


Nuclei

Unlike most other eukaryotes, ciliates have two different sorts of nuclei: a tiny, diploid
micronucleus Micronucleus is the name given to the small nucleus that forms whenever a chromosome or a fragment of a chromosome is not incorporated into one of the daughter nuclei during cell division. It usually is a sign of genotoxic events and chromosomal i ...
(the "generative nucleus", which carries the germline of the cell), and a large, ampliploid macronucleus (the "vegetative nucleus", which takes care of general cell regulation, expressing the phenotype of the organism). The latter is generated from the micronucleus by amplification of the genome and heavy editing. The micronucleus passes its genetic material to offspring, but does not express its genes. The macronucleus provides the
small nuclear RNA Small nuclear RNA (snRNA) is a class of small RNA molecules that are found within the splicing speckles and Cajal bodies of the cell nucleus in eukaryotic cells. The length of an average snRNA is approximately 150 nucleotides. They are transcribe ...
for vegetative growth. Division of the macronucleus occurs in most ciliate species, apart from those in class Karyorelictea, whose macronuclei are replaced every time the cell divides. Macronuclear division is accomplished by amitosis, and the segregation of the chromosomes occurs by a process whose mechanism is unknown. After a certain number of generations (200–350, in ''Paramecium aurelia'', and as many as 1,500 in ''Tetrahymena'') the cell shows signs of aging, and the macronuclei must be regenerated from the micronuclei. Usually, this occurs following '' conjugation'', after which a new macronucleus is generated from the post-conjugal micronucleus.


Cytoplasm

Food vacuole The food vacuole, or digestive vacuole, is an organelle found in simple eukaryotes such as protists. This organelle is essentially a lysosome. During the stage of the symbiont parasites' lifecycle where it resides within a human (or other mammali ...
s are formed through
phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...
and typically follow a particular path through the cell as their contents are digested and broken down by
lysosomes A lysosome () is a membrane-bound organelle found in many animal cells. They are spherical vesicles that contain hydrolytic enzymes that can break down many kinds of biomolecules. A lysosome has a specific composition, of both its membrane prote ...
so the substances the vacuole contains are then small enough to
diffuse Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
through the membrane of the food vacuole into the cell. Anything left in the food vacuole by the time it reaches the cytoproct ( anal pore) is discharged by exocytosis. Most ciliates also have one or more prominent contractile vacuoles, which collect water and expel it from the cell to maintain osmotic pressure, or in some function to maintain ionic balance. In some genera, such as ''
Paramecium '' ''Paramecium'' ( , ; also spelled ''Paramoecium'') is a genus of eukaryotic, unicellular ciliates, commonly studied as a representative of the ciliate group. ''Paramecia'' are widespread in freshwater, brackish, and marine environments and a ...
'', these have a distinctive star shape, with each point being a collecting tube.


Specialized structures in ciliates

Mostly, body cilia are arranged in ''mono-'' and '' dikinetids'', which respectively include one and two kinetosomes (basal bodies), each of which may support a cilium. These are arranged into rows called ''kineties'', which run from the anterior to posterior of the cell. The body and oral kinetids make up the ''infraciliature'', an organization unique to the ciliates and important in their classification, and include various fibrils and microtubules involved in coordinating the cilia. In some forms there are also body polykinetids, for instance, among the
spirotrich The spirotrichs are a large and diverse group of ciliate protozoa. They typically have prominent oral cilia in the form of a series of polykinetids, called the adoral zone of membranelles, beginning anterior to the oral cavity and running down ...
s where they generally form bristles called cirri. The infraciliature is one of the main components of the cell cortex. Others are the ''alveoli'', small vesicles under the cell membrane that are packed against it to form a pellicle maintaining the cell's shape, which varies from flexible and contractile to rigid. Numerous mitochondria and extrusomes are also generally present. The presence of alveoli, the structure of the cilia, the form of mitosis and various other details indicate a close relationship between the ciliates,
Apicomplexa The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic alveolates. Most of them possess a unique form of organelle that comprises a type of non-photosynthetic plastid called an apicoplast, and an apical complex structure. Th ...
, and dinoflagellates. These superficially dissimilar groups make up the alveolates.


Feeding

Most ciliates are heterotrophs, feeding on smaller organisms, such as
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
and
alga Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...
e, and detritus swept into the oral groove (mouth) by modified oral cilia. This usually includes a series of membranelles to the left of the mouth and a paroral membrane to its right, both of which arise from ''polykinetids'', groups of many cilia together with associated structures. The food is moved by the cilia through the mouth pore into the gullet, which forms food vacuoles. Feeding techniques vary considerably, however. Some ciliates are mouthless and feed by absorption (
osmotrophy Osmotrophy is a feeding mechanism involving the movement of dissolved organic compounds by osmosis for nutrition. Organisms that use osmotrophy are called osmotrophs. Some mixotrophic microorganisms use osmotrophy to derive some of their energy. ...
), while others are predatory and feed on other protozoa and in particular on other ciliates. Some ciliates parasitize animals, although only one species, '' Balantidium coli'', is known to cause disease in humans.


Reproduction and sexual phenomena


Reproduction

Ciliates reproduce asexually, by various kinds of fission. During fission, the micronucleus undergoes mitosis and the macronucleus elongates and undergoes amitosis (except among the Karyorelictean ciliates, whose macronuclei do not divide). The cell then divides in two, and each new cell obtains a copy of the micronucleus and the macronucleus. Typically, the cell is divided transversally, with the anterior half of the ciliate (the ''proter'') forming one new organism, and the posterior half (the ''opisthe'') forming another. However, other types of fission occur in some ciliate groups. These include '' budding'' (the emergence of small ciliated offspring, or "swarmers", from the body of a mature parent); '' strobilation'' (multiple divisions along the cell body, producing a chain of new organisms); and ''palintomy'' (multiple fissions, usually within a
cyst A cyst is a closed sac, having a distinct envelope and division compared with the nearby tissue. Hence, it is a cluster of cells that have grouped together to form a sac (like the manner in which water molecules group together to form a bubble) ...
). Fission may occur spontaneously, as part of the vegetative cell cycle. Alternatively, it may proceed as a result of self-fertilization (
autogamy Autogamy, or self-fertilization, refers to the fusion of two gametes that come from one individual. Autogamy is predominantly observed in the form of self-pollination, a reproductive mechanism employed by many flowering plants. However, species o ...
), or it may follow conjugation, a sexual phenomenon in which ciliates of compatible mating types exchange genetic material. While conjugation is sometimes described as a form of reproduction, it is not directly connected with reproductive processes, and does not directly result in an increase in the number of individual ciliates or their progeny.


Conjugation

;Overview Ciliate conjugation is a sexual phenomenon that results in genetic recombination and nuclear reorganization within the cell. During conjugation, two ciliates of a compatible mating type form a bridge between their cytoplasms. The micronuclei undergo
meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately r ...
, the macronuclei disappear, and haploid micronuclei are exchanged over the bridge. In some ciliates (peritrichs, chonotrichs and some
suctoria Suctoria are ciliates that become sessile in their developed stage and then lose their redundant cilia. They feed by extracellular digestion. They were originally thought to feed by suction – hence their name. In fact, they use specialize ...
ns), conjugating cells become permanently fused, and one conjugant is absorbed by the other. In most ciliate groups, however, the cells separate after conjugation, and both form new macronuclei from their micronuclei. Conjugation and autogamy are always followed by fission. In many ciliates, such as ''Paramecium'', conjugating partners (gamonts) are similar or indistinguishable in size and shape. This is referred to as "isogamontic" conjugation. In some groups, partners are different in size and shape. This is referred to as "anisogamontic" conjugation. In sessile peritrichs, for instance, one sexual partner (the microconjugant) is small and mobile, while the other (macroconjugant) is large and
sessile Sessility, or sessile, may refer to: * Sessility (motility), organisms which are not able to move about * Sessility (botany), flowers or leaves that grow directly from the stem or peduncle of a plant * Sessility (medicine), tumors and polyps that ...
. ;Stages of conjugation In '' Paramecium caudatum'', the stages of conjugation are as follows (see diagram at right): # Compatible mating strains meet and partly fuse # The micronuclei undergo meiosis, producing four haploid micronuclei per cell. # Three of these micronuclei disintegrate. The fourth undergoes mitosis. # The two cells exchange a micronucleus. # The cells then separate. # The micronuclei in each cell fuse, forming a diploid micronucleus. # Mitosis occurs three times, giving rise to eight micronuclei. # Four of the new micronuclei transform into macronuclei, and the old macronucleus disintegrates. # Binary fission occurs twice, yielding four identical daughter cells.


DNA rearrangements (gene scrambling)

Ciliates contain two types of nuclei: somatic " macronucleus" and the germline "
micronucleus Micronucleus is the name given to the small nucleus that forms whenever a chromosome or a fragment of a chromosome is not incorporated into one of the daughter nuclei during cell division. It usually is a sign of genotoxic events and chromosomal i ...
". Only the DNA in the micronucleus is passed on during sexual reproduction (conjugation). On the other hand, only the DNA in the macronucleus is actively expressed and results in the phenotype of the organism. Macronuclear DNA is derived from micronuclear DNA by amazingly extensive DNA rearrangement and amplification. The macronucleus begins as a copy of the micronucleus. The micronuclear chromosomes are fragmented into many smaller pieces and amplified to give many copies. The resulting macronuclear chromosomes often contain only a single gene. In ''
Tetrahymena ''Tetrahymena'', a unicellular eukaryote, is a genus of free-living ciliates. The genus Tetrahymena is the most widely studied member of its phylum. It can produce, store and react with different types of hormones. Tetrahymena cells can recog ...
'', the micronucleus has 10 chromosomes (five per haploid genome), while the macronucleus has over 20,000 chromosomes. In addition, the micronuclear genes are interrupted by numerous "internal eliminated sequences" (IESs). During development of the macronucleus, IESs are deleted and the remaining gene segments, macronuclear destined sequences (MDSs), are spliced together to give the operational gene. ''Tetrahymena'' has about 6,000 IESs and about 15% of micronuclear DNA is eliminated during this process. The process is guided by small RNAs and epigenetic chromatin marks. In
spirotrich The spirotrichs are a large and diverse group of ciliate protozoa. They typically have prominent oral cilia in the form of a series of polykinetids, called the adoral zone of membranelles, beginning anterior to the oral cavity and running down ...
ciliates (such as '' Oxytricha''), the process is even more complex due to "gene scrambling": the MDSs in the micronucleus are often in different order and orientation from that in the macronuclear gene, and so in addition to deletion, DNA inversion and translocation are required for "unscrambling". This process is guided by long RNAs derived from the parental macronucleus. More than 95% of micronuclear DNA is eliminated during spirotrich macronuclear development.


Aging

ln clonal populations of ''Paramecium'', aging occurs over successive generations leading to a gradual loss of vitality, unless the cell line is revitalized by conjugation or
autogamy Autogamy, or self-fertilization, refers to the fusion of two gametes that come from one individual. Autogamy is predominantly observed in the form of self-pollination, a reproductive mechanism employed by many flowering plants. However, species o ...
. In ''Paramecium tetraurelia'', the clonally aging line loses vitality and expires after about 200 fissions, if the cell line is not rejuvenated by conjugation or self-fertilization. The basis for clonal aging was clarified by the transplantation experiments of Aufderheide in 1986 who demonstrated that the macronucleus, rather than the cytoplasm, is responsible for clonal aging. Additional experiments by Smith-Sonneborn, Holmes and Holmes, and Gilley and Blackburn demonstrated that, during clonal aging, DNA damage increases dramatically. Thus, DNA damage appears to be the cause of aging in ''P. tetraurelia''.


Fossil record

Until recently, the oldest ciliate fossils known were tintinnids from the
Ordovician period The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. Th ...
. In 2007, Li ''et al.'' published a description of fossil ciliates from the
Doushantuo Formation The Doushantuo Formation (formerly transcribed as Toushantuo or Toushantou, from ) is a geological formation in western Hubei, eastern Guizhou, southern Shaanxi, central Jiangxi, and other localities in China. It is known for the fossil Lagerst ...
, about 580 million years ago, in the
Ediacaran period The Ediacaran Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and th ...
. These included two types of tintinnids and a possible ancestral suctorian. A fossil '' Vorticella'' has been discovered inside a leech cocoon from the Triassic period, about 200 million years ago.


Phylogeny

According to the 2016 phylogenetic analysis, Mesodiniea is consistently found as the sister group to all other ciliates. Additionally, two big sub-groups are distinguished inside subphylum Intramacronucleata: SAL ( Spirotrichea+
Armophorea Armophorea is a class of ciliates in the subphylum Intramacronucleata. . It was first resolved in 2004 and comprises three orders: Metopida, Clevelandellida, and Armophorida. Previously members of this class were thought to be heterotrichs beca ...
+
Litostomatea The Litostomatea are a class of ciliates. The group consists of three subclasses: Haptoria, Trichostomatia and Rhynchostomatia. Haptoria includes mostly carnivorous forms such as '' Didinium'', a species of which preys primarily on the ciliate ' ...
) and CONthreeP or
Ventrata Ventrata is an infraphylum of ciliates inside the subphylum Intramacronucleata that unites the classes Phyllopharyngea, Colpodea, Nassophorea, Prostomatea, Plagiopylea and Oligohymenophorea. It is equivalent to the clade CONthreeP or Cont ...
(
Colpodea The Colpodea are a class of ciliate The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present ...
+ Oligohymenophorea+ Nassophorea+
Phyllopharyngea The Phyllopharyngea are a class of ciliates, some of which are extremely specialized. Motile cells typically have cilia restricted to the ventral surface, or some part thereof, arising from monokinetids with a characteristic ultrastructure. In ...
+ Plagiopylea+
Prostomatea Prostomatea is a class of ciliates. It includes the genera ''Coleps ''Coleps'' is a genus of ciliates in the class Prostomatea with barrel-shaped bodies surrounded by regularly arranged plates composed of calcium carbonate. Description S ...
). The class
Protocruziea Protocruziea is a class of ciliate The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present i ...
is found as the sister group to
Ventrata Ventrata is an infraphylum of ciliates inside the subphylum Intramacronucleata that unites the classes Phyllopharyngea, Colpodea, Nassophorea, Prostomatea, Plagiopylea and Oligohymenophorea. It is equivalent to the clade CONthreeP or Cont ...
/ CONthreeP. The class Cariacotrichea was excluded from the analysis, but it was originally established as part of Intramacronucleata.
The odontostomatids were identified in 2018 as its own class Odontostomatea, related to
Armophorea Armophorea is a class of ciliates in the subphylum Intramacronucleata. . It was first resolved in 2004 and comprises three orders: Metopida, Clevelandellida, and Armophorida. Previously members of this class were thought to be heterotrichs beca ...
.


Classification

Several different classification schemes have been proposed for the ciliates. The following scheme is based on a molecular
phylogenetic analysis In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
of up to four genes from 152 species representing 110 families: * Class Mesodiniea (e.g. '' Mesodinium'')


Subphylum Postciliodesmatophora

* Class Heterotrichea (e.g. ''
Stentor In Greek mythology, Stentor (Ancient Greek: Στέντωρ; ''gen''.: Στέντορος) was a herald of the Greek forces during the Trojan War. Mythology Stentor is mentioned briefly in Homer's ''Iliad'' in which Hera in the guise of Stent ...
'') * Class Karyorelictea


Subphylum Intramacronucleata

* Class
Armophorea Armophorea is a class of ciliates in the subphylum Intramacronucleata. . It was first resolved in 2004 and comprises three orders: Metopida, Clevelandellida, and Armophorida. Previously members of this class were thought to be heterotrichs beca ...
* Class Odontostomatea (e.g. '' Discomorphella'', '' Saprodinium'') * Class Cariacotrichea (only one species, '' Cariacothrix caudata'') * Class
Muranotrichea Muranotrichea is a class of free-living marine anaerobic ciliates, that, together with the classes Parablepharismea and Armophorea, form a major clade of obligate anaerobes within the SAL group ( Spirotrichea, Armophorea, and Litostomatea). Mu ...
* Class
Parablepharismea Parablepharismea is a class of free-living marine and brackish anaerobic ciliates that form a major clade of obligate {{wiktionary, obligate As an adjective, obligate means "by necessity" (antonym ''facultative'') and is used mainly in biology in ...
* Class
Colpodea The Colpodea are a class of ciliate The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present ...
(e.g. ''
Colpoda Colpoda is a genus of ciliates in the class Colpodea, order Tetrahymenida, Colpodida, and family Colpodidae. Description ''Colpoda'' are distinctly reniform (kidney-shaped) and are strongly convex on one side, concave on the other. The concav ...
'') * Class
Litostomatea The Litostomatea are a class of ciliates. The group consists of three subclasses: Haptoria, Trichostomatia and Rhynchostomatia. Haptoria includes mostly carnivorous forms such as '' Didinium'', a species of which preys primarily on the ciliate ' ...
** Subclass Haptoria (e.g. '' Didinium'') ** Subclass Rhynchostomatia ** Subclass Trichostomatia (e.g. '' Balantidium'') * Class Nassophorea * Class
Phyllopharyngea The Phyllopharyngea are a class of ciliates, some of which are extremely specialized. Motile cells typically have cilia restricted to the ventral surface, or some part thereof, arising from monokinetids with a characteristic ultrastructure. In ...
** Subclass Chonotrichia ** Subclass Cyrtophoria ** Subclass Rhynchodia ** Subclass
Suctoria Suctoria are ciliates that become sessile in their developed stage and then lose their redundant cilia. They feed by extracellular digestion. They were originally thought to feed by suction – hence their name. In fact, they use specialize ...
(e.g. '' Podophyra'') ** Subclass Synhymenia * Class Oligohymenophorea ** Subclass Apostomatia ** Subclass Astomatia ** Subclass Hymenostomatia (e.g. ''
Tetrahymena ''Tetrahymena'', a unicellular eukaryote, is a genus of free-living ciliates. The genus Tetrahymena is the most widely studied member of its phylum. It can produce, store and react with different types of hormones. Tetrahymena cells can recog ...
'') ** Subclass Peniculia (e.g. ''
Paramecium '' ''Paramecium'' ( , ; also spelled ''Paramoecium'') is a genus of eukaryotic, unicellular ciliates, commonly studied as a representative of the ciliate group. ''Paramecia'' are widespread in freshwater, brackish, and marine environments and a ...
'') ** Subclass
Peritrich The peritrichs (Latin: Peritrichia) are a large and distinctive group of ciliates. The peritrichs were first defined by Friedrich von Stein in 1859. Initially they were considered spirotrichs, then treated as a separate category, before receivi ...
ia (e.g. '' Vorticella'') ** Subclass Scuticociliatia * Class Plagiopylea * Class
Prostomatea Prostomatea is a class of ciliates. It includes the genera ''Coleps ''Coleps'' is a genus of ciliates in the class Prostomatea with barrel-shaped bodies surrounded by regularly arranged plates composed of calcium carbonate. Description S ...
(e.g. ''
Coleps ''Coleps'' is a genus of ciliates in the class Prostomatea with barrel-shaped bodies surrounded by regularly arranged plates composed of calcium carbonate. Description Species of ''Coleps'' can grow up to 250 µm in length, but are usua ...
'') * Class
Protocruziea Protocruziea is a class of ciliate The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present i ...
* Class Spirotrichea **Subclass Choreotrichia **Subclass Euplotia **Subclass Hypotrichia **Subclass Licnophoria **Subclass Oligotrichia **Subclass Phacodiniidea **Subclass Protohypotrichia


Other

Some old classifications included Opalinidae in the ciliates. The fundamental difference between multiciliate flagellates (e.g., hemimastigids, '' Stephanopogon'', '' Multicilia'', opalines) and ciliates is the presence of macronuclei in ciliates alone.


Pathogenicity

The only member of the ciliate phylum known to be pathogenic to humans is '' Balantidium coli'', which causes the disease balantidiasis. It is not pathogenic to the domestic pig, the primary reservoir of this pathogen.


References


Further reading

* * *


External links

* {{Authority control