Pneumocystis jirovecii
Encyclopedia
Pneumocystis jirovecii is a yeast-like fungus
of the genus Pneumocystis. The causative organism of Pneumocystis pneumonia
, it is an important human pathogen, particularly among immunocompromised hosts. Prior to its discovery as a human-specific pathogen, P. jirovecii was known as P. carinii.
(multilobed) and closely associated with host cells. Globular cysts eventually form that have a thicker wall. Within these ascus
-like cysts, eight spores form which are released through rupture of the cyst wall. The cysts often collapse forming crescent-shaped bodies visible in stained tissue. It is not known for certain if meiosis
takes place within the cysts, or what the genetic status is of the various cell types.
is an important disease of immunocompromised humans, particularly patients with HIV
, but also patients with a severely suppressed immune system (for example, following a bone marrow transplant
).
In humans with a normal immune system, it is an extremely common silent infection.
-specific. In 1976 the name Pneumocystis jiroveci was proposed for the first time, to distinguish the organism found in humans from variants of Pneumocystis in other animals. The organism was named thus in honor of Otto Jirovec
, who described Pneumocystis pneumonia in humans in 1952. After DNA analysis showed significant differences in the human variant, the proposal was made again in 1999 and has come into common use.
The name was spelt according to the International Code of Zoological Nomenclature
since the organism was believed to be a protozoan. After it became clear that it was a fungus, the name was changed to Pneumocystis jirovecii, according to the International Code of Botanical Nomenclature (ICBN), which requires the name to be spelled with double ii.
Both spellings are commonly used, but according to the ICBN P. jirovecii has priority.
A change in the ICBN in 2005 now recognizes the validity of the 1976 publication, making the 1999 proposal redundant, and cites Pneumocystis and P. jirovecii as examples of the change in ICBN Article 45, Ex 8. The name P. jirovecii is typified (both lectotypified and epitypified) by samples from human autopsies dating from the 1960s.
The term PCP, which was widely used by practitioners and patients, has been retained for convenience, with the rationale that it now stands for the more general Pneumocystis pneumonia rather than Pneumocystis carinii pneumonia.
The name P. carinii is incorrect for the human variant, but still describes the species found in rats and that name is typified by an isolate from rats.
in 1909 who discovered it in experimental animals but confused it with part of the life-cycle of Trypanosoma cruzi
(causal agent of Chagas disease
) and later called both organisms 'Schizotrypanum cruzi' a form of trypanosome
infecting humans. The rediscovery of Pneumocystis cysts was reported by Antonio Carini
in 1910 also in Brazil
. The genus was again discovered in 1912 by Delanoë and Delanoë this time at the Pasteur Institute
in Paris, France who found it in rats and who proposed the genus and species name Pneumocystis carinii after Carini.
Pneumocystis was redescribed as a human pathogen in 1942 by two Dutch investigators, van der Meer and Brug who found it in three new cases: a 3-month-old infant with congenital heart disease and in 2 of 104 autopsy
cases - a 4-month-old infant and a 21-year-old adult. There being only one described species
in the genus
, they considered the human parasite to be P. carinii. Nine years later (1951) Dr. Josef Vanek at Charles University in Prague
in Prague
, Czechoslovakia
showed in a study of lung sections from sixteen children that the organism labelled "P. carinii" was the causative agent of pneumonia in these children. The following year (1952) Jírovec reported "P. carinii" as the cause of interstitial pneumonia in neonates. Following the realization that Pneumocystis from humans could not infect experimental animals such as rats, and that the rat form of Pneumocystis differed physiologically and had different antigen
ic properties, Frenkel was the first to recognize the human pathogen as a distinct species. He named it Pneumocystis jirovecii (see nomenclature above). There has been controversy over the relabeling of P. carinii in humans as P. jirovecii, which is why both names still appear in publications. However, only the name P. jirovecii is used exclusively for the human pathogen, whereas the name P. carinii has had a broader application to many species. Frenkel and those before him, believed that all Pneumocystis were protozoans, but soon afterwards evidence began accumulating that Pneumocystis was a fungal genus. Recent studies show it to be an unusual, in some ways a primitive genus of Ascomycota
, related to a group of yeast
s. Every tested primate
, including humans, appears to have their own type of Pneumocystis that is incapable of cross-infecting other host species and has co-evolved
with each mammal
species. Currently only 5 species have been formally named: P. jirovecii from humans, P. carinii as originally named from rats, P. murina from mice, P. wakefieldiae also from rats, and P. oryctolagi from rabbits.
Historical and even recent reports of P. carinii from humans are based upon older classifications (still used by many, or those still debating the recognition of distinct species in the genus Pneumocystis) which does not mean that the true P. carinii from rats actually infects humans. In an intermediate classification system, the various taxa in different mammals have been called formae speciales or forms. For example the human "form" was called Pneumocystis carinii f. [or f. sp.] hominis, while the original rat infecting form was called Pneumocystis carinii f. [or f. sp.] carinii. This terminology is still used by some researchers. The species of Pneumocystis originally seen by Chagas have not yet been named as distinct species. Many other undescribed species presumably exist and those that have been detected in many mammals are only known from molecular sample detection from lung tissue or fluids, rather than by direct physical observation. As of yet, they are cryptic taxa.
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...
of the genus Pneumocystis. The causative organism of Pneumocystis pneumonia
Pneumocystis pneumonia
Pneumocystis pneumonia or pneumocystosis is a form of pneumonia, caused by the yeast-like fungus Pneumocystis jirovecii...
, it is an important human pathogen, particularly among immunocompromised hosts. Prior to its discovery as a human-specific pathogen, P. jirovecii was known as P. carinii.
Life-cycle
The complete life-cycles of any of the species of Pneumocystis are not known, but presumably all resemble the others in the genus. The terminology follows zoological terms, rather than mycological terms, reflecting the initial misdetermination as a protozoan parasite. It is an obligate intracellular pathogen. All stages are found in lungs and because they cannot be cultured ex-vivo, direct observation of living Pneumocystis is difficult. The trophozoite stage is thought to be equivalent to the so-called vegetative state of other species (such as Schizosaccharomyces pombe) which, like Pneumocystis belong to the Taphrinomycotina branch of the fungal kingdom. The Trophozoite stage is single-celled and appears amoeboidAmoeboid
Amoeboids are single-celled life-forms characterized by an irregular shape."Amoeboid" and "amœba" are often used interchangeably even by biologists, and especially refer to a creature moving by using pseudopodia. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to...
(multilobed) and closely associated with host cells. Globular cysts eventually form that have a thicker wall. Within these ascus
Ascus
An ascus is the sexual spore-bearing cell produced in ascomycete fungi. On average, asci normally contain eight ascospores, produced by a meiotic cell division followed, in most species, by a mitotic cell division. However, asci in some genera or species can number one , two, four, or multiples...
-like cysts, eight spores form which are released through rupture of the cyst wall. The cysts often collapse forming crescent-shaped bodies visible in stained tissue. It is not known for certain if meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
takes place within the cysts, or what the genetic status is of the various cell types.
Medical relevance
Pneumocystis pneumoniaPneumocystis pneumonia
Pneumocystis pneumonia or pneumocystosis is a form of pneumonia, caused by the yeast-like fungus Pneumocystis jirovecii...
is an important disease of immunocompromised humans, particularly patients with HIV
HIV
Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...
, but also patients with a severely suppressed immune system (for example, following a bone marrow transplant
Bone marrow transplant
Hematopoietic stem cell transplantation is the transplantation of multipotent hematopoietic stem cell or blood, usually derived from bone marrow, peripheral blood stem cells, or umbilical cord blood...
).
In humans with a normal immune system, it is an extremely common silent infection.
Nomenclature
At first, the name Pneumocystis carinii was applied to the organisms found in both rats and humans, as it was not yet known that the parasite was hostHost (biology)
In biology, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...
-specific. In 1976 the name Pneumocystis jiroveci was proposed for the first time, to distinguish the organism found in humans from variants of Pneumocystis in other animals. The organism was named thus in honor of Otto Jirovec
Otto Jirovec
Otto Jírovec was a Czech professor of parasitology and protozoology.A significant fungus parasite of humans, Pneumocystis jirovecii, is named in his honour. Pneumocystis jirovecii Otto Jírovec (January 31, 1907–March 7, 1972) was a Czech professor of parasitology and protozoology.A significant...
, who described Pneumocystis pneumonia in humans in 1952. After DNA analysis showed significant differences in the human variant, the proposal was made again in 1999 and has come into common use.
The name was spelt according to the International Code of Zoological Nomenclature
International Code of Zoological Nomenclature
The International Code of Zoological Nomenclature is a widely accepted convention in zoology that rules the formal scientific naming of organisms treated as animals...
since the organism was believed to be a protozoan. After it became clear that it was a fungus, the name was changed to Pneumocystis jirovecii, according to the International Code of Botanical Nomenclature (ICBN), which requires the name to be spelled with double ii.
Both spellings are commonly used, but according to the ICBN P. jirovecii has priority.
A change in the ICBN in 2005 now recognizes the validity of the 1976 publication, making the 1999 proposal redundant, and cites Pneumocystis and P. jirovecii as examples of the change in ICBN Article 45, Ex 8. The name P. jirovecii is typified (both lectotypified and epitypified) by samples from human autopsies dating from the 1960s.
The term PCP, which was widely used by practitioners and patients, has been retained for convenience, with the rationale that it now stands for the more general Pneumocystis pneumonia rather than Pneumocystis carinii pneumonia.
The name P. carinii is incorrect for the human variant, but still describes the species found in rats and that name is typified by an isolate from rats.
Pneumocystis genome
Pneumocystis species cannot be grown in culture. Therefore, there is a limitation to the availability of the human disease causing agent, P. jirovecii. Hence, investigation of the whole genome of a Pneumocystis is largely based upon true P. carinii available from experimental rats which can be maintained with infections. Genetic material of other species, such as P. jirovecii can be compared to the genome of P. carinii.History
The earliest report of this genus appears to have been that of Carlos ChagasCarlos Chagas
Carlos Justiniano Ribeiro Chagas, or Carlos Chagas , was a Brazilian sanitary physician, scientist and bacteriologist who worked as a clinician and researcher. He discovered Chagas disease, also called American trypanosomiasis in 1909, while working at the Oswaldo Cruz Institute in Rio de Janeiro...
in 1909 who discovered it in experimental animals but confused it with part of the life-cycle of Trypanosoma cruzi
Trypanosoma cruzi
Trypanosoma cruzi is a species of parasitic euglenoid trypanosomes. This species causes the trypanosomiasis diseases in humans and animals in America...
(causal agent of Chagas disease
Chagas disease
Chagas disease is a tropical parasitic disease caused by the flagellate protozoan Trypanosoma cruzi. T. cruzi is commonly transmitted to humans and other mammals by an insect vector, the blood-sucking insects of the subfamily Triatominae most commonly species belonging to the Triatoma, Rhodnius,...
) and later called both organisms 'Schizotrypanum cruzi' a form of trypanosome
Trypanosoma
Trypanosoma is a genus of kinetoplastids , a monophyletic group of unicellular parasitic flagellate protozoa. The name is derived from the Greek trypano and soma because of their corkscrew-like motion. All trypanosomes are heteroxenous and are transmitted via a vector...
infecting humans. The rediscovery of Pneumocystis cysts was reported by Antonio Carini
Antonio Carini
Antonio Carini was an Italian physician, bacteriologist and professor. He worked in the public health services of São Paulo, Brazil for over forty years....
in 1910 also in Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
. The genus was again discovered in 1912 by Delanoë and Delanoë this time at the Pasteur Institute
Pasteur Institute
The Pasteur Institute is a French non-profit private foundation dedicated to the study of biology, micro-organisms, diseases, and vaccines. It is named after Louis Pasteur, who made some of the greatest breakthroughs in modern medicine at the time, including pasteurization and vaccines for anthrax...
in Paris, France who found it in rats and who proposed the genus and species name Pneumocystis carinii after Carini.
Pneumocystis was redescribed as a human pathogen in 1942 by two Dutch investigators, van der Meer and Brug who found it in three new cases: a 3-month-old infant with congenital heart disease and in 2 of 104 autopsy
Autopsy
An autopsy—also known as a post-mortem examination, necropsy , autopsia cadaverum, or obduction—is a highly specialized surgical procedure that consists of a thorough examination of a corpse to determine the cause and manner of death and to evaluate any disease or injury that may be present...
cases - a 4-month-old infant and a 21-year-old adult. There being only one described species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
in the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
, they considered the human parasite to be P. carinii. Nine years later (1951) Dr. Josef Vanek at Charles University in Prague
Charles University in Prague
Charles University in Prague is the oldest and largest university in the Czech Republic. Founded in 1348, it was the first university in Central Europe and is also considered the earliest German university...
in Prague
Prague
Prague is the capital and largest city of the Czech Republic. Situated in the north-west of the country on the Vltava river, the city is home to about 1.3 million people, while its metropolitan area is estimated to have a population of over 2.3 million...
, Czechoslovakia
Czechoslovakia
Czechoslovakia or Czecho-Slovakia was a sovereign state in Central Europe which existed from October 1918, when it declared its independence from the Austro-Hungarian Empire, until 1992...
showed in a study of lung sections from sixteen children that the organism labelled "P. carinii" was the causative agent of pneumonia in these children. The following year (1952) Jírovec reported "P. carinii" as the cause of interstitial pneumonia in neonates. Following the realization that Pneumocystis from humans could not infect experimental animals such as rats, and that the rat form of Pneumocystis differed physiologically and had different antigen
Antigen
An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...
ic properties, Frenkel was the first to recognize the human pathogen as a distinct species. He named it Pneumocystis jirovecii (see nomenclature above). There has been controversy over the relabeling of P. carinii in humans as P. jirovecii, which is why both names still appear in publications. However, only the name P. jirovecii is used exclusively for the human pathogen, whereas the name P. carinii has had a broader application to many species. Frenkel and those before him, believed that all Pneumocystis were protozoans, but soon afterwards evidence began accumulating that Pneumocystis was a fungal genus. Recent studies show it to be an unusual, in some ways a primitive genus of Ascomycota
Ascomycota
The Ascomycota are a Division/Phylum of the kingdom Fungi, and subkingdom Dikarya. Its members are commonly known as the Sac fungi. They are the largest phylum of Fungi, with over 64,000 species...
, related to a group of yeast
Yeast
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...
s. Every tested primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...
, including humans, appears to have their own type of Pneumocystis that is incapable of cross-infecting other host species and has co-evolved
Co-evolution
In biology, coevolution is "the change of a biological object triggered by the change of a related object." Coevolution can occur at many biological levels: it can be as microscopic as correlated mutations between amino acids in a protein, or as macroscopic as covarying traits between different...
with each mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
species. Currently only 5 species have been formally named: P. jirovecii from humans, P. carinii as originally named from rats, P. murina from mice, P. wakefieldiae also from rats, and P. oryctolagi from rabbits.
Historical and even recent reports of P. carinii from humans are based upon older classifications (still used by many, or those still debating the recognition of distinct species in the genus Pneumocystis) which does not mean that the true P. carinii from rats actually infects humans. In an intermediate classification system, the various taxa in different mammals have been called formae speciales or forms. For example the human "form" was called Pneumocystis carinii f. [or f. sp.] hominis, while the original rat infecting form was called Pneumocystis carinii f. [or f. sp.] carinii. This terminology is still used by some researchers. The species of Pneumocystis originally seen by Chagas have not yet been named as distinct species. Many other undescribed species presumably exist and those that have been detected in many mammals are only known from molecular sample detection from lung tissue or fluids, rather than by direct physical observation. As of yet, they are cryptic taxa.