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{{short description|Microparasite that can grow and reproduce in a host cell}}
'''Intracellular parasites''' are [[microparasite]]s that are capable of growing and reproducing inside the cells of a [[Host (biology)|host]].
'''Intracellular parasites''' are [[microparasite]]s that are capable of growing and reproducing inside the cells of a [[Host (biology)|host]].<ref>{{cite journal |last1=Horta |first1=Maria Fátima |last2=Andrade |first2=Luciana Oliveira |last3=Martins-Duarte |first3=Érica Santos |last4=Castro-Gomes |first4=Thiago |title=Cell invasion by intracellular parasites – the many roads to infection |journal=Journal of Cell Science |date=15 February 2020 |volume=133 |issue=4 |doi=10.1242/jcs.232488|pmid=32079731 |doi-access=free }}</ref> They are also called '''intracellular''' [[pathogen]]s.<ref name=":1"/><ref>{{cite journal |last1=Thakur |first1=A |last2=Mikkelsen |first2=H |last3=Jungersen |first3=G |title=Intracellular Pathogens: Host Immunity and Microbial Persistence Strategies. |journal=Journal of Immunology Research |date=2019 |volume=2019 |pages=1356540 |doi=10.1155/2019/1356540 |doi-access=free |pmid=31111075}}</ref>


== Types of Parasites ==
== Types ==
There are two main types of intracellular parasites: Facultative and Obligate.<ref name=":1">{{Cite journal |last1=Leon-Sicairos |first1=Nidia |last2=Reyes-Cortes |first2=Ruth |last3=Guadrón-Llanos |first3=Alma M. |last4=Madueña-Molina |first4=Jesús |last5=Leon-Sicairos |first5=Claudia |last6=Canizalez-Román |first6=Adrian |date=2015 |title=Strategies of Intracellular Pathogens for Obtaining Iron from the Environment |journal=BioMed Research International |language=en |volume=2015 |pages=1–17 |doi=10.1155/2015/476534 |issn=2314-6133|doi-access=free |pmid=26120582 |pmc=4450229 }}</ref>
There are two main types of intracellular parasites: Facultative and Obligate.


Facultative intracellular parasites are capable of living and reproducing in or outside of host cells. Obligate intracellular parasites, on the other hand, need a host cell to live and reproduce. Many of these types of cells require specialized host types, and invasion of host cells occurs in different ways.
Facultative intracellular parasites are capable of living and reproducing in or outside of host cells. Obligate intracellular parasites, on the other hand, need a host cell to live and reproduce. Many of these types of cells require specialized host types, and invasion of host cells occurs in different ways.<ref name=":1" />


===Facultative===
===Facultative===
Line 10: Line 11:


Bacterial examples include:
Bacterial examples include:
{{div col|colwidth=22em|content=
*''[[Bartonella henselae]]''<ref>{{cite web|url=http://www.vetmed.auburn.edu/wp-content/uploads/2015/03/Bartonella-henselae.pdf|title=Bartonella henselae}}</ref>
* ''[[Bartonella henselae]]''<ref name=":2">{{cite web|url=http://www.vetmed.auburn.edu/wp-content/uploads/2015/03/Bartonella-henselae.pdf|title=Bartonella henselae}}</ref>
*''[[Francisella tularensis]]''
* ''[[Francisella tularensis]]''
*''[[Listeria monocytogenes]]''<ref>{{Cite journal|last=Dramsi|first=Shaynoor|last2=Cossart|first2=Pascale|date=2002-03-18|title=Listeriolysin O|journal=The Journal of Cell Biology|volume=156|issue=6|pages=943–946|doi=10.1083/jcb.200202121|issn=0021-9525|pmc=2173465|pmid=11901162}}</ref>
*[[Salmonella typhi|''Salmonella'' Typhi]]<ref>{{Cite journal | last1 = Jantsch | first1 = J. | last2 = Chikkaballi | first2 = D. | last3 = Hensel | first3 = M. | title = Cellular aspects of immunity to intracellular Salmonella enterica | doi = 10.1111/j.1600-065X.2010.00981.x | journal = Immunological Reviews | volume = 240 | issue = 1 | pages = 185–195 | year = 2011 | pmid = 21349094}}</ref>
* ''[[Listeria monocytogenes]]''<ref>{{Cite journal|last1=Dramsi|first1=Shaynoor|last2=Cossart|first2=Pascale|date=2002-03-18|title=Listeriolysin O|journal=The Journal of Cell Biology|volume=156|issue=6|pages=943–946|doi=10.1083/jcb.200202121|issn=0021-9525|pmc=2173465|pmid=11901162}}</ref>
* [[Salmonella typhi|''Salmonella'' Typhi]]<ref>{{Cite journal | last1 = Jantsch | first1 = J. | last2 = Chikkaballi | first2 = D. | last3 = Hensel | first3 = M. | title = Cellular aspects of immunity to intracellular Salmonella enterica | doi = 10.1111/j.1600-065X.2010.00981.x | journal = Immunological Reviews | volume = 240 | issue = 1 | pages = 185–195 | year = 2011 | pmid = 21349094| s2cid = 19344119 }}</ref>
⚫
*''[[Brucella]]''
*''[[Legionella]]''
* ''[[Brucella]]''
* ''[[Escherichia coli]]'', specifically uropathogenic ones<ref name=Cossart/>
*''[[Mycobacterium]]''
*''[[Nocardia]]''
* ''[[Legionella]]''
*''[[Neisseria]]''
* ''[[Mycobacterium]]''
⚫
* ''[[Nocardia]]''
⚫
*''[[Rhodococcus equi]]''<ref>{{Cite journal | last1 = Kelly | first1 = B. G. | last2 = Wall | first2 = D. M. | last3 = Boland | first3 = C. A. | last4 = Meijer | first4 = W. G. | title = Isocitrate lyase of the facultative intracellular pathogen Rhodococcus equi | journal = Microbiology | volume = 148 | issue = Pt 3 | pages = 793–798 | year = 2002 | pmid = 11882714 | doi=10.1099/00221287-148-3-793| doi-access = free }}</ref>
*''[[Yersinia]]''
* ''[[Neisseria]]''
⚫
* ''[[Rhodococcus equi]]''<ref>{{Cite journal | last1 = Kelly | first1 = B. G. | last2 = Wall | first2 = D. M. | last3 = Boland | first3 = C. A. | last4 = Meijer | first4 = W. G. | title = Isocitrate lyase of the facultative intracellular pathogen Rhodococcus equi | journal = Microbiology | volume = 148 | issue = Pt 3 | pages = 793–798 | year = 2002 | pmid = 11882714 | doi=10.1099/00221287-148-3-793| doi-access = free }}</ref>
⚫
*''[[Staphylococcus aureus]]''<ref>{{Cite journal | last = Bravo-Santano | title = Intracellular Staphylococcus aureus Modulates Host Central Carbon Metabolism To Activate Autophagy | journal = American Society for Microbiology | doi=10.1128/mSphere.00374-18 | display-authors = etal| doi-access = free | volume = 3 | pages = e00374–18 }}</ref>
* ''[[Yersinia]]''
⚫
* ''[[Staphylococcus aureus]]''<ref>{{Cite journal | last = Bravo-Santano | title = Intracellular Staphylococcus aureus Modulates Host Central Carbon Metabolism To Activate Autophagy | journal = American Society for Microbiology | year = 2018 | doi=10.1128/mSphere.00374-18 | display-authors = etal| doi-access = free | volume = 3 | issue = 4 | pages = e00374–18 | pmid = 30089650 | pmc = 6083095 }}</ref>
* ''[[Mycobacterium tuberculosis]]'' (within the macrophages)<ref>{{Cite journal |last1=Daffé |first1=M. |last2=Etienne |first2=G. |date=June 1999 |title=The capsule of Mycobacterium tuberculosis and its implications for pathogenicity |url=https://linkinghub.elsevier.com/retrieve/pii/S0962847998902003 |journal=Tubercle and Lung Disease |language=en |volume=79 |issue=3 |pages=153–169 |doi=10.1054/tuld.1998.0200|pmid=10656114 }}</ref>
}}


Fungal examples include:
Fungal examples include:
{{div col|colwidth=22em|content=
*''[[Histoplasma capsulatum]]''.<ref name="pmid11082066">{{cite journal |vauthors=Sebghati TS, Engle JT, Goldman WE |title=Intracellular parasitism by Histoplasma capsulatum: fungal virulence and calcium dependence |journal=Science |volume=290 |issue=5495 |pages=1368–72 |date=November 2000 |pmid=11082066 |doi= 10.1126/science.290.5495.1368|bibcode=2000Sci...290.1368S }}</ref>
* ''[[Histoplasma capsulatum]]''.<ref name="pmid11082066">{{cite journal |vauthors=Sebghati TS, Engle JT, Goldman WE |title=Intracellular parasitism by Histoplasma capsulatum: fungal virulence and calcium dependence |journal=Science |volume=290 |issue=5495 |pages=1368–72 |date=November 2000 |pmid=11082066 |doi= 10.1126/science.290.5495.1368|bibcode=2000Sci...290.1368S }}</ref>
*''[[Cryptococcus neoformans]]''<ref>{{Cite journal | last1 = Alvarez | first1 = M. | last2 = Burns | first2 = T. | last3 = Luo | first3 = Y. | last4 = Pirofski | first4 = L. A. |authorlink4=Liise-anne Pirofski | last5 = Casadevall | first5 = A. | title = The outcome of Cryptococcus neoformans intracellular pathogenesis in human monocytes | journal = BMC Microbiology | volume = 9 | pages = 51 | year = 2009 | pmid = 19265539 | pmc = 2670303 | doi = 10.1186/1471-2180-9-51}}</ref>
* ''[[Cryptococcus neoformans]]''<ref>{{Cite journal | last1 = Alvarez | first1 = M. | last2 = Burns | first2 = T. | last3 = Luo | first3 = Y. | last4 = Pirofski | first4 = L. A. |authorlink4=Liise-anne Pirofski | last5 = Casadevall | first5 = A. | title = The outcome of Cryptococcus neoformans intracellular pathogenesis in human monocytes | journal = BMC Microbiology | volume = 9 | pages = 51 | year = 2009 | pmid = 19265539 | pmc = 2670303 | doi = 10.1186/1471-2180-9-51 | doi-access = free }}</ref>
* ''[[Blastomyces dermatitidis]]''{{Efn|Only in animal study at initial stages of infection.<ref>{{Cite journal |last1=Sterkel |first1=Alana K. |last2=Mettelman |first2=Robert |last3=Wüthrich |first3=Marcel |last4=Klein |first4=Bruce S. |date=2015-02-15 |title=The unappreciated intracellular lifestyle of Blastomyces dermatitidis |journal=Journal of Immunology |volume=194 |issue=4 |pages=1796–1805 |doi=10.4049/jimmunol.1303089 |issn=1550-6606 |pmc=4373353 |pmid=25589071}}</ref>}}
}}


=== Obligate ===
=== Obligate ===
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'''Obligate intracellular parasites''' cannot reproduce outside their host cell, meaning that the parasite's [[reproduction]] is entirely reliant on [[intracellular]] resources.
'''Obligate intracellular parasites''' cannot reproduce outside their host cell, meaning that the parasite's [[reproduction]] is entirely reliant on [[intracellular]] resources.


All [[virus]]es are obligate intracellular parasites.
Obligate intracellular parasites of [[human]]s include:


Bacterial examples (that affect humans) include:
*[[Virus]]es
⚫
* [[Chlamydia (bacterium)|''Chlamydia'']], and closely related species.<ref name="pmid8979345">{{cite journal |vauthors=Amann R, Springer N, Schönhuber W, Ludwig W, Schmid EN, Müller KD, Michel R|title=Obligate intracellular bacterial parasites of acanthamoebae related to Chlamydia spp |journal=Applied and Environmental Microbiology |volume=63 |issue=1 |pages=115–21 |date=January 1997 |pmid=8979345 |pmc=168308 |doi= 10.1128/AEM.63.1.115-121.1997|bibcode=1997ApEnM..63..115A }}</ref>
*Certain [[bacteria]], including:
* ''[[Rickettsia]]''
⚫
**[[Chlamydia (bacterium)|''Chlamydia'']], and closely related species.<ref name="pmid8979345">{{cite journal |vauthors=Amann R, Springer N, Schönhuber W, Ludwig W, Schmid EN, Müller KD, Michel R|title=Obligate intracellular bacterial parasites of acanthamoebae related to Chlamydia spp |journal=Applied and Environmental Microbiology |volume=63 |issue=1 |pages=115–21 |date=January 1997 |pmid=8979345 |pmc=168308 |doi= 10.1128/AEM.63.1.115-121.1997}}</ref>
**''[[Rickettsia]]''
* ''[[Coxiella (bacterium)|Coxiella]]''
⚫
* Certain species of ''[[Mycobacterium]]'' such as ''[[Mycobacterium leprae]]'', that survive in [[phagocyte]]s
**''[[Coxiella (bacterium)|Coxiella]]''
* ''[[Anaplasma phagocytophilum]]''{{Efn|Some sources say that it's parasite, but some not.}}<ref>{{Cite journal |last1=Foley |first1=Janet E. |last2=Nieto |first2=Nathan C. |last3=Barbet |first3=Anthony |last4=Foley |first4=Patrick |date=2009-12-15 |title=Antigen diversity in the parasitic bacterium Anaplasma phagocytophilum arises from selectively-represented, spatially clustered functional pseudogenes |journal=PLOS ONE |volume=4 |issue=12 |pages=e8265 |doi=10.1371/journal.pone.0008265 |issn=1932-6203 |pmc=2789410 |pmid=20016821|doi-access=free |bibcode=2009PLoSO...4.8265F }}</ref>
⚫
** Certain species of ''[[Mycobacterium]]'' such as ''[[Mycobacterium leprae]]''
*Certain [[protozoa]], including:
⚫
**[[Apicomplexan]]s (''[[Plasmodium]]'' spp., ''[[Toxoplasma gondii]]'' and ''[[Cryptosporidium parvum]]''<ref>{{Cite journal | doi = 10.1016/j.ijpara.2003.10.001 | last1 = Deng | first1 = M. | last2 = Lancto | first2 = C. A. | last3 = Abrahamsen | first3 = M. S. | title = Cryptosporidium parvum regulation of human epithelial cell gene expression | journal = International Journal for Parasitology | volume = 34 | issue = 1 | pages = 73–82 | year = 2004 | pmid = 14711592}}</ref>)
⚫
**[[Trypanosomatid]]s (''[[Leishmania]]'' spp. and ''[[Trypanosoma cruzi]]'')
*Certain [[fungi]]
⚫
**''[[Pneumocystis jirovecii]]''<ref>{{cite book|page=28|title=Rook's Textbook of Dermatology. Vol. 4|editor=David Anthony Burns|editor2=Stephen M. Breathnach|editor3=Neil H. Cox|editor4=Christopher E. M. Griffiths|place=Chichester|publisher=Wiley-Blackwell|year=2010|edition=8th|isbn=978-1-4051-6169-5}}</ref>


[[Protozoa]]n examples (that affect humans) include:
⚫
The [[mitochondria]] in eukaryotic cells may also have originally been such parasites, but ended up forming a [[mutualism (biology)|mutualistic]] relationship ([[endosymbiotic theory]]).<ref>{{cite journal | author=Lynn Sagan | year=1967 | title=On the origin of mitosing cells | journal=[[J Theor Biol]] | volume=14 | issue=3 | pages=255–274 | pmid=11541392 | doi=10.1016/0022-5193(67)90079-3}}</ref>
⚫
* [[Apicomplexan]]s (''[[Plasmodium]]'' spp., ''[[Toxoplasma gondii]]'' and ''[[Cryptosporidium parvum]]''<ref>{{Cite journal | doi = 10.1016/j.ijpara.2003.10.001 | last1 = Deng | first1 = M. | last2 = Lancto | first2 = C. A. | last3 = Abrahamsen | first3 = M. S. | title = Cryptosporidium parvum regulation of human epithelial cell gene expression | journal = International Journal for Parasitology | volume = 34 | issue = 1 | pages = 73–82 | year = 2004 | pmid = 14711592}}</ref>)
⚫
* [[Trypanosomatid]]s (''[[Leishmania]]'' spp. and ''[[Trypanosoma cruzi]]'')


[[Fungal]] examples (that affect humans) include:
⚫
Study of obligate pathogens is difficult because they cannot usually be reproduced outside the host. However, in 2009 scientists reported a technique allowing the [[Q-fever]] pathogen ''[[Coxiella burnetii]]'' to grow in an [[axenic]] culture and suggested the technique may be useful for study of other pathogens.<ref>{{cite journal |title=Host cell-free growth of the Q fever bacterium Coxiella burnetii |vauthors=Omsland A, Cockrell DC, Howe D, Fischer ER, Virtaneva K, Sturdevant DE, Porcella SF, Heinzen RA|journal=Proceedings of the National Academy of Sciences USA |date=March 17, 2009|volume=106 |issue=11 |pages=4430–4 |pmid=19246385 |doi=10.1073/pnas.0812074106 |pmc=2657411| bibcode=2009PNAS..106.4430O }}</ref>
⚫
* ''[[Pneumocystis jirovecii]]''<ref>{{cite book|page=28|title=Rook's Textbook of Dermatology. Vol. 4|editor=David Anthony Burns|editor2=Stephen M. Breathnach|editor3=Neil H. Cox|editor4=Christopher E. M. Griffiths|place=Chichester|publisher=Wiley-Blackwell|year=2010|edition=8th|isbn=978-1-4051-6169-5}}</ref>


⚫
The [[mitochondria]] in eukaryotic cells may also have originally been such parasites, but ended up forming a [[mutualism (biology)|mutualistic]] relationship ([[endosymbiotic theory]]).<ref>{{cite journal | author=Lynn Sagan | year=1967 | title=On the origin of mitosing cells | journal=[[J Theor Biol]] | volume=14 | issue=3 | pages=255–274 | pmid=11541392 | doi=10.1016/0022-5193(67)90079-3| bibcode=1967JThBi..14..225S }}</ref>
=== Exceptions ===

[[Polypodium (animal)|Polypodium]] is a metazoan intracellular parasite, distinct from most if not all other intracellular parasites for this reason.
⚫
Study of obligate pathogens is difficult because they cannot usually be reproduced outside the host. However, in 2009 scientists reported a technique allowing the [[Q-fever]] pathogen ''[[Coxiella burnetii]]'' to grow in an [[axenic]] culture and suggested the technique may be useful for study of other pathogens.<ref>{{cite journal |title=Host cell-free growth of the Q fever bacterium Coxiella burnetii |vauthors=Omsland A, Cockrell DC, Howe D, Fischer ER, Virtaneva K, Sturdevant DE, Porcella SF, Heinzen RA|journal=Proceedings of the National Academy of Sciences USA |date=March 17, 2009|volume=106 |issue=11 |pages=4430–4 |pmid=19246385 |doi=10.1073/pnas.0812074106 |pmc=2657411| bibcode=2009PNAS..106.4430O |doi-access=free}}</ref>

=== Unusual examples ===
[[Polypodium (animal)|''Polypodium'']] is a rare metazoan (animal) intracellular parasite, distinct from most if not all other intracellular parasites for this reason. It lives inside the unfertilized egg cells (oocytes) of fish.<ref name="Evans 2008">{{Cite journal |author1=Evans N. M. |author2=Lindner A. |author3=Raikova E. V. |author4=Collins A. G. |author5=Cartwright P. |title=Phylogenetic placement of the enigmatic parasite, ''Polypodium hydriforme'', within the Phylum Cnidaria |journal=[[BMC Evolutionary Biology]] |volume=8 |pages=139 |year=2008 |issue=1 |pmid=18471296 |pmc=2396633 |doi=10.1186/1471-2148-8-139 |url= |doi-access=free |bibcode=2008BMCEE...8..139E }}</ref>


== Invasion ==
== Invasion ==
When an intracellular parasite goes to enter a host cell, it is particular about the type of host cell. This is because most intracellular parasites are able to infect a few different cell types.<ref name=":0">{{Cite journal|last=Leirião|first=Patrícia|last2=Rodrigues|first2=Cristina D|last3=Albuquerque|first3=Sónia S|last4=Mota|first4=Maria M|date=December 2004|title=Survival of protozoan intracellular parasites in host cells|journal=EMBO Reports|language=en|volume=5|issue=12|pages=1142–1147|doi=10.1038/sj.embor.7400299|issn=1469-221X|pmc=1299194|pmid=15577928}}</ref> The entrance of these host cells will differ between intracellular parasites. Not all intracellular parasites will enter a cell the same way. Some will work with specific components in or on the host cell, an example being ''[[Trypanosoma cruzi]]''. This parasite will attach itself to the host cell while increasing the intracellular calcium, which in turn disrupts the actin at the site of attachment, causing the host cell to create a lysosomal-barrier around the disruption. The parasite will take advantage of this membrane and produce a vacuole into the host cell. Other intracellular parasites have developed other ways to enter a host cell that are not requiring a specific component or action from within the host cell. An example of this is that some intracellular parasites use a method called gliding motility. This is the use of an actin-myosin motor that is connected the intracellular parasites cytoskeleton.<ref name=":0" />
When an intracellular parasite goes to enter a host cell, it is particular about the type of host cell. This is because most intracellular parasites are able to infect only a few different cell types.<ref name=":0">{{Cite journal|last1=Leirião|first1=Patrícia|last2=Rodrigues|first2=Cristina D|last3=Albuquerque|first3=Sónia S|last4=Mota|first4=Maria M|date=December 2004|title=Survival of protozoan intracellular parasites in host cells|journal=EMBO Reports|language=en|volume=5|issue=12|pages=1142–1147|doi=10.1038/sj.embor.7400299|issn=1469-221X|pmc=1299194|pmid=15577928}}</ref>

* Viruses use a number of host receptors to gain entry to the cell, usually by causing [[endocytosis]].<ref name=Cossart>{{cite journal |last1=Cossart |first1=P. |last2=Helenius |first2=A. |title=Endocytosis of Viruses and Bacteria |journal=Cold Spring Harbor Perspectives in Biology |date=1 August 2014 |volume=6 |issue=8 |pages=a016972 |doi=10.1101/cshperspect.a016972|pmid=25085912 |pmc=4107984 }}</ref> See [[viral entry]] for more on this well-studied topic.
* Bacteria are also generally small enough to be engulfed by endocytosis, which they trigger with adhesins. Unlike viruses, they can and often do manipulate the cell's behavior beforehand, by injecting effector proteins into the cytosol.<ref name=Cossart/>
* Protists are generally too big to enter through endocytosis; they use alternate ways.<ref name=Horta>{{cite journal |last1=Horta |first1=Maria Fátima |last2=Andrade |first2=Luciana Oliveira |last3=Martins-Duarte |first3=Érica Santos |last4=Castro-Gomes |first4=Thiago |title=Cell invasion by intracellular parasites – the many roads to infection |journal=Journal of Cell Science |date=15 February 2020 |volume=133 |issue=4 |doi=10.1242/jcs.232488|pmid=32079731 |doi-access=free }}</ref>
** ''Plasmodium'' and ''Toxoplasma gondii'' are [[apicomplexans]], named for the fact they have a "apical complex", used for gaining entry into the cell. The apicomplexan first moves on the cell looking for an ideal receptor. When the receptor is found, it re-orients itself so the apical complex points at the cell. It then secretes a number of proteins to form a ''moving junction'', through which it gains entry.<ref name=Horta/>
** ''[[Trypanosoma cruzi]]'' and ''[[Leishmania]]'' enter by subverting the pathways for [[plasma membrane]] repair. All nucleated cells use calcium concentration as a signal for membrane damage. ''T. cruzi'' attaches to the target cell then increases the calcium concentration inside, disrupting the actin network and triggering the repair mechanism. Lysosomes are recruited to this disruption and release their contents to the extracellular side, as a way to replenish the plasma membrane. ''T. cruzi'' take advantage of the excess membrane to form a vacuole in the host cell, gaining entry.<ref name=":0" /> Because this repair mechanism is universal to all cells with a nucleus, ''T. cruzi'' is not picky about the target cell type. ''Leishmania'' also uses this mechanism.<ref name=Horta/>
** ''Leishmania'' can also trigger [[phagocytosis]]. It is able to withstand the degradation process the cell carries out following phagocytosis.<ref name=Horta/>
** Microsporidians, which are tiny protozoans related to fungi, seems to form "polar tubes" that poke into the target cell.<ref name=Horta/>

Other intracellular parasites have developed different ways to enter a host cell that do not require a specific component or action from within the host cell. An example is intracellular parasites using a method called gliding motility. This is the use of an actin-myosin motor that is connected to the intracellular parasites' cytoskeleton.{{cn|date=February 2024}}


== Nutrition ==
== Nutrition ==
The majority of intracellular parasites must keep host cells alive as long as possible while they are reproducing and growing. In order to grow, they need nutrients that might be scarce in their free form in the cell. To study the mechanism that intracellular parasites use to obtain nutrients, ''[[Legionella pneumophila]]'', a facultative intracellular parasite, has been used as a model. It is known that ''Legionella pneumophila'' obtains nutrients by promoting host [[Proteasome|proteasomal]] degradation. Self-degradation of host proteins into [[amino acids]] provides the parasite with its primary carbon and energy source.<ref name=pmid22096100>{{cite journal |doi=10.1126/science.1212868 |pmid=22096100 |title=Host Proteasomal Degradation Generates Amino Acids Essential for Intracellular Bacterial Growth |journal=Science |volume=334 |issue=6062 |pages=1553–7 |year=2011 |last1=Price |first1=C. T. D |last2=Al-Quadan |first2=T |last3=Santic |first3=M |last4=Rosenshine |first4=I |last5=Abu Kwaik |first5=Y |bibcode=2011Sci...334.1553P }}</ref>
The majority of intracellular parasites must keep host cells alive as long as possible while they are reproducing and growing. In order to grow, they need nutrients that might be scarce in their free form in the cell. To study the mechanism that intracellular parasites use to obtain nutrients, ''[[Legionella pneumophila]]'', a bacterial facultative intracellular parasite, has been used as a model. It is known that ''Legionella pneumophila'' obtains nutrients by promoting host [[Proteasome|proteasomal]] degradation. Self-degradation of host proteins into [[amino acids]] provides the parasite with its primary carbon and energy source.<ref name=pmid22096100>{{cite journal |doi=10.1126/science.1212868 |pmid=22096100 |title=Host Proteasomal Degradation Generates Amino Acids Essential for Intracellular Bacterial Growth |journal=Science |volume=334 |issue=6062 |pages=1553–7 |year=2011 |last1=Price |first1=C. T. D |last2=Al-Quadan |first2=T |last3=Santic |first3=M |last4=Rosenshine |first4=I |last5=Abu Kwaik |first5=Y |bibcode=2011Sci...334.1553P |s2cid=206537041 }}</ref>


== Susceptibility ==
== Susceptibility ==
Line 63: Line 83:
==See also==
==See also==
* [[Myzocytosis]]
* [[Myzocytosis]]
* [[Phagocyte#Pathogen evasion and resistance|Pathogen evasion and resistance]]

== Explanatory notes ==
{{Notelist}}


== References ==
== References ==

Latest revision as of 16:49, 13 March 2024

Intracellular parasites are microparasites that are capable of growing and reproducing inside the cells of a host.[1] They are also called intracellular pathogens.[2][3]

Types

[edit]

There are two main types of intracellular parasites: Facultative and Obligate.[2]

Facultative intracellular parasites are capable of living and reproducing in or outside of host cells. Obligate intracellular parasites, on the other hand, need a host cell to live and reproduce. Many of these types of cells require specialized host types, and invasion of host cells occurs in different ways.[2]

Facultative

[edit]

Facultative intracellular parasites are capable of living and reproducing either inside or outside cells.

Bacterial examples include:

Fungal examples include:

Obligate

[edit]
Two apicomplexans, Toxoplasma gondii, within their host cell. Transmission electron microscopy

Obligate intracellular parasites cannot reproduce outside their host cell, meaning that the parasite's reproduction is entirely reliant on intracellular resources.

All viruses are obligate intracellular parasites.

Bacterial examples (that affect humans) include:

Protozoan examples (that affect humans) include:

Fungal examples (that affect humans) include:

The mitochondria in eukaryotic cells may also have originally been such parasites, but ended up forming a mutualistic relationship (endosymbiotic theory).[18]

Study of obligate pathogens is difficult because they cannot usually be reproduced outside the host. However, in 2009 scientists reported a technique allowing the Q-fever pathogen Coxiella burnetii to grow in an axenic culture and suggested the technique may be useful for study of other pathogens.[19]

Unusual examples

[edit]

Polypodium is a rare metazoan (animal) intracellular parasite, distinct from most if not all other intracellular parasites for this reason. It lives inside the unfertilized egg cells (oocytes) of fish.[20]

Invasion

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When an intracellular parasite goes to enter a host cell, it is particular about the type of host cell. This is because most intracellular parasites are able to infect only a few different cell types.[21]

  • Viruses use a number of host receptors to gain entry to the cell, usually by causing endocytosis.[7] See viral entry for more on this well-studied topic.
  • Bacteria are also generally small enough to be engulfed by endocytosis, which they trigger with adhesins. Unlike viruses, they can and often do manipulate the cell's behavior beforehand, by injecting effector proteins into the cytosol.[7]
  • Protists are generally too big to enter through endocytosis; they use alternate ways.[22]
    • Plasmodium and Toxoplasma gondii are apicomplexans, named for the fact they have a "apical complex", used for gaining entry into the cell. The apicomplexan first moves on the cell looking for an ideal receptor. When the receptor is found, it re-orients itself so the apical complex points at the cell. It then secretes a number of proteins to form a moving junction, through which it gains entry.[22]
    • Trypanosoma cruzi and Leishmania enter by subverting the pathways for plasma membrane repair. All nucleated cells use calcium concentration as a signal for membrane damage. T. cruzi attaches to the target cell then increases the calcium concentration inside, disrupting the actin network and triggering the repair mechanism. Lysosomes are recruited to this disruption and release their contents to the extracellular side, as a way to replenish the plasma membrane. T. cruzi take advantage of the excess membrane to form a vacuole in the host cell, gaining entry.[21] Because this repair mechanism is universal to all cells with a nucleus, T. cruzi is not picky about the target cell type. Leishmania also uses this mechanism.[22]
    • Leishmania can also trigger phagocytosis. It is able to withstand the degradation process the cell carries out following phagocytosis.[22]
    • Microsporidians, which are tiny protozoans related to fungi, seems to form "polar tubes" that poke into the target cell.[22]

Other intracellular parasites have developed different ways to enter a host cell that do not require a specific component or action from within the host cell. An example is intracellular parasites using a method called gliding motility. This is the use of an actin-myosin motor that is connected to the intracellular parasites' cytoskeleton.[citation needed]

Nutrition

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The majority of intracellular parasites must keep host cells alive as long as possible while they are reproducing and growing. In order to grow, they need nutrients that might be scarce in their free form in the cell. To study the mechanism that intracellular parasites use to obtain nutrients, Legionella pneumophila, a bacterial facultative intracellular parasite, has been used as a model. It is known that Legionella pneumophila obtains nutrients by promoting host proteasomal degradation. Self-degradation of host proteins into amino acids provides the parasite with its primary carbon and energy source.[23]

Susceptibility

[edit]

People with T cell deficiencies are particularly susceptible to intracellular pathogens.[24]

See also

[edit]

Explanatory notes

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  1. ^ Only in animal study at initial stages of infection.[13]
  2. ^ Some sources say that it's parasite, but some not.

References

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  1. ^ Horta, Maria Fátima; Andrade, Luciana Oliveira; Martins-Duarte, Érica Santos; Castro-Gomes, Thiago (15 February 2020). "Cell invasion by intracellular parasites – the many roads to infection". Journal of Cell Science. 133 (4). doi:10.1242/jcs.232488. PMID 32079731.
  2. ^ a b c Leon-Sicairos, Nidia; Reyes-Cortes, Ruth; Guadrón-Llanos, Alma M.; Madueña-Molina, Jesús; Leon-Sicairos, Claudia; Canizalez-Román, Adrian (2015). "Strategies of Intracellular Pathogens for Obtaining Iron from the Environment". BioMed Research International. 2015: 1–17. doi:10.1155/2015/476534. ISSN 2314-6133. PMC 4450229. PMID 26120582.
  3. ^ Thakur, A; Mikkelsen, H; Jungersen, G (2019). "Intracellular Pathogens: Host Immunity and Microbial Persistence Strategies". Journal of Immunology Research. 2019: 1356540. doi:10.1155/2019/1356540. PMID 31111075.
  4. ^ "Bartonella henselae" (PDF).
  5. ^ Dramsi, Shaynoor; Cossart, Pascale (2002-03-18). "Listeriolysin O". The Journal of Cell Biology. 156 (6): 943–946. doi:10.1083/jcb.200202121. ISSN 0021-9525. PMC 2173465. PMID 11901162.
  6. ^ Jantsch, J.; Chikkaballi, D.; Hensel, M. (2011). "Cellular aspects of immunity to intracellular Salmonella enterica". Immunological Reviews. 240 (1): 185–195. doi:10.1111/j.1600-065X.2010.00981.x. PMID 21349094. S2CID 19344119.
  7. ^ a b c Cossart, P.; Helenius, A. (1 August 2014). "Endocytosis of Viruses and Bacteria". Cold Spring Harbor Perspectives in Biology. 6 (8): a016972. doi:10.1101/cshperspect.a016972. PMC 4107984. PMID 25085912.
  8. ^ Kelly, B. G.; Wall, D. M.; Boland, C. A.; Meijer, W. G. (2002). "Isocitrate lyase of the facultative intracellular pathogen Rhodococcus equi". Microbiology. 148 (Pt 3): 793–798. doi:10.1099/00221287-148-3-793. PMID 11882714.
  9. ^ Bravo-Santano; et al. (2018). "Intracellular Staphylococcus aureus Modulates Host Central Carbon Metabolism To Activate Autophagy". American Society for Microbiology. 3 (4): e00374–18. doi:10.1128/mSphere.00374-18. PMC 6083095. PMID 30089650.
  10. ^ Daffé, M.; Etienne, G. (June 1999). "The capsule of Mycobacterium tuberculosis and its implications for pathogenicity". Tubercle and Lung Disease. 79 (3): 153–169. doi:10.1054/tuld.1998.0200. PMID 10656114.
  11. ^ Sebghati TS, Engle JT, Goldman WE (November 2000). "Intracellular parasitism by Histoplasma capsulatum: fungal virulence and calcium dependence". Science. 290 (5495): 1368–72. Bibcode:2000Sci...290.1368S. doi:10.1126/science.290.5495.1368. PMID 11082066.
  12. ^ Alvarez, M.; Burns, T.; Luo, Y.; Pirofski, L. A.; Casadevall, A. (2009). "The outcome of Cryptococcus neoformans intracellular pathogenesis in human monocytes". BMC Microbiology. 9: 51. doi:10.1186/1471-2180-9-51. PMC 2670303. PMID 19265539.
  13. ^ Sterkel, Alana K.; Mettelman, Robert; Wüthrich, Marcel; Klein, Bruce S. (2015-02-15). "The unappreciated intracellular lifestyle of Blastomyces dermatitidis". Journal of Immunology. 194 (4): 1796–1805. doi:10.4049/jimmunol.1303089. ISSN 1550-6606. PMC 4373353. PMID 25589071.
  14. ^ Amann R, Springer N, Schönhuber W, Ludwig W, Schmid EN, Müller KD, Michel R (January 1997). "Obligate intracellular bacterial parasites of acanthamoebae related to Chlamydia spp". Applied and Environmental Microbiology. 63 (1): 115–21. Bibcode:1997ApEnM..63..115A. doi:10.1128/AEM.63.1.115-121.1997. PMC 168308. PMID 8979345.
  15. ^ Foley, Janet E.; Nieto, Nathan C.; Barbet, Anthony; Foley, Patrick (2009-12-15). "Antigen diversity in the parasitic bacterium Anaplasma phagocytophilum arises from selectively-represented, spatially clustered functional pseudogenes". PLOS ONE. 4 (12): e8265. Bibcode:2009PLoSO...4.8265F. doi:10.1371/journal.pone.0008265. ISSN 1932-6203. PMC 2789410. PMID 20016821.
  16. ^ Deng, M.; Lancto, C. A.; Abrahamsen, M. S. (2004). "Cryptosporidium parvum regulation of human epithelial cell gene expression". International Journal for Parasitology. 34 (1): 73–82. doi:10.1016/j.ijpara.2003.10.001. PMID 14711592.
  17. ^ David Anthony Burns; Stephen M. Breathnach; Neil H. Cox; Christopher E. M. Griffiths, eds. (2010). Rook's Textbook of Dermatology. Vol. 4 (8th ed.). Chichester: Wiley-Blackwell. p. 28. ISBN 978-1-4051-6169-5.
  18. ^ Lynn Sagan (1967). "On the origin of mitosing cells". J Theor Biol. 14 (3): 255–274. Bibcode:1967JThBi..14..225S. doi:10.1016/0022-5193(67)90079-3. PMID 11541392.
  19. ^ Omsland A, Cockrell DC, Howe D, Fischer ER, Virtaneva K, Sturdevant DE, Porcella SF, Heinzen RA (March 17, 2009). "Host cell-free growth of the Q fever bacterium Coxiella burnetii". Proceedings of the National Academy of Sciences USA. 106 (11): 4430–4. Bibcode:2009PNAS..106.4430O. doi:10.1073/pnas.0812074106. PMC 2657411. PMID 19246385.
  20. ^ Evans N. M.; Lindner A.; Raikova E. V.; Collins A. G.; Cartwright P. (2008). "Phylogenetic placement of the enigmatic parasite, Polypodium hydriforme, within the Phylum Cnidaria". BMC Evolutionary Biology. 8 (1): 139. Bibcode:2008BMCEE...8..139E. doi:10.1186/1471-2148-8-139. PMC 2396633. PMID 18471296.
  21. ^ a b Leirião, Patrícia; Rodrigues, Cristina D; Albuquerque, Sónia S; Mota, Maria M (December 2004). "Survival of protozoan intracellular parasites in host cells". EMBO Reports. 5 (12): 1142–1147. doi:10.1038/sj.embor.7400299. ISSN 1469-221X. PMC 1299194. PMID 15577928.
  22. ^ a b c d e Horta, Maria Fátima; Andrade, Luciana Oliveira; Martins-Duarte, Érica Santos; Castro-Gomes, Thiago (15 February 2020). "Cell invasion by intracellular parasites – the many roads to infection". Journal of Cell Science. 133 (4). doi:10.1242/jcs.232488. PMID 32079731.
  23. ^ Price, C. T. D; Al-Quadan, T; Santic, M; Rosenshine, I; Abu Kwaik, Y (2011). "Host Proteasomal Degradation Generates Amino Acids Essential for Intracellular Bacterial Growth". Science. 334 (6062): 1553–7. Bibcode:2011Sci...334.1553P. doi:10.1126/science.1212868. PMID 22096100. S2CID 206537041.
  24. ^ Bannister, Barbara A.; Gillespie, Stephen H.; Jones, Jane (2006). "Chapter 22". Infection: Microbiology and Management. Malden, MA: Wiley-Blackwell. p. 432. ISBN 1-4051-2665-5.
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