Acanthamoeba

Taxobox
color = khaki
name = "Acanthamoeba"
regnum = Protista
phylum = Amoebozoa
genus = "Acanthamoeba"

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image_caption = Phase contrast micrograph of an "Acanthamoeba polyphaga" cyst.

"Acanthamoeba" is a genus of amoebae, one of the most common protozoa in soil, and also frequently found in fresh water and other habitats. The cells are small, usually 15 to 35 μm in length and oval to triangular in shape when moving. The pseudopods form a clear hemispherical lobe at the anterior, and there are various short filose extensions from the margins of the body. These give it a spiny appearance, which is what the name "Acanthamoeba" refers to. Cysts are common. Most species are free-living bacterivores, but some are opportunists that can cause infections in humans and other animals.

Human pathogen

Diseases caused by "Acanthamoeba" include amoebic keratitis and encephalitis. [cite journal | first = C | last = Di Gregorio | coauthors = Rivasi F, Mongiardo N, De Rienzo B, Wallace S, Visvesvara GS | year = 1992 | month = Dec | title = "Acanthamoeba" meningoencephalitis in a patient with acquired immunodeficiency syndrome | journal = Archives of Pathology & Laboratory Medicine | volume = 116 | issue = 12 | pages = 1363–5 | pmid = 1456885 | accessdate = 2007-02-12] The latter is caused by "Acanthamoeba" entering cuts and spreading to the central nervous system. The former is a rare disease where amoebae invade the cornea of the eye. In the United States, it is nearly always associated with contact lens use, as "Acanthamoeba" can survive in the space between the lens and the eye. [cite journal | first = JD | last = Auran | coauthors = Starr MB, Jakobiec FA | year = 1987 | title = "Acanthamoeba" keratitis. A review of the literature | journal = Cornea | volume = 6 | issue = 1 | pages = 2–26 | pmid = 3556011 | accessdate = 2007-02-12 | doi = 10.1097/00003226-198706010-00002] [JOHN D.T. (1993) Opportunistically pathogenic free-living amebae. In: J.P. Kreier and J.R. Baker (Eds.), "Parasitic Protozoa. Vol. 3". Academic Press, New York, pp. 143–246.] [cite journal | first = PR | last = Badenoch | coauthors = Adams M, Coster DJ | year = 1995 | month = Feb | title = Corneal virulence, cytopathic effect on human keratocytes and genetic characterization of "Acanthamoeba" | journal = International journal for parasitology | volume = 25 | issue = 2 | pages = 229–39 | pmid = 7622330| accessdate = 2007-02-12 | doi = 10.1016/0020-7519(94)00075-Y] [cite journal | first = JY | last = Niederkorn | coauthors = Alizadeh H, Leher H, McCulley JP | year = 1999 | month = May | title = The pathogenesis of "Acanthamoeba" keratitis | journal = Microbes and Infection | volume = 1 | issue = 6 | pages = 437–43 | pmid = 10602676 | accessdate = 2007-02-12 | doi = 10.1016/S1286-4579(99)80047-1] However, elsewhere in the world, many cases of "Acanthamoeba" present in non-contact lens wearers.Sharma S, Garg P, Rao GN. [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Retrieve&list_uids=11004092 "Patient characteristics, diagnosis, and treatment of non-contact lens related Acanthamoeba keratitis."] The British Journal of Ophthalmology. 2000 Oct;84(10):1103-8. PMID: 11004092] For this reason, contact lenses must be properly disinfected before wearing, and should be removed when swimming or surfing.

To detect "Acanthamoeba" on a contact lens in a laboratory, a sheep blood agar plate with a layer (a lawn) of "E. coli" is made. Part of the contact lens is placed on the agar plate. If "Acanthamoeba" are present, they will ingest the bacteria, leaving a clear patch on the plate around the area of the lens. Polymerase chain reaction can also be used to confirm a diagnosis of "Acanthamoeba" keratitis, especially when contact lenses are not involved.cite journal | last = Pasricha | first = Gunisha | coauthors = Savitri Sharma, Prashant Garg, Ramesh K. Aggarwal | year = 2003 | month = July | title = Use of 18S rRNA Gene-Based PCR Assay for Diagnosis of Acanthamoeba Keratitis in Non-Contact Lens Wearers in India| volume = 41 | issue = 7 | pages = 3206–3211 | doi = 10.1128/JCM.41.7.3206-3211.2003 | journal = Journal of Clinical Microbiology | url = http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=165372 | pmid = 12843065]

Acanthamoeba granulomatous encephalitis is an opportunistic protozoan pathogen that rarely causes disease in humans. Approximately 400 cases have been reported worldwide with a survival rate of only two to three percent. Infection usually occurs in patients with an immunodeficiency, diabetes, malignancies, malnutrition, systemic lupus erythematosus, or alcoholism. The parasite's portal of entry is via lesions in the skin or the upper respiratory tract or via inhalation of airborne cysts. The parasite then spreads hematogenously into the central nervous system. "Acanthamoeba" crosses the blood brain barrier by means that are not yet understood. Subsequent invasion of the connective tissue and induction of pro-inflammatory responses leads to neuronal damage which can be fatal within days. A post-mortem biopsy reveals severe oedema and hemorrhagic necrosis. [cite journal | first = N | last = Khan | year = 2006 | month = Nov | title = "Acanthamoeba" invasion of the central nervous system | journal = International Journal of Parasitology | volume = 37 | issue = 2 | pages = 131–8 | pmid = 17207487 | accessdate = 2008-05-01 | doi = 10.1016/j.ijpara.2006.11.010] A patient that has contracted this illness usually displays subacute symptoms including altered mental status, headaches, fever, neck stiffness, seizures, focal neurological signs such as cranial nerve palsies and coma all leading to death within one week to several months. [cite journal | first = V | last = Kaushal | coauthors = Chhina DK, Kumar R, Pannu HS, Dhooria HPS, Chhina RS | year = 2007 | month = Mar | title = "Acanthamoeba" Encephalitis | journal = Indian Journal of Medical Microbiology | volume = 26 | issue = 2 | pages = 182–4 | pmid = 18445961 | accessdate = 2008-05-01] Due to the rarity of this parasite and our lack of knowledge there are currently no good diagnoses or treatments for "Acanthamoeba".

Infection usually mimics that of bacterial leptomeningitis, tuberculous meningitis, or viral encephalitis. The misdiagnosis often leads to erroneous treatment that is ineffective. In the case that "Acanthamoeba" is diagnosed correctly, the current treatments such as amphotericin-B, rifampicin, trimethroprim-sulfamethoxazole, ketokonazole, fluconazole, sulfadiazine, albendazole are only tentatively successful. Correct and timely diagnosis as well as improved treatment methods as well as understanding of the parasite are important factors in improving the outcome of infection by "Acanthamoeba".

MRSA

Methicillin-resistant Staphylococcus aureus (MRSA) is an important pathogen in the hospital setting due to its resistance to many antibiotics. Recent findings from the University of Bath demonstrate that MRSA can infect and replicate inside of "Acanthamoeba polyphaga"; this "Acanthamoeba" species is widespread throughout the environment. Since "A. polyphaga" can form cysts, cysts infected with MRSA can act as a mode of airborne dispersal for MRSA. Additionally, it is noted that "evidence with other pathogens suggests that pathogens that emerge from amoeba are more resistant to antibiotics and more virulent." [cite press release | publisher=University of Bath | date=2006-02-28 | title=MRSA use amoeba to spread, sidestepping hospital protection measures, new research shows | url=http://www.bath.ac.uk/news/articles/releases/mrsaamoeba280206.html | accessdate=2007-02-12] It has been observed that "Acanthamoeba" can increase MRSA numbers by 1000-fold. [cite press release | publisher=Blackwell Publishing | date=2006-03-01 | title=Single Cell Amoeba Increases MRSA Numbers One Thousand Fold | url=http://www.blackwellpublishing.com/press/pressitem.asp?ref=694 | accessdate=2007-02-12]

Importance in soil ecology

"A. castellanii" can be found at high densities in various soil ecosystems. It preys on bacteria, but also fungi and other protozoa.

This species is able to lyse bacteria and produce a wide range of enzymes such as cellulases or chitinases [cite journal | first = I. J. | last = Anderson | coauthors = Watkins, R. F., Samuelson, J., Spencer, D. F., Majoros, W. H., Gray, M. W. and Loftus, B. J. | year = 2005 | month = Aug | title = Gene Discovery in the "Acanthamoeba castellanii" Genome | journal = Protist | volume = 156 | issue = 2 | pages = 203–14 | pmid = 16171187 | accessdate = 2007-02-12 | doi = 10.1016/j.protis.2005.04.001] and probably contributes to the break down of organic matter in soil, contributing to the microbial loop.

pecies

Species of "Acanthamoeba" are distinguished mainly on the form of cysts, and include the following; those marked with an asterisk are known to cause infections.

*"A. astronyxis"*
*"A. castellanii"*
*"A. comandoni"
*"A. culbertsoni"*
*"A. divionensis"
*"A. griffini"
*"A. hatchetti"*
*"A. healyi"
*"A. jacobsi"
*"A. lenticulata"
*"A. lugdunensis"*
*"A. mauritaniensis"
*"A. palestinensis"*
*"A. pearcei"
*"A. polyphaga"*
*"A. pustulosa"
*"A. quina"*
*"A. rhysodes"*
*"A. royreba"
*"A. terricola" (renamed A.castellanii Poussard)
*"A. triangularis"
*"A. tubiashi"

Endosymbiontes of "Acanthamoeba"

"Acanthamoeba" sp. contain diverse bacterial endosymbionts which are similar to human pathogens. Because of this they are considered to be potential emerging human pathogens. [cite journal | first = M | last = Horn | coauthors = Wagner, M | year = 2004 | month = Sep-Oct | title = Bacterial Endosymbionts of Free-living Amoebae | journal = Journal of Eukaryotic Microbiology | volume = 51 | issue = 5 | pages = 509–14 | pmid = 15537084 | accessdate = 2007-02-12 | doi = 10.1111/j.1550-7408.2004.tb00278.x] The exact nature of these symbionts and the benefit they represent for the amoebal host still have to be clarified.

See also

* Legionella
* Momus (artist)

References

* Khan, N. A. (2006) "Acanthamoeba": biology and increasing importance in human health. Fems Microbiology Reviews 30, 564-595.

External links

* [http://cmr.asm.org/cgi/content/full/16/2/273 Acanthamoeba spp. as Agents of Disease in Humans] - Clinical Microbiology Reviews, accessed on 4 February 2006
* [http://www.bms.ed.ac.uk/research/others/smaciver/acanthamoeba.htm Comprehensive resource on Amoeba] -
* [http://www.ukoptometry.co.uk/blog/2007/03/26/swimming-around-in-a-pool-of-acanthamoeba/ Eye health and Acanthamoeba]
* [http://www.caister.com/acanthamoeba/ Acanthamoeba pictures and illustrations]