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''Acanthamoeba'' is a genus of amoebae that are commonly recovered from soil,
fresh water Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does incl ...
, and other
habitats In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...
. ''Acanthamoeba'' has two evolutive forms, the metabolically active trophozoite and a dormant, stress-resistant cyst. Trophozoites are small, usually 15 to 25 μm in length and amoeboid in shape. In nature, ''Acanthamoeba'' species are free-living bacterivores, but in certain situations, they can cause infections (acanthamebiasis) in humans and other animals.


Distribution

''Acanthamoeba'' spp. are among the most prevalent protozoa found in the environment. They are distributed worldwide, and have been isolated from soil, air, sewage, seawater, chlorinated swimming pools, domestic tap water, bottled water, dental treatment units, hospitals, air-conditioning units, and contact lens cases. Additionally, they have been isolated from human skin, nasal cavities, throats, and intestines, as well as plants and other mammals.


Role in disease

Diseases caused by ''Acanthamoeba'' include
keratitis Keratitis is a condition in which the eye's cornea, the clear dome on the front surface of the eye, becomes inflamed. The condition is often marked by moderate to intense pain and usually involves any of the following symptoms: pain, impaired e ...
and granulomatous amoebic encephalitis (GAE). The latter is often but not always seen in immunosuppressed patients. GAE is caused by the amoebae entering the body through an open wound and then spreading to the brain. The combination of host immune responses and secreted amoebal
protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the ...
s causes massive brain swelling resulting in death in about 95% of those infected.


Granulomatous amoebic encephalitis (GAE)

Granulomatous amoebic encephalitis (GAE) is caused by amoebic infection of the central nervous system (CNS). It is characterized by neurological symptoms including headache, seizures, and mental-status abnormalities. These worsen progressively over weeks to months, leading to death in most patients. Infection is generally associated with underlying conditions such as
immunodeficiency Immunodeficiency, also known as immunocompromisation, is a state in which the immune system's ability to fight infectious diseases and cancer is compromised or entirely absent. Most cases are acquired ("secondary") due to extrinsic factors that a ...
, diabetes, malignancies, malnutrition,
systemic lupus erythematosus Lupus, technically known as systemic lupus erythematosus (SLE), is an autoimmune disease in which the body's immune system mistakenly attacks healthy tissue in many parts of the body. Symptoms vary among people and may be mild to severe. Commo ...
, and alcoholism. The parasite enters the body through cuts in the skin or by being inhaled into the upper respiratory tract. The parasite then spreads through the blood into the CNS. ''Acanthamoeba'' crosses the
blood–brain barrier The blood–brain barrier (BBB) is a highly selective semipermeable border of endothelial cells that prevents solutes in the circulating blood from ''non-selectively'' crossing into the extracellular fluid of the central nervous system where ne ...
by means that are not yet understood. Subsequent invasion of the connective tissue and induction of pro-inflammatory responses leads to neuronal damage that can be fatal within days. Pure granulomatous lesions are rare in patients with AIDS and other related immunodeficiency states, as the patients do not have adequate numbers of CD+ve T-cells to mount a granulomatous response to ''Acanthamoeba'' infection in CNS and other organs and tissues. A perivascular cuffing with amoebae in necrotic tissue is usual finding in the AIDS and related T-cell immunodeficiency conditions. Brain biopsy normally reveals severe oedema and hemorrhagic
necrosis Necrosis () is a form of cell injury which results in the premature death of cells in living tissue by autolysis. Necrosis is caused by factors external to the cell or tissue, such as infection, or trauma which result in the unregulated diges ...
. A patient who has contracted this illness usually displays subacute symptoms, including altered mental status, headaches, fever, neck stiffness, seizures, and focal neurological signs (such as cranial nerve palsies and coma), all leading to death within one week to several months. Due to the rarity of this parasite and a lack of knowledge, no good diagnoses or treatments for ''Acanthamoeba'' infection are now known. ''Acanthamoeba'' keratitis cases in the past have resolved from a therapy consisting of atropine and some other drugs with no antimicrobial effects. Recent publications show
atropine Atropine is a tropane alkaloid and anticholinergic medication used to treat certain types of nerve agent and pesticide poisonings as well as some types of slow heart rate, and to decrease saliva production during surgery. It is typically given ...
to interfere with the protist's CHRM1 receptor, causing cell death. Infection usually mimics that of bacterial
leptomeningitis Meningitis is acute or chronic inflammation of the protective membranes covering the brain and spinal cord, collectively called the meninges. The most common symptoms are fever, headache, and neck stiffness. Other symptoms include confusion or ...
, tuberculous
meningitis Meningitis is acute or chronic inflammation of the protective membranes covering the brain and spinal cord, collectively called the meninges. The most common symptoms are fever, headache, and neck stiffness. Other symptoms include confusion or ...
, or viral encephalitis. The misdiagnosis often leads to erroneous, ineffective treatment. In the case that the ''Acanthamoeba'' is diagnosed correctly, the current treatments, such as
amphotericin B Amphotericin B is an antifungal medication used for serious fungal infections and leishmaniasis. The fungal infections it is used to treat include mucormycosis, aspergillosis, blastomycosis, candidiasis, coccidioidomycosis, and cryptococcosis ...
,
rifampicin Rifampicin, also known as rifampin, is an ansamycin antibiotic used to treat several types of bacterial infections, including tuberculosis (TB), ''Mycobacterium avium'' complex, leprosy, and Legionnaires’ disease. It is almost always used t ...
, trimethoprim- sulfamethoxazole,
ketoconazole Ketoconazole, sold under the brand name Nizoral among others, is an antiandrogen and antifungal medication used to treat a number of fungal infections. Applied to the skin it is used for fungal skin infections such as tinea, cutaneous ca ...
,
fluconazole Fluconazole is an antifungal medication used for a number of fungal infections. This includes candidiasis, blastomycosis, coccidiodomycosis, cryptococcosis, histoplasmosis, dermatophytosis, and pityriasis versicolor. It is also used to prev ...
, sulfadiazine, or albendazole, are only tentatively successful. Correct and timely diagnosis, as well as improved treatment methods and an understanding of the parasite, are important factors in improving the outcome of infection by ''Acanthamoeba''. A paper published in 2013 has shown substantial effects of some FDA-approved drugs with an ''in vitro'' kill rate above 90%. These results were ''in vitro'' effects, but as the drugs are already approved, human infections can be targeted after dose calculations in clinical trials done with these diverse groups of drugs.


Acanthamoebic keratitis

When present in the eye, ''Acanthamoeba'' strains can cause acanthamoebic keratitis, which may lead to corneal ulcers or even blindness. This condition occurs most often among
contact lens Contact lenses, or simply contacts, are thin lenses placed directly on the surface of the eyes. Contact lenses are ocular prosthetic devices used by over 150 million people worldwide, and they can be worn to correct vision or for cosmeti ...
wearers who do not properly disinfect their lenses, exacerbated by a failure to wash hands prior to handling the lenses. Multipurpose contact lens solutions are largely ineffective against ''Acanthamoeba'', whereas hydrogen peroxide-based solutions have good disinfection characteristics. The first cure of a corneal infection was achieved in 1985 at Moorfields Eye Hospital. In May 2007, Advanced Medical Optics, manufacturer of Complete Moisture Plus Contact Lens Solution products, issued a voluntary recall of their Complete Moisture Plus solutions. The fear was that contact lens wearers who used their solution were at higher risk of acanthamoebic keratitis than contact lens wearers who used other solutions. The manufacturer recalled the product after the Centers for Disease Control in the United States found that 21 people had possibly received an ''Acanthamoeba'' infection after using Complete Moisture Plus in the month prior to diagnosis.


As a bacterial reservoir

Several species of bacteria that can cause human disease are also able to infect and replicate within ''Acanthamoeba'' species. These include ''
Legionella pneumophila ''Legionella pneumophila'' is a thin, aerobic, pleomorphic, flagellated, non-spore-forming, Gram-negative bacterium of the genus '' Legionella''. ''L. pneumophila'' is the primary human pathogenic bacterium in this group and is the causative ...
'', ''
Pseudomonas aeruginosa ''Pseudomonas aeruginosa'' is a common encapsulated, gram-negative, aerobic– facultatively anaerobic, rod-shaped bacterium that can cause disease in plants and animals, including humans. A species of considerable medical importance, ''P. a ...
'', and some strains of ''
Escherichia coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Esc ...
'' and ''
Staphylococcus aureus ''Staphylococcus aureus'' is a Gram-positive spherically shaped bacterium, a member of the Bacillota, and is a usual member of the microbiota of the body, frequently found in the upper respiratory tract and on the skin. It is often posit ...
''. For some of these bacteria, replication inside ''Acanthamoeba'' has been associated with enhanced growth in macrophages, and increased resistance to some antibiotics. Furthermore, due to the high prevalence of ''Acanthamoeba'' in the environment, these amoebae have been proposed to serve as an environmental reservoir for some human pathogens.


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, and probably contributes to the breakdown of organic matter in soil, contributing to the microbial loop.


Physiology


Role as a model organism

Because ''Acanthamoeba'' does not differ greatly at the ultrastructural level from a mammalian cell, it is an attractive model for cell-biology studies; it is important in cellular microbiology, environmental biology, physiology, cellular interactions, molecular biology, biochemistry, and evolutionary studies, due to the organisms' versatile roles in the ecosystem and ability to capture prey by phagocytosis, act as vectors and reservoirs for microbial pathogens, and to produce serious human infections. In addition, ''Acanthamoeba'' has been used extensively to understand the molecular biology of
cell motility Motility is the ability of an organism to move independently, using metabolic energy. Definitions Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...
and cancer cell dormancy by in-depth exploration of the process of encystation. The recently available ''Acanthamoeba''
genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ...
sequence revealed several orthologs of genes employed in
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 ...
of sexual
eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacter ...
s. These genes included ''
Spo11 Spo11 is a protein that in humans is encoded by the ''SPO11'' gene. Spo11, in a complex with mTopVIB, creates double strand breaks to initiate meiotic recombination. Its active site contains a tyrosine which ligates and dissociates with DNA to pr ...
, Mre11, Rad50,
Rad51 DNA repair protein RAD51 homolog 1 is a protein encoded by the gene ''RAD51''. The enzyme encoded by this gene is a member of the RAD51 protein family which assists in repair of DNA double strand breaks. RAD51 family members are homologous to th ...
, Rad52, Mnd1,
Dmc1 Meiotic recombination protein DMC1/LIM15 homolog is a protein that in humans is encoded by the ''DMC1'' gene. Meiotic recombination protein Dmc1 is a homolog of the bacterial strand exchange protein RecA. Dmc1 plays the central role in homologou ...
, Msh'', and '' Mlh''. This finding suggests that ''Acanthamoeba'' is capable of some form of meiosis and may be able to undergo sexual reproduction. Furthermore, since ''Acanthamoeba'' diverged early from the eukaryotic family tree, these results suggest that meiosis was present early in eukaryotic evolution. Owing to its ease and economy of cultivation, the Neff strain of ''A. castellanii'', discovered in a pond in
Golden Gate Park Golden Gate Park, located in San Francisco, California, United States, is a large urban park consisting of of public grounds. It is administered by the San Francisco Recreation & Parks Department, which began in 1871 to oversee the developm ...
in the 1960s, has been effectively used as a classic model organism in the field of cell biology. From just 30 L of simple medium inoculated with ''A. castellanii'', about 1 kg of cells can be obtained after several days of aerated culture at room temperature. Pioneered in the laboratory of Edward D. Korn at the
National Institutes of Health The National Institutes of Health, commonly referred to as NIH (with each letter pronounced individually), is the primary agency of the United States government responsible for biomedical and public health research. It was founded in the lat ...
(NIH), many important biological molecules have been discovered and their pathways elucidated using the ''Acanthamoeba'' model. Thomas Dean Pollard applied this model at the NIH,
Harvard Medical School Harvard Medical School (HMS) is the graduate medical school of Harvard University and is located in the Longwood Medical Area of Boston, Massachusetts. Founded in 1782, HMS is one of the oldest medical schools in the United States and is cons ...
, Johns Hopkins University School of Medicine, and the
Salk Institute for Biological Studies The Salk Institute for Biological Studies is a scientific research institute located in the La Jolla community of San Diego, California, U.S. The independent, non-profit institute was founded in 1960 by Jonas Salk, the developer of the polio va ...
to discover and characterize many proteins that are essential for cell motility, not only in amoebae, but also in many other eukaryotic cells, especially those of the human nervous and immune systems, the developing embryo, and cancer cells. ''Acanthamoeba'' also has served as a model to study the evolution of certain G-proteins. This unicellular eukaryote expresses few GPCRs over its cell membrane that serve vital role for the microorganism, structural homology bioinformatics tools have been used to show the presence of a homolog of human M1-muscarinic receptor in ''A. castellanii''. Blocking these muscarinic receptors in past studies has proven to be amoebicidal in Acanthamoeba spp. More recently, voltage-gated calcium channels in ''Acanthamoeba'' spp. (CavAc) have been reported to have similarities with human voltage-gated calcium channels such as TPC-1 and L-type calcium channels and respond to Ca-channel blockers such as loperamide. This model microbe has been studied to understand complex neurodegenerative states including Alzheimer's disease. Scientists have isolated a neurotransmitter acetylcholine in ''Acanthamoeba'' and the enzymatic machinery needed for its synthesis.


Endosymbionts

''Acanthamoeba'' spp. contain diverse bacterial endosymbionts that are similar to human pathogens, so they are considered to be potential emerging human pathogens. The exact nature of these symbionts and the benefit they represent for the amoebic host still have to be clarified. These include '' Legionella'' and ''Legionella''-like pathogens.


Giant viruses

The giant viruses '' Mimivirus,
Megavirus Megavirus is a viral genus containing a single identified species named ''Megavirus chilensis'', phylogenetically related to Acanthamoeba polyphaga Mimivirus (APMV). In colloquial speech, Megavirus chilensis is more commonly referred to as jus ...
'', and '' Pandoravirus'' infect ''Acanthamoeba''. Members of the genus ''Acanthamoeba'' are unusual in serving as hosts for a variety of giant viruses (that have more than 1000 protein-coding genes; for instance, ''Pandoravirus'', which has about 2500 protein-coding genes in its genome).


Diversity

''Acanthamoeba'' can be distinguished from other genera of amoebae based on morphological characteristics. However, differentiating one species of ''Acanthamoeba'' from another by morphology has proven difficult. Based on 18S rDNA sequencing, known ''Acanthamoeba'' strains can be organized into 12 groups, denoted T1-T12. Most disease-causing isolates belong to type T4. Below is a list of described species of ''Acanthamoeba'', with sequence types noted where known. Species that have been identified in diseased patients are marked with *. *''A. astronyxis'' (Ray & Hayes 1954) Page 1967 * (T7) *''A. byersi'' Qvarnstrom, Nerad & Visvesvara 2013 * *''A. castellanii'' Volkonski 1931 * (T4) 'A. terricola'' Pussard 1964*''A. comandoni'' Pussard 1964 (T9) *''A. culbertsoni'' (Singh & Das 1970) Griffin 1972 * (T10) *''A. divionensis'' Pussard & Pons 1977 (T4) *''A. echinulata'' Pussard & Pons 1977 *''A. gigantea'' Schmöller 1964 *''A. glebae'' (Dobell 1914) *''A. gleichenii'' Volkonsky 1931 *''A. griffini'' Sawyer 1971 (T3) *''A. hatchetti'' Sawyer, Visvesvara & Harke 1977 * (T11) *''A. healyi'' Moura, Wallace & Visvesvara 1992 (T12) *''A. hyalina'' Dobel & O'connor 1921 *''A. jacobsi'' Sawyer, Nerad & Visvesvara 1992 *''A. keratitis'' * *''A. lenticulata'' Molet & Ermolieff-braun 1976 (T3) *''A. lugdunensis'' Pussard & Pons 1977 * (T4) *''A. mauritaniensis'' Pussard & Pons 1977 (T4) *''A. micheli'' Corsaro et al. 2015 *''A. palestinensis'' (Reich 1933) Page 1977 * (T1) *''A. paradivionensis'' Pussard & Pons 1977 (T4) *''A. pearcei'' Nerad et al. 1995 *''A. polyphaga'' (Puschkarew 1913) Volkonsky 1931 * (T4) *''A. pustulosa'' Pussard & Pons 1977 (T2) *''A. pyriformis'' (Olive & Stoianovitch 1969) Spiegel & Shadwick 2016 *''A. quina'' Pussard & Pons 1977 * *''A. rhysodes'' (Singh 1952) Griffin 1972 * (T4) *''A. royreba'' Willaert, Stevens & Tyndall 1978 *''A. sohi'' Kyung-il & Shin 2003 *''A. stevensoni'' Sawyer et al. 1993 (T11) *''A. triangularis'' Pussard & Pons 1977 (T4) *''A. tubiashi'' Lewis & Sawyer 1979 (T8)


Etymology

From the Greek ''akantha'' (spike/thorn), which was added before "amoeba" (change) to describe this organism as having a spine-like structure (acanthopodia). This organism is now well known as Acanthamoeba, an amphizoic, opportunistic, and nonopportunistic protozoan protist widely distributed in the environment.


See also

* ''Acanthamoeba'' infection * ''
Balamuthia mandrillaris ''Balamuthia mandrillaris'' is a free-living amoeba that causes the rare but deadly neurological condition granulomatous amoebic encephalitis (GAE). ''B. mandrillaris'' is a soil-dwelling amoeba and was first discovered in 1986 in the brain of a ...
''


References


External links


''Acanthamoeba''
Centers for Disease Control and Prevention The Centers for Disease Control and Prevention (CDC) is the national public health agency of the United States. It is a United States federal agency, under the Department of Health and Human Services, and is headquartered in Atlanta, Georg ...

Video of Acanthamoeba from contact lens keratitis
*

on
Amoeba An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudop ...

Eye health and AcanthamoebaAcanthamoeba pictures and illustrations
{{Taxonbar, from=Q1152218 Amoebozoa genera Discosea Parasitic amoebozoa