Acanthamoeba is a genus of
amoebaeAmoeboids 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...
, one of the most common
protozoaProtozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...
in soil, and also frequently found in
fresh waterFresh Water is the debut album by Australian rock and blues singer Alison McCallum, released in 1972. Rare for an Australian artist at the time, it came in a gatefold sleeve...
and other
habitatsA habitat is an ecological or environmental area that is inhabited by a particular species of animal, plant or other type of organism...
. 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
bacterivoreBacterivores are free-living, generally heterotrophic organisms, exclusively microscopic, which obtain energy and nutrients primarily or entirely from the consumption of bacteria. Many species of amoeba are bacterivores, as well as other types of protozoans. In common all species of bacteria will...
s, but some are opportunists that can cause infections in humans and other animals.
Human pathogen
Diseases caused by
Acanthamoeba include amoebic
keratitisKeratitis is a condition in which the eye's cornea, the front part of the eye, becomes inflamed. The condition is often marked by moderate to intense pain and usually involves impaired eyesight.-Types:...
and
encephalitisEncephalitis is an acute inflammation of the brain. Encephalitis with meningitis is known as meningoencephalitis. Symptoms include headache, fever, confusion, drowsiness, and fatigue...
. The latter is caused by
Acanthamoeba entering cuts and spreading to the
central nervous systemThe central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
.
Acanthamoeba granulomatous encephalitis
This 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
immunodeficiencyImmunodeficiency is a state in which the immune system's ability to fight infectious disease is compromised or entirely absent. Immunodeficiency may also decrease cancer immunosurveillance. Most cases of immunodeficiency are acquired but some people are born with defects in their immune system,...
, diabetes, malignancies, malnutrition,
systemic lupus erythematosusSystemic lupus erythematosus , often abbreviated to SLE or lupus, is a systemic autoimmune disease that can affect any part of the body. As occurs in other autoimmune diseases, the immune system attacks the body's cells and tissue, resulting in inflammation and tissue damage...
, 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
biopsyA biopsy is a medical test involving sampling of cells or tissues for examination. It is the medical removal of tissue from a living subject to determine the presence or extent of a disease. The tissue is generally examined under a microscope by a pathologist, and can also be analyzed chemically...
reveals severe oedema and hemorrhagic
necrosisNecrosis is the premature death of cells in living tissue. Necrosis is caused by factors external to the cell or tissue, such as infection, toxins, or trauma. This is in contrast to apoptosis, which is a naturally occurring cause of cellular death...
. 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. Due to the rarity of this parasite and a lack of knowledge there are currently no good diagnoses or treatments for
Acanthamoeba.
Infection usually mimics that of bacterial leptomeningitis, tuberculous
meningitisMeningitis is inflammation of the protective membranes covering the brain and spinal cord, known collectively as the meninges. The inflammation may be caused by infection with viruses, bacteria, or other microorganisms, and less commonly by certain drugs...
, 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 aureusMethicillin-resistant Staphylococcus aureus is a bacterium responsible for several difficult-to-treat infections in humans. It is also called multidrug-resistant Staphylococcus aureus and oxacillin-resistant Staphylococcus aureus...
(MRSA) is an important pathogen in the hospital setting due to its
resistanceAntibiotic resistance is a type of drug resistance where a microorganism is able to survive exposure to an antibiotic. While a spontaneous or induced genetic mutation in bacteria may confer resistance to antimicrobial drugs, genes that confer resistance can be transferred between bacteria in a...
to many
antibioticAn antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...
s. Recent findings from the
University of BathThe University of Bath is a campus university located in Bath, United Kingdom. It received its Royal Charter in 1966....
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." It has been observed that
Acanthamoeba can increase MRSA numbers by 1000-fold.
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 and probably contributes to the breakdown of organic matter in soil, contributing to the
microbial loopThe microbial loop describes a trophic pathway in the marine microbial food web where dissolved organic carbon is returned to higher trophic levels via the incorporation into bacterial biomass, and coupled with the classic food chain formed by phytoplankton-zooplankton-nekton. The term microbial...
.
Species
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. keratitis*
- 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
endosymbiontAn endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis...
s which are similar to human pathogens. Because of this they are considered to be potential emerging human pathogens. The exact nature of these symbionts and the benefit they represent for the amoebal host still have to be clarified.
These include
LegionellaLegionella is a pathogenic Gram negative bacterium, including species that cause legionellosis or Legionnaires' disease, most notably L. pneumophila. It may be readily visualized with a silver stain....
and Legionella-like pathogens.
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.
Acanthamoeba is important in cellular microbiology, environmental biology, physiology, cellular interactions, molecular biology, biochemistry, and evolutionary studies. This is due to the organism's versatile roles in the ecosystem and its ability to capture prey by phagocytosis, act as vector, reservoir for microbial pathogens, and to produce serious human infections. In addition,
Acanthamoeba has been used extensively to understand the molecular biology of cell motility.
Owing to its ease and economy of cultivation, the Neff strain of
A. castellanii discovered in a pond in
Golden Gate ParkGolden Gate Park, located in San Francisco, California, is a large urban park consisting of of public grounds. Configured as a rectangle, it is similar in shape but 20% larger than Central Park in New York, to which it is often compared. It is over three miles long east to west, and about half a...
in the 1960s, has been effectively utilized as a classic model organism in the field of cell biology. From just 30 liters of simple medium inoculated with
A. castellanii, approximately one kilogram of cells can be obtained after several days of aerated culture at room temperature. Pioneered in the laboratory of Dr. Edward D. Korn at the
National Institutes of HealthThe National Institutes of Health are an agency of the United States Department of Health and Human Services and are the primary agency of the United States government responsible for biomedical and health-related research. Its science and engineering counterpart is the National Science Foundation...
(NIH), many important biological molecules have been discovered and their pathways elucidated using the
Acanthamoeba model. Dr. Thomas Dean Pollard applied this model at the NIH,
Harvard Medical SchoolHarvard Medical School is the graduate medical school of Harvard University. It is located in the Longwood Medical Area of the Mission Hill neighborhood of Boston, Massachusetts....
, Johns Hopkins University School of Medicine, and the
Salk Institute for Biological StudiesThe Salk Institute for Biological Studies is a premier independent, non-profit, scientific research institute located in La Jolla, California. It was founded in 1960 by Jonas Salk, the developer of the polio vaccine; among the founding consultants were Jacob Bronowski and Francis Crick. Building...
to discover and characterize many proteins that are essential for cell motility not only in amoebas but in many other eukaryotic cells, especially those of the human nervous and immune systems, the developing embryo, and cancer cells.
Recall from Advanced Medical Optics
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 Complete Moisture Plus were at higher risk than those who used other contact lens solutions to develop acanthamoeba keratitis.
The manufacturer recalled the product after the Centers for Disease Control in the United States found that 21 individuals had possibly received an acanthamoeba infection after using Complete Moisture Plus in the month prior to diagnosis.
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