What is an amoeba?
Amoebae are single-celled microbes that "crawl" and can sometimes eat your brain.
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| Amoeba proteus with algae filled vacuoles for food production. (Image credit: Getty Images ) |
"Amoeba" is a term that describes a simple eukaryotic organism that moves in a characteristic crawling manner. However, a comparison of the genetic content of different amoebae shows that these organisms are not necessarily closely related.
WHAT DOES AN AMOEBA LOOK LIKE?
All living organisms can be broadly divided into two groups — prokaryotes and eukaryotes — characterized by the relative complexity of their cells. Eukaryotes are highly organized unicellular or multicellular organisms such as animals and plants. Prokaryotes, on the other hand, are basic single-celled organisms such as bacteria and archaea.
Amoebae are eukaryotes. Their individual cells, like those of other eukaryotes, have certain distinguishing features: Their cell contents are enclosed in a cell membrane, and their DNA is packaged in a central cell compartment called the nucleus, according to a 2014 report published in the journal BMC Biology. (opens in a new tab). In addition, they contain specialized structures called organelles that perform a variety of cellular functions including energy production and protein transport.
Most of these organelles are common to all eukaryotic cells, but there are a few exceptions. For example, the parasitic amoebae Entamoeba histolytica, which cause amoebic dysentery in humans, lack the Golgi apparatus, an organelle responsible for modifying and transporting proteins, according to a 2005 study published in The Journal of Biological Chemistry (opens in a new tab) . The researchers found that E. histolytica instead contains Golgi compartments or vesicles—small fluid-filled sacs—that perform similar functions.
There are also amoebae that do not have mitochondria, the organelles responsible for generating cellular energy, because they live in oxygen-poor or "anoxic conditions," Sutherland Maciver, a reader in the Department of Biomedical Sciences at the University of Edinburgh. , he told Live Science.
According to a 2014 review published in the journal Biochemistry (opens in new tab), these mitochondria-less organisms may contain organelles called hydrogenosomes or mitosomes, which are related to mitochondria but are thought to be highly altered versions of the organelles. This is the case of E. histolytica and the free-living amoeba Mastigamoeba balamuthi, whose survival does not depend on other organisms.
HOW DOES AN AMOEBA MOVE?
In structure, amoebas are very similar to the cells of higher organisms. "They're like our cells, and in fact, when they're moving, they look very similar to our white blood cells," Maciver said. (White blood cells are immune cells that help defend the body against disease.)
Like our white blood cells, amoebae move using pseudopodia, which is Latin for "false legs." These short-lived protrusions of cytoplasm—the semi-fluid material inside the cell membrane—help the amoebae grip the surface and move forward. As the pseudopodium moves across the surface in one direction, the rear end of the amoeba contracts, Maciver said.
"As it closes, it does two things," he said. "The contraction pushes the cytoplasm forward to fill the expanding pseudopod, but the contraction also pulls out the adhesions at the back end of the cell." Maciver describes these adhesions between the amoeba and the surface on which it moves as physical molecular adhesions, constantly forming at the anterior end and breaking at the posterior. This movement, using pseudopodia, is a characteristic that unites various amoebae and distinguishes them from other protists—simple eukaryotic organisms such as amoebae that are not plants, animals, or fungi.
According to Human Parasitology (opens in new tab) (Academic Press/Elsevier, 2019), four different types of pseudopodia are seen among amoebae: filopodia, lobopodia, rhizopodia, and axopodia. The most common form of parasitic amoebae has lobopodia, which are broad, blunt cytoplasmic projections, while filopodia are thin, thread-like projections.
Rhizopodia, also known as reticulopodia, are thin thread-like projections that fit together, and axopodia are stiff and reinforced by a series of microtubular structures called axonemes, according to Ecology and Classification of North American Freshwater Invertebrates (opens in new tab) (Academic Press, 2001) . Other pseudopods are supported by structural tubular elements known as microtubules, which are responsible for carrying out cellular movements.
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Amoebae can also use their pseudopodia for feeding. A 1995 report published in the journal Applied and Environmental Microbiology (opens in a new tab) gives the example of the soil amoeba Acanthamoeba castellanii, which takes in both solids and liquids using its pseudopodia. The process of ingesting solid material is called phagocytosis, and the process of engulfing liquid droplets is known as pinocytosis, also known as cell drinking, according to Dosage Form Design Considerations (opens in new tab) (Academic Press/Elsevier, 2018).
"Most known amoebae eat bacteria," Maciver told Live Science. He explained that amoebae have receptors on their cell surface that bind to bacteria, which are then taken up by phagocytosis into the amoebae, usually at the back of the cell.
In the case of giant amoebae, such as Amoeba proteus, the process of phagocytosis is slightly different, according to Maciver. Giant amoebae engulf their prey by "deliberately gathering pseudopods around bacteria". In both cases, as the bacterium is drawn in, the cell membrane surrounding it contracts to form an intracellular compartment called a vacuole.
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| Entamoeba histolytica is a human intestinal parasite. It can cause colitis, severe diarrhea and dysentery. (Image credit: CDC/ Dr. Mae Melvin) |
HOW ARE AMOEBAS CLASSIFIED?
For centuries, various classification systems for organisms, including amoebae, have been based on similarities in observable characteristics and morphology. "There really isn't a coherent group of organisms called amoebae," Maciver said. "Amoebae are rather any protozoan cells that move by crawling." (The term "protozoa" refers to a subgroup of protists, which are again simple eukaryotic organisms that are not plants, animals or fungi, Live Science previously reported.)
Historically, amoebae were classified together in a single taxonomic group called Sarcodina, which was characterized by the use of pseudopodia. Sarcodina amoebae were then classified based on the specific type of pseudopodia they used, according to a 2008 article published in the journal Protistology (opens in new tab). However, this system of classification did not capture the evolutionary relationships between the various amoebae—it was not a family tree, so to speak.
Molecular phylogenetics changed the course of the taxonomic classification of eukaryotes. By comparing similarities and differences in specific DNA sequences in organisms, scientists were able to discern how closely related they are, according to a 2020 review in the journal Trends in Ecology & Evolution (opens in a new tab).
The first analyzes compared the DNA sequences that code for part of the ribosome, the site of protein synthesis in the cell; specifically, the researchers looked at the genes for the so-called 18S ribosomal subunit, or "SSU rDNA." Based on analyzes of SSU rDNA and other DNA sequences, eukaryotic organisms are now organized in a way that better represents their evolutionary relationships—a phylogenetic tree, according to a 2008 Protistology article.
Each lineage in the phylogenetic tree is represented by a branched structure. In this system, the first levels are known as "supergroups". Fabien Burki, author of a 2014 review article published in the journal Cold Spring Harbor Perspectives in Biology (opens in a new tab), described these supergroups as the "building blocks" of the tree.
Burki listed five supergroups for eukaryotic organisms: Ophiskontha, Amoebozoa, Excavata, Archaeplastida, and SAR, which includes three subgroups named Stramenopiles, Alveolata, and Rhizaria. Animals and fungi are in the group Ophiskontha. Amoeboid protists and some parasitic lineages that lack mitochondria are part of the Amoebozoa. Together, Ophiskontha and Amoebozoa form a larger supergroup called Amorphea, according to a review in the journal Trends in Ecology & Evolution.
Heterotrophic protists—organisms that receive nutrients from other organisms—are part of the Excavata, while plants and most other photosynthetic organisms are part of the Archaeplastida, according to The Encyclopedia of Evolutionary Biology (opens in new tab) (Academic Press/Elsevier, 2016) .
"If you look at the great diversity of protists, you can see that there are amoebae in virtually every group," Maciver said. "There's even an amoeboid organism in the brown algae [Labyrinthula]. However, most amoebae are present in the Amoebozoa group, Maciver said. In addition, he noted that amoebae are also present in Rhizaria and Excavata. Nuclearoids, a group of amoeba with filopodia, for example, belong to the supergroup Opisthokonta, and Labyrinthulids belong to the Stramenopilia.
WHY ARE AMOEBAS IMPORTANT?
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| A 3D illustration of the brain-eating amoeba, Naegleria fowleri. (Image credit: Getty Images) |
Amoebae are known to cause a number of human diseases. Amebiasis, or amoebic dysentery, is an infection caused by E. histolytica, a human intestinal parasite, according to the Centers for Disease Control and Prevention (CDC). According to the medical database StatPearls , E. histolytica can invade the wall of the colon and cause colitis, where the lining of the colon becomes inflamed and the parasite can cause severe diarrhea and dysentery.
Although E. histolytica infection can occur anywhere in the world, it is most common in tropical regions that have inadequate sanitation systems and overcrowded conditions.
Contact lens wearers are potentially at risk of a rare corneal infection called Acanthamoeba keratitis. According to the CDC, Acanthamoeba species are free-living and commonly found in soil, air, and water. Poor hygiene practices for contact lenses, such as improper storage, handling and disinfection or swimming with lenses, are some of the risk factors for the disease, according to the CDC. Contact lens wearers can reduce their risk of infection by wearing and cleaning lenses as prescribed by their eye care provider and removing lenses before any activity involving contact with water, including showering, using a hot tub, or swimming.
While initial symptoms include redness, itching and blurred vision, if the infection is left untreated, it can cause severe pain and lead to vision loss, according to the CDC.
Amoebae also cause various brain infections. Naegleria fowleri, which has been called the "brain-eating amoeba," causes primary amoebic meningoencephalitis (PAM). Although rare, the disease is almost always fatal, according to the CDC. Early symptoms include fever and vomiting, and the disease eventually progresses to more severe symptoms such as hallucinations and coma. N. fowleri is found in warm freshwater bodies such as hot springs, lakes and rivers, or in poorly chlorinated swimming pools or contaminated hot tap water. These amoebae enter from the nose and travel to the brain. However, the infection cannot be contracted by swallowing water, according to the CDC.
Another amoeba, Balamuthia mandrillaris, can cause an infection of the brain known as granulomatous amoebic encephalitis (GAE). Balamuthia infections are rare but often fatal. Current estimates suggest the infection has a 90% fatality rate, according to the CDC .
Early symptoms include headache, nausea and low-grade fever, partial paralysis, seizures, and difficulty speaking. B. mandrillaris is found in soil and can enter the body through open wounds or when people inhale contaminated dust, according to the CDC. Since the discovery of the amoeba in the 1980s, about 200 cases of infection have been reported worldwide; this includes more than 100 confirmed cases in the US.
Amoebae can also host and aid in the spread of bacteria that are pathogenic to humans. According to a 2018 report in the journal Front Cell Infection Microbiology, bacterial pathogens such as Legionella, which can cause pneumonia- and flu-like illnesses, can resist digestion when consumed by amoebae. Instead, the bacteria are released intact from the vacuoles into the cytoplasm of the amoeba, where they multiply inside the cell. In such cases, the bacteria can become resistant to treatments designed to control their numbers, including treating the water with chlorine.
Maciver gives the example of cooling towers as places where amoebas and these bacteria can grow. Cooling towers tend to expel water droplets that can be inhaled by passers-by. "It is known that on many occasions we breathe in a drop of water containing an amoeba that is full of these [Legionella] pathogens," he said. If bacteria enter the body of an immunocompromised individual in such a way, they can eventually infect macrophages, one of the immune system's many defense cells.
"Not only does a macrophage look like an amoeba, but its biochemical pathways and cell biology are quite similar," Maciver said. "So the same programmed events that allow the bacteria to escape from the amoeba now work to allow Legionella to escape from the macrophages."
In addition to their role in human disease, amoebae are also an important part of the soil ecosystem. Amoebae feed on harmful bacteria and regulate their population in the soil, according to a 2021 review in the journal Applied and Environmental Microbiology.
Amoebae are also important for recycling nutrients in the soil. According to Maciver, when the nutrients are available, they are taken up by the bacteria, which "effectively locks all the nutrients in the bacterial mass." When bacteria are consumed, nutrients are released back into the soil. "If you have a cycle where the amoeba eats the bacteria, the overall effect is to increase the availability of nutrients to the plants," Maciver said.