What Is an Amoeba?

An amoeba under a microscope — Amoeba proteus with algae filled vacuoles for food production.

(Image credit: Getty Images)

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"Amoeba" is a term that describes a simple eukaryotic organism that moves in a characteristic crawling fashion. However, a comparison of the genetic content of the various amoebas shows that these organisms are not necessarily closely related to each other.

What does an amoeba look like?

Amoebas are eukaryotes. Their single cells, like those of other eukaryotes, possess certain characteristic features: Their cellular contents are enclosed within a cell membrane, and their DNA is packaged into a central cellular compartment called the nucleus, according to a 2014 report published in the journal BMC Biology. In addition, they contain specialized structures called organelles, which perform a range 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 amoebas Entamoeba histolytica, which cause amoebic dysentery in humans, do not have the golgi apparatus, the organelle responsible for modifying and transporting proteins, according to a 2005 study published in The Journal of Biological Chemistry. Researchers found that E. histolytica instead contain golgi-like compartments or vesicles — small fluid-filled pouches — that execute similar functions.

"As it contracts, it does two things," he said. "The contraction pushes the cytoplasm forward to fill the expanding pseudopod, but the contraction also pulls up adhesions at the back end of the cell." Maciver describes these adhesions between an amoeba and the surface on which it moves as physical molecular adhesions, which are constantly formed at the front end and broken at the back. This movement, using pseudopodia, is a characteristic that unites various amoebas and distinguishes them from other protists — simple eukaryotic organisms like amoebas that are not plants, animals or fungi.

Rhizopodia, also known as reticulopodia, are thin filament-like projections that mesh together, and axopodia are rigid and strengthened by an array of microtubular structures called axonemes, according to Ecology and Classification of Northern American Freshwater Invertebrates (Academic Press, 2001). Other pseudopods are supported by structural tube-shaped elements known as microtubules, which are responsible for executing cell movements.

Amoebas can also use their pseudopodia to feed. A 1995 report, published in the journal Applied and Environmental Microbiology, gives the example of the soil-dwelling amoeba Acanthamoeba castellanii, which ingests both solids and liquids using its pseudopodia. The process of ingesting solid material is called phagocytosis, and the process of engulfing drops of liquid is known as pinocytosis, also known as cell drinking, according to Dosage Form Design Considerations (Academic Press/Elsevier, 2018).

For centuries, the various systems of classifying organisms, including amoebas, were based on similarities in observable characteristics and morphology. "There isn't actually a coherent group of organisms called the amoebae," Maciver said. "Rather, amoebae are any protozoan cells that move by crawling." (The term "protozoa" refers to a subset of protists, which again are simple eukaryotic organisms that are not plants, animals or fungi, Live Science previously reported.)

Historically, amoebas were classified together in a single taxonomic group called Sarcodina, distinguished by their use of pseudopodia. Sarcodina amoebas were then subdivided based on the specific type of pseudopodia they used, according to a 2008 article published in the journal Protistology. However, this system of classification didn't capture the evolutionary relationships between the various amoebas — it was not a family tree, so to speak.

Early analyses compared the DNA sequences that encode part of the ribosome, the site of protein synthesis in a cell; specifically, scientists looked at the genes for the so-called 18S subunit of ribosomes, or "SSU rDNA." Based on the analyses of SSU rDNA and other DNA sequences, eukaryotic organisms are now organized in a manner that better represents their evolutionary relationships — the phylogenetic tree, according to the 2008 Protistology article.

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 Amoebozoa. Together, Ophiskontha and Amoebozoa form a larger supergroup called Amorphea, according to the review in the journal Trends in Ecology & Evolution.

"If you look at the great diversity of the protists, you can see that there are amoebae in virtually all the groups," Maciver said. "There's even an amoeboid organism within the brown algae [Labyrinthula]." Most amoebas are present within the Amoebozoa group, though, Maciver said. In addition, he noted that amoebas are also present within Rhizaria and Excavata. Nucleariids, a group of amoebae with filopodia, belong to the Opisthokonta supergroup, for example, and Labyrinthulids fit within the Stramenopiles.

Amoebas are known to cause a range 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 colon wall and cause colitis, where the inner lining of the colon becomes inflamed, and the parasite can cause severe diarrhea and dysentery.

Contact lens wearers are potentially at risk of a rare infection of the cornea called Acanthamoeba keratitis. According to the CDC, species in the Acanthamoeba genus are free-living and are commonly found in soil, air and water. Poor contact lens hygiene practices, such as improper storage, handling and disinfection or swimming with lenses, are some of the risk factors for the disease, the CDC states. Contact lens users can reduce their risk of infection by wearing and cleaning their lenses as prescribed by their eye care provider, and removing their lenses before any activity involving contact with water, including showering, using a hot tub or swimming.

Amoebas also cause different infections of the brain. Naegleria fowleri, which has been dubbed "the brain-eating amoeba," causes primary amoebic meningoencephalitis (PAM). Though the disease is rare, it is almost always fatal, according to the CDC. Early symptoms include fever and vomiting, and the disease ultimately progresses to more severe symptoms such as hallucinations and coma. N. fowleri is present in warm freshwater bodies such as hot springs, lakes and rivers, or in poorly chlorinated swimming pools or contaminated, hot tap water. These amoebas enter from the nose and travel to the brain. However, the infection can’t be contracted by swallowing water, according to the CDC.

Early symptoms include headaches, nausea and low-grade fever, partial paralysis, seizures and speech difficulties. B. mandrillaris is found in the soil and can enter the body through open wounds or when people breathe in contaminated dust, according to the CDC. Since the amoeba was discovered in the 1980s, about 200 cases of the infection have been reported worldwide; this includes more than 100 confirmed cases in the U.S.

Amoebas can also play host to bacteria that are pathogenic to humans, and help those bacteria spread. Bacterial pathogens, such as Legionella, which can cause pneumonia- and flu-like illnesses, can resist digestion when consumed by amoebas, according to a 2018 report in the journal Front Cell Infection Microbiology. Instead, the bacteria are released intact from vacuoles into an amoeba's cytoplasm, where they proliferate inside the cell. In such cases, bacteria can become resistant to treatments designed to control their numbers, including chlorine treatment of water.

Maciver cites the example of cooling towers as a place where both amoebas and these bacteria can grow. Cooling towers tend to expel water droplets, which passersby can breathe in. "What's known to happen on many occasions, is that we breathe in a droplet of water containing an amoeba that is full of these pathogens [Legionella]," he said. If bacteria enter the body of an immunocompromised individual in such a manner, they can ultimately infect macrophages, one of the immune system's many defensive cells.

Amoebas are also important for recycling nutrients in the soil. According to Maciver, when nutrients become available they are taken up by bacteria, which "effectively lock up all the nutrients in bacterial mass." When bacteria are consumed, nutrients are released back into the soil. "If you have a cycle whereby amoeba eat bacteria, the overall effect is to increase nutrient availability for plants," Maciver said.

Aparna Vidyasagar is a freelance science journalist who specializes in health and life sciences. Aparna has written for a number of publications, including New Scientist, Science, PBS SoCal, Mental Floss, and several others. Aparna has a doctorate in Cellular and Molecular Pathology from the University of Wisconsin-Madison, and also received a master’s degree and bachelor’s degree from the same university.

What is an amoeba?

What does an amoeba look like?

How does an amoeba move?

How are amoebas classified?

Why are amoebas important?

Additional resources and readings

Bibliography