What is an amoeba?

Amoebas use weapons called pseudopods to acquire food.

An amoeba is a type of single-celled organism usually found in water around decaying vegetation, in damp soil, and in animals such as humans. It is relatively advanced and can extend and retract bubble-like arms called pseudopods that can be used to grab food particles and propel itself through microscopic terrain. The name “amoeba” can mean a particular genus of microorganism or any member of a large family of similar life forms. While most types are harmless, some can cause serious illness in humans.


The transparency of most amoebas makes them ideal objects for microscopic study.

These organisms are types of protists, which are a group of simple life forms that are neither plants nor animals. Most are microscopic, but some species are large enough to be visible to the naked eye. The amoeba proteus is the best known species and measures about 0.028 inches (0.7 mm), which is barely visible. Some species can reach 0.1 inch (about 3 millimeters) or more in diameter. German naturalist August von Rosenhof is thought to have been the first to observe these microorganisms in 1757, using an ancient microscope.


Protists are a group of life forms that are neither plant nor animal.

Amoebas are easy to study because they are relatively large and almost transparent. A basic light microscope can therefore reveal its internal structure and organs. The organism’s mobility and complexity make it much more interesting to study than more simplistic protists. Scientists often use the amoeba for testing and observation, calling it a “model organism”.

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Dysentery is caused by amoebas that live in contaminated water.

An amoeba consists of a thin, flexible cell membrane that contains a fluid called cytoplasm and various organs. The nucleus contains the genetic material. The contractile vacuole is used to regulate the amount of water in the cell, and this important organ can absorb water from the cytoplasm and expel it from the cell through the membrane. Food vacuoles are used to store and digest food that the amoeba has absorbed.

Habits and Life Cycle

It is believed that amoebas were first seen under a microscope in the mid-18th century.

Many species of amoeba are found in fresh water, often in mud, at the bottom of swimming pools, or among decaying vegetation. Others live in the human digestive system. These include those that can cause illness, but most simply live on food in the gut without causing any problems. Certain types live in the skin, feeding on dead skin cells.

Amoebas use their pseudopods to get around and obtain food. They can use these extensions to wrap around tiny particles or smaller organisms and incorporate them into the cell, where they are digested. Most species feed on small pieces of organic matter and other microorganisms such as bacteria. Some are parasites that devour the cells of the host animal.

Depending on the salt content of the water the organism is in, it will shrink or swell in an attempt to match its density with that of its surroundings. If one is placed in unsalted water, it can swell so much that it will explode. When an amoeba encounters harsh conditions, such as drought or low temperatures, it can shrink and wrap itself in a tough protective layer. This is known as a cyst. It may exist for long periods in this state until conditions improve, but it may not survive indefinitely.

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Amoebas reproduce asexually by dividing into two “daughter” cells. The nucleus divides first, forming two genetically identical nuclei, a complex process known as mitosis. The organism then divides into two separate cells, each containing a nucleus, a process called cytokinesis.

amoebas and diseases

Some amoebas can cause disease in humans, the best known being amoebic dysentery, or amoebiasis, caused by the organism Entamoeba histolytica. It is transmitted through sewage contaminated water. Humans can be exposed to it by eating food that has come into contact with contaminated water and has not been properly washed. The body attacks the intestines, causing pain and diarrhea. The disease is very unpleasant and can be serious, but it is treatable.

Primary amoebic meningoencephalitis (PAM) is an extremely serious, but fortunately very rare, brain disease caused by an organism called Naegleria fowleri, sometimes called the “brain-eating” amoeba. The infection is almost always fatal. The organism lives in warm lakes and pools in many parts of the world where the water temperature is high enough, including the southern states of the US and Australia.

People swimming or playing in water containing the organism are at risk of infection. It enters the body via the nostrils, migrating to the brain, where it consumes cells, causing damage to brain tissue and inflammation. The disease such as is best prevented by still avoiding likely habitats, warm pools or by wearing nose plugs if swimming in such areas.

Observing Amoebas

For anyone with a microscope, these microorganisms make a fascinating subject for viewing. They are sometimes hard to find, however. One way to obtain living specimens is to use a jar to skim the top layer of mud from the bottom of a pond. Another method is to take a sample of pond water, including some plants, and place a microscope slide cover slip on the surface, so that it floats. After a day or so, bacteria will attach themselves to the lower surface, attracting tiny predators, including amoebas.

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