Amoebas in Pond Water, Dysentery and Meningoencephalitis
Amoebas are fascinating liitle creatures with a body made of just one cell. Despite this apparent limitation, they have many abilities and can live in a wide variety of habitats. They eat other organisms and catch their prey by surrounding and engulfing them. Most amoebas are microscopic, but some have a diameter of several millimetres and are visible without a microscope.
"Amoebas" are generally considered to be one-celled creatures that move by a process called amoeboid movement in at least one stage of their life cycle. Some belong to the genus Amoeba, but others belong to related genera. During amoeboid movement, amoebas extend projections called pseudopods from their cell membrane and then slowly flow into the pseudopods. The pseudopods can be extended from different parts of the cell and in different directions. As a result, an amoeba's shape is constantly changing.
Some amoebas live in pond water. They may also be found in other bodies of fresh water, in salt water and in wet soil. They even live in our bodies. Unfortunately, some amoebas can cause disease, including dysentery (inflammation in the intestine) and meningoencephalitis (inflammation in the brain).
Diagram of a Typical Amoeba
The word "amoeba" is sometimes spelled "ameba". The plural form is either amoebas, amebas, amoebae or amebae. All the variations are considered to be correct.
Structure of an Amoeba
As in animal and human cells, the outermost layer of an amoeba is the cell membrane. Inside the membrane are the nucleus and the cytoplasm. The cytoplasm consists of organelles and granules embedded in a fluid called the cytosol. Organelles are structures that perform specific functions in a cell.
Two important organelles in the cytoplasm of an amoeba are the food vacuole and the contractile vacuole. The food vacuole traps and digests the prey. Water is continually entering the amoeba's body from its aqueous environment. The contractile vacuole absorbs this water and expands as it does so. When it's full, the vacuole releases the water to the outside through the cell membrane. The nucleus is also an important organelle in the cell. It contains the genes, which in turn contain the genetic code that controls many aspects of the cell's structure and function.
The thin, outermost layer of an amoeba's cytoplasm is known as ectoplasm while the innermost layer is called the endoplasm. The ectoplasm has a thicker consistency than the endoplasm, which is less viscous.
A Moving Amoeba
The words pseudopodium (singular) and pseudopodia (plural) are sometimes used instead of pseudopod and pseudopods. All of the terms are considered to be correct.
A pseudopod starts as a bulge on the cell membrane of the amoeba. The surface of this bulge consists of ectoplasm and the inner part is composed of endoplasm. The ectoplasm appears to form an enclosed tube or sleeve around the endoplasm as the pseudopod extends. The endoplasm can be seen streaming forward in the centre of a pseudopod when a moving amoeba is observed.
Amoeboid movement is a complex process that isn't well understood. It's known that endoplasm (sometimes known as plasmasol) can be converted into ectoplasm (plasmagel), and vice versa. A leading theory says that as the flowing endoplasm reaches the tip of a pseudopod, it's deflected and converted to ectoplasm. At the other end of the pseudopod, ectoplasm becomes endoplasm and somehow contributes to the forward flow of fluid in the pseudopod. There are unanswered questions about this process, however.
Scientists have discovered that pseudopods contain two proteins called actin and myosin. They think that these proteins are responsible for the cytoplasmic motion in pseudopods, although their mechanism of action hasn't been confirmed. The proteins are involved in the movement of structures and materials in our cells and in the cells of other organisms. They are also involved in the movement of muscles.
Pseudopods Catching Prey - Phagocytosis
Amoeba proteus is a microscopic organism with a large cell. It's available at most science supply companies that sell living organisms and is often used by schools. In nature it lives in bodies of fresh water, such as ponds. It feeds on smaller one-celled creatures, algae and rotifers. Rotifers are multicellular creatures but are smaller than many unicellular organisms.
An amoeba appears to extend pseudopods with purpose when it's trying to catch its prey, but it's actually responding to specific stimuli created by the prey's presence. Researchers have discovered that pseudopods are extended due to a variety of stimuli, including chemical gradients, temperature gradients and electric fields.
The amoeba extends its pseudopods around and over its prey, trapping it in a drop of water. Cell membrane surrounds the prey on all sides, forming a food vacuole, which is incorporated into the amoeba's body. This method of feeding is called phagocytosis. Once the prey is trapped, digestive enzymes enter the food vacuole. These kill the prey and digest it.
People are sometimes amazed that such a slow-moving creature as an amoeba can catch such fast-swimming creatures as those shown in the video above. The amoeba's lack of speed is actually an advantage because its prey may not detect that it's being surrounded.
Amoeba proteus reproduces by a process called binary fission, which is a type of asexual reproduction. First the nucleus of the cell divides into two nuclei, and then the cell divides into two cells. One nucleus goes into each of the "daughter" cells, which soon enlarge to become the size of the parent cell.
A Living Amoeba proteus
Entamoeba histolytica and Amoebic Dysentery
Dysentery is a disorder in which the intestine becomes inflamed due to an infection. It's accompanied by abdominal pain, diarrhea and feces containing blood and mucus. There may also be a fever and vomiting. One cause of dysentery is an amoeba named Entamoeba histolytica. An intestinal infection by this bacterium is known as amoebiasis or amebiasis.
The disease begins when a person swallows food or liquid contaminated with Entamoeba histolytica cysts. A cyst is a thick-walled structure containing an inactive cell. There may not be any symptoms of the amoeba infection, but sometimes dysentery develops.
Once inside the digestive tract the cysts may open up, releasing trophozoites, which are amoeboid cells. A trophozoite is the feeding stage of a parasite. The Entamoeba histolytica trophozoites divide to make new cells. The cells then adhere to the lining of the large intestine, where they feed on bacteria and cellular debris. They may sometimes penetrate the surface of the lining, producing an ulcer. The activities of the bacteria cause colitis, which is an inflammation of the colon, the largest part of the large intestine. Colitis leads to the unpleasant symptoms of dysentery.
Some trophozoites in the lumen (cavity) of the intestine don't attach to the intestinal lining but form cysts instead. The cysts pass out of the body in the feces and can infect other people when they are swallowed in contaminated food or water.
Treating and Preventing Amoebic Dysentery
Amoebic dysentery is usually treatable by medications. Occasionally, however, the trophozoites pass through the intestinal lining. enter the bloodstream and become invasive. They may reach the liver, the lungs or the brain via the circulatory system. This condition is much harder to treat and can be very dangerous.
Amoebic dysentery occurs around the world, but it's most common in countries with poor sanitation. If someone is travelling in one of these countries it's important that they boil or otherwise disinfect water and avoid eating uncooked vegetables or unpeeled fruit.
Entamoeba histolytica Life Cycle
The Brain-Eating Amoeba - Naegleria fowleri
Although Naegleria fowleri is sometimes called the "brain-eating amoeba", it doesn't normally live in the human body. In addition, some people feel that it shouldn't be called an amoeba at all, since its life cycle is very different from that of other amoebas.
Naegleria fowleri usually lives in warm, freshwater habitats, including ponds, lakes, moist soil and even hot springs. The brain may become infected with the organism when water containing the parasite enters a person's nose. There have also been cases of people becoming infected from contaminated water in a neti pot. Neti pots are used to clean out sinuses. The parasite can travel from the sinuses to the brain.
Naegleria fowleri exists in three different forms during its life cycle: an inactive cyst, a motile trophozoite that somewhat resembles a typical amoeba, and another motile form with two long, whip-like extensions at one end. Each of these extensions is called a flagellum. The moving flagella propel the organism through a liquid.
- The cyst is an inactive form surrounded by a tough, protective wall. It forms during unfavourable environmental conditions, such as when the surroundings become too cold or too dry.
- The trophozoite is released from the cyst when the environmental conditions become favourable again. This is the main stage of the life cycle and is the cell that feeds.
- The flagellated form develops from the trophozoite when it's exposed to certain environmental conditions, such as insufficient food in an area. In experiments, scientists have observed that the flagellated form develops when Naegleria fowleri enters distilled water. The flagellated cell is a temporary and non-feeding stage.
Stages of the Naegleria fowleri Life Cycle
Naegleria fowleri - A Radio Interview with a CDC Doctor
Meningoencephalitis is a combination of meningitis and encephalitis. Meningitis is inflammation of the membranes around the brain, which are known as the meninges. Encephalitis is inflammation of the brain itself.
Primary Amoebic Meningoencephalitis, or PAM
Naegleria fowleri occasionally enters the brain and cause a disease called primary amoebic meningoencephalitis, or PAM. The parasite causes brain swelling and destruction of brain tissue. Thankfully, the infection is rare. From 2006 to 2015, only thirty-seven people became infected with the parasite in the United States (as far as the CDC knows). Sadly, though, for those who do get sick, the death rate is very high, as the quote below shows.
On average, symptoms appear around five days after the initial infection. The first symptoms may include a headache, nausea, vomiting and a fever. Later other symptoms may develop, including a stiff neck and symptoms indicating brain problems, such as confusion, balance problems, hallucinations and seizures.
There are strategies that may lower the risk of contracting PAM. Avoiding entering fresh water during the warmest time of the year is the best method of protection. Other helpful methods may include using a nose clip when entering water, keeping the head above the water surface and avoiding stirring up bottom sediments. However, as the CDC says, although these are "common sense" recommendations, they haven't been tested scientifically to see if they can actually prevent infection.
The fatality rate (from PAM) is over 97%. Only 3 people out of 138 known infected individuals in the United States from 1962 to 2015 have survived.— CDC (Centers for Disease Control and Prevention)
Other Types of Amoebas
There are many other types of amoebas in existence. For example, Entamoeba coli lives harmlessly in our large intestine and Entamoeba gingivalis lives inside our mouth in gum pockets, possibly harmlessly but perhaps contributing to gum disease. Chaos carolinensis is a giant freshwater amoeba that may be two millimetres in diameter or even larger. It has hundreds of nuclei instead of only one.
Some amoeboid organisms have an outer test, or shell, which is either secreted by the cell or made from outside materials that stick to the surface of the cell. The pseudopods are narrow and needle-like and extend through openings in the test.
The amoeboid way of life seems to be very successful. Even some of the white blood cells in our body move by amoeboid movement and engulf bacteria by phagocytosis. Understanding how an amoeba functions may help us understand how other cells function, including those of humans.
© 2013 Linda Crampton
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