Amoeba

Amoebas are protozoans characterized by cytoplasmic extensions that can change the overall shape of the organism. They consist of a mass of cellular fluid surrounded by a membrane, and contain one or more nuclei (depending upon the species), as well as other cell organelles, such as food vacuoles.

The word amoeba is derived from the Greek word ameibein (to change), which describes the amoeba’s most easily distinguishable feature, the continuous changing of shape by repeated formation of pseudopodia (Greek: false feet).

Pseudopodal movement is based on a continual change in the state of protoplasm flowing into the foot-like appendage. An interior fluid (endoplasm), under pressure from an exterior gel (ectoplasm), flows forward in the cell. When the endoplasm reaches the tip of a developing pseudopod, the fluid is forced backward against the ectoplasm, and is turned into a gel. After returning to the body of the cell, the newly formed ectoplasm gel is converted back to fluid endoplasm, and again flows forward under the pressure of the exterior gel.

Pseudopodia serve two important functions— locomotion and food capture, activities that are often interrelated. Amoebas use their pseudopodia to ingest food by a method called phagocytosis (Greek: phagein, to eat). The streaming of protoplasm inside the pseudopodia moves the amoeba forward. When the organism contacts a food particle, the pseudopodia surround the particle. After the amoeba corrals the food, the food passes into the cell via food vacuoles. Inside the cell, the food is digested by enzymes within food vacuoles and assimilated by the amoeba. Later, the amoeba expels parts of the food that are not acceptable.

Organisms referred to by the term “amoeba” belong to the Domain Eukarya, Kingdom Protista, Subkingdom Protozoa, which almost exclusively includes single-celled eukaryotes that get nutrition from other organisms. Amoebas belong to the class Sarcodina, which has as its principle-distinguishing feature the presence of pseudopodia.

The Rhizopoda (in some classifications a superclass of Sarcodina) contains all organisms that form lobopodia, which are broad pseudopodia, or filopodia, which are thread-like psuedopodia. These organisms move by means of pseudopodial movement. Rhizopoda includes the subclass Lobosea. Lobosea includes both naked amoebas and testaceous forms (those with tests, or shells).

Amoebas that have tests are sometimes referred to as shelled. These tests include coatings of various kinds, such as scales, mucoid layers called glycocalyces, or complex filaments much smaller than scales.

Amoebas are further classified within the order Amoebida. Amoebida are characterized as free-living in fresh or saltwater or soil. Most Amoebida ingest bacteria, but larger members also feed on algae and other protozoans. Amoebida is further divided into several families including Amoebidae, Hartmannellidae, Entamoebidae, and Acanthamoebidae.

The family Amoebidae includes mostly freshwater species of amoeba, whose pseudopodal movement is either monopodial (the entire protoplasmic mass moves forward) or polypodial (several pseudopodia advance simultaneously). One member, Amoeba proteus is commonly used for teaching and cell biology research. Chaos carolinense, one of the larger species of amoeba, has multiple nuclei and can reach a length of 0.12 in (3 mm).

The Hartmannellidae family includes small and medium-sized amoebas that move forward monopodially, advancing by means of a steady cytoplasmic flow. They feed on bacteria, although some species of the genus Saccamoeba also feed on unicellular algae.

The family Entamoebidae includes most of the obligately endozoic (parasitic inside a host) Amoebida organisms, including Entamoeba histolytica. Amebiasis (infection with E. histolytica ) is a serious intestinal disease also called amoebic dysentery. It is characterized by diarrhea, fever, and dehydration. Although amebiasis is usually limited to the intestine, it can spread to other areas of the body, especially the liver.

E. histolytica exists as either a trophozoite or cyst. The trophozoite is motile, possesses a single nucleus, and lives in the intestine. It is passed from the body in diarrhea, but cannot survive outside the host. The cyst form, consisting of condensed protoplasm surrounded by a protective wall, is produced in the intestine, can survive outside the host, and even withstands the acid of the stomach when it is ingested with food or contaminated water. Once inside the intestine, E. histolytica multiplies by means of binary fission.

Acanthamoebidae, another family of Amoebida, includes the genus Acanthamoeba, members of which are often isolated from fresh water and soil. Acanthamoeba cause primary amebic meningoencephalitis (PAM, inflammation of the brain and its protective membranes), especially in individuals who are ill and whose immune systems are weakened. Acanthamoeba infections have been traced to fresh water, hot tubs, soil, and homemade contact lens solutions. In the latter case, contamination of contact lens solution with the organism has caused keratitis, an inflammation of the cornea accompanied by pain and blurred vision. Severe cases can require a corneal transplant or even removal of the eye.

An amoeba of the order Schizopyrenida, Naegleria fowleri is an especially dangerous human parasite, causing rapidly fatal PAM in people swimming in heated water, or warm, freshwater ponds and lakes, mainly in the southern United States. Both Naegleria and Acanthamoeba enter through the nasal mucosa and spread to the brain along nerves.