An amoeba is a highly motile eukaryotic, unicellular organism. It usually belongs to the kingdom protozoa and moves in an “amoeboid” manner. Therefore, microbiologists often use the term “amoeba” to refer to a specific type of movement and amoeba interchangeably.
Interestingly, amoebas are not a distinct taxonomic group but are characterized by their “amoeba-like” movement rather than different morphological characters.
Furthermore, even members of the same species can appear different. Species of amoebas are found in all major eukaryotic lineages, including fungi, algae, and even animals.
Amoebas contain an endoplasm that is naturally granular. This granular endoplasm contains the nucleus and various enclosed food vacuoles.
In addition, amoebas are eukaryotic by definition, possessing a unique nucleus containing a central karyosome with a thin layer of pearl-shaped chromatin covering the inner nuclear membrane. However, unlike many eukaryotes, amoebas are anaerobic. So, amoebas do not contain mitochondria and produce ATP exclusively in an anaerobic way.
Amoebas can be classified as free-living and parasitic. Parasitic amoebas are ubiquitous and often parasitize higher vertebrates and invertebrates alike. Only a limited number of amoeba species can infect humans and typically invade the gut.
Specifically, only Entamoebahistolytica represents a true human pathogen infecting the gastrointestinal tract. A second enteric pathogen, Dientamoeba fragilis, is commonly mistaken for an amoeba under a light microscope because of its similar morphology.
In fact, D. fragilis was originally misclassified as an amoeba; However, modern methods have identified it as a trichomonas parasite without flagella. Interestingly, some free-living amoebae can cause opportunistic infections in humans, resulting in ocular infections, as well as various neurological and skin infections.
As a class of organisms, amoebas are defined by their unique movement patterns. This motion strategy creates forward motion through the following three steps:
1. “Inflate” the plasma membrane forward. This marked rearrangement is known as the pseudopod, or “false foot,” which is very similar in nature to the lamellipodium produced in higher vertebrates;
2. the pseudopod attaches to the substrate and becomes filled with cytosol;
3. The rear part of the amoeba loosens its attachment to the substrate and is propelled forward.
During amoeboid motion, the viscosity of the cytosol alternates between a fluid-like sol flowing from the central region of the cytoplasm, known as the endoplasm, into the pseudopod at the front of the cell. Once this occurs, the endoplasm becomes an ectoplasm containing a gel-like substance that forms the cortex beneath the plasma membrane.
As the amoeba moves forward, the ectoplasmic gel is again converted into the endoplasmic sol, and the cycle repeats as the cell moves on. This transition between the gel and sol states occurs after the breakdown and reassembly of networks of actin microfilaments located in the cytosol.
In particular, cofinin is responsible for breaking down actin filaments to form the sol, while profilin leads to actin polymerization and the gel is formed from α-actinin and filamin.
Amoeba Size and Shape
Amoebas vary in both size and shape, and even members of the same species can vary widely morphologically. While the earliest identified amoebas were approximately 400 to 600 microns in size, both extremely small (between 2 and 3 microns) and exceptionally large amoebae (20 cm; visible to the naked eye) have been documented to date.
Therefore, amoeba species exhibit a wide range of sizes. Typically, when scientists study amoebas, the samples are passed through a filter about 0.45 microns in size, and the remains on the filter are used for culturing.
Because amoebas use pseudopodia to move and feed, they are classified based on the morphology and internal structure of their pseudopodia.
For example, amoebozoic species (e.g., amoebas) have bulbous pseudopodia with a tubular central portion and rounded ends; Cercozoa amoebas (e.g. Euglypha and Gromia) have pseudopods that appear thin and filamentous; Foraminifera produce slender pseudopodia that branch and fuse together to form web-like structures; others are characterized by rigid, needle-like pseudopodia with a complex network of microtubules.
Free-living amoebas (which do not require a host) are either “testat” or “naked”. Testate amoeba contains a hard shell, while naked amoeba does not. The shells of testate amoebas typically consist of calcium, silica, chitin, or other components (e.g. grains of sand).
Another component typically found in freshwater amoebas is a contractile vacuole. This vacuole is needed to expel excess water from the cell and maintain osmotic balance. Since the concentration of solutes in freshwater is lower than the inner cytosol of the amoeba, water flows through the cell membrane by osmosis.
Therefore, the contractile vacuole pumps this excess water out of the cell to ensure the cell does not rupture. In contrast, most marine amoebas do not possess a contractile vacuole because the cytosol and water outside the amoeba are balanced.
Due to the extremely diverse nature of amoebas, the different species of amoeba reproduce using a variety of different methods. These methods include spores, binary fission, and even sexual.
By far the most common form of asexual reproduction in amoebas is binary fission. In preparation for reproduction, the amoeba retracts its pseudopodia and forms a spherical shape.
In the nucleus, mitosis is observed, and the cytoplasm divides and separates in the center of the cell, forming two daughter cells. Because this process merely copies the genetic information to form a second cell, the two resulting daughter cells are identical clones of the parent cell.
The cell nucleus is therefore absolutely necessary for this form of reproduction. This has been verified in experiments where an amoeba was cut in half or the nucleus extracted from the amoeba. In both situations, the cell eventually dies without a nucleus.
Multiple fission and encystment
Under conditions of food scarcity, amoebas reproduce by multiple fission. This process involves the production of multiple daughter cells by:
- the pseudopodia are retracted and the amoeba forms a spherical shape;
- the amoeba secretes a substance that hardens and encapsulates the cell, forming a cyst (encystment);
- the amoeba protected by the cyst undergoes multiple mitosis, producing multiple daughter cells;
- When favorable conditions return, the cyst wall ruptures and releases the daughter cells. Inside a host, the amoeba is encapsulated on its way through the large intestine to protect against dehydration, ensuring its survival outside the host.
Individual haploid amoebas (known as myxamoebas, or “social amoebas”) live on decaying vegetation (eg, tree trunks), eat bacteria, and reproduce asexually by binary fission, as described above. Unlike the amoebae, which encapsulate when the food supply is exhausted, tens of thousands of myxamoebae fuse, forming a moving stream of cells that converge at a central location.
In this area, the cells stack on top of each other, forming a conical mound called a “dense aggregate.” Next, a spike rises from the top of the conical mound, and the tight aggregate folds into a motile “grex” (also called a pseudoplasmodium or snail), 2–4 mm long and surrounded by a slimy substance.
The grex then migrates to an illuminated area, where it differentiates into a fruiting body composed of a tubular stalk (about 15%–20% of the total cell population) and spore cells. This process involves the secretion of an extracellular coating and the elongation of a tube through the grex by prestalk cells located in the anterior portion of the grex.
When the prestalk cells differentiate into stalk cells, they vacuole and enlarge. This serves to elevate the prespore cells in the posterior portion of the grex. The elevated prespore cells differentiate into spore cells and disperse, each representing a new myxamoeba, while the stalk cells die.
Myxamoebae is also unique in that they can also reproduce sexually. This happens when two myxamoebs fuse into a giant cell. This giant cell then engulfs all the other cells in a myxamoeba aggregate.
After absorbing all of its neighbors, the giant cell will incyst itself and undergo several meiotic and mitotic divisions under the cyst’s protective covering.
When suitable environmental conditions are met, the cyst ruptures and releases new myxamoebas. Because this process involves meiosis and the genetic information of two amoebas, the resulting daughter cells are genetically different from the parent cells.
temperature and reproduction
Temperature is a critical factor affecting amoeba growth. While several amoeba species have been found to grow over a wide temperature range from 10°C to 37°C, pathogenic strains have been found to survive more efficiently at higher temperatures (between 32°C and 37°C).
This suggests that amoebas are highly resilient to temperature changes and most are adapted to survive in humans. Hence, this may have pathogenetic implications as amoebic cysts are extremely resistant to microbicides and can infect humans via contaminated drinking water.