Amoeba in the context of "Mesohyl"

A multicellular organism is an organism that consists of more than one cell, and more than one cell type, unlike unicellular organisms. All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- and partially multicellular, like slime molds and social amoebae such as the genus Dictyostelium.

Multicellular organisms arise in various ways, for example by cell division or by aggregation of many single cells. Colonial organisms are the result of many identical individuals joining together to form a colony. However, it can often be hard to separate colonial protists from true multicellular organisms, because the two concepts are not distinct; colonial protists have been dubbed "pluricellular" rather than "multicellular". There are also macroscopic organisms that are multinucleate though technically unicellular, such as the Xenophyophorea that can reach 20 cm.

Protozoa (sg.: protozoan or protozoon; alternative plural: protozoans) are a polyphyletic group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic debris. Historically, protozoans were regarded as "one-celled animals".

When first introduced by Georg Goldfuss, in 1818, the taxon Protozoa was erected as a class within the Animalia, with the word 'protozoa' meaning "first animals", because they often possess animal-like behaviours, such as motility and predation, and lack a cell wall, as found in plants and many algae.

Dictyostelium is a genus of single- and multi-celled eukaryotic, phagotrophic bacterivores. Though they are Protista and in no way fungal, they traditionally are known as "slime molds". They are present in most terrestrial ecosystems as a normal and often abundant component of the soil microflora, and play an important role in the maintenance of balanced bacterial populations in soils.

The genus Dictyostelium is in the order Dictyosteliida, the so-called cellular slime molds or social amoebae. In turn the order is in the infraphylum Mycetozoa. Members of the order are of great theoretical interest in biology because they have aspects of both unicellularity and multicellularity. The individual cells in their independent phase are common on organic detritus or in damp soils and caves. In this phase they are amoebae. Typically, the amoebal cells grow separately and wander independently, feeding mainly on bacteria. However, they interact to form multi-cellular structures following starvation. Groups of up to about 100,000 cells signal each other by releasing chemoattractants such as cyclic AMP (cAMP) or glorin. They then coalesce by chemotaxis to form an aggregate that becomes surrounded by an extracellular matrix. The aggregate forms a fruiting body, with cells differentiating individually into different components of the final structure. In some species, the whole aggregate may move collectively – forming a structure known as a grex or "slug" – before finally forming a fruiting body. Basic processes of development such as differential cell sorting, pattern formation, stimulus-induced gene expression, and cell-type regulation are common to Dictyostelium and metazoans. For further detail see family Dictyostelid.

Amoebozoa is a major taxonomic group containing about 2,400 described species of amoeboid protists, often possessing blunt, fingerlike, lobose pseudopods and tubular mitochondrial cristae. In traditional classification schemes, Amoebozoa is usually ranked as a phylum within either the kingdom Protista or the kingdom Protozoa. In the classification favored by the International Society of Protistologists, it is retained as an unranked "supergroup" within Eukaryota. Molecular genetic analysis supports Amoebozoa as a monophyletic clade. Modern studies of eukaryotic phylogenetic trees identify it as the sister group to Opisthokonta, another major clade which contains both fungi and animals as well as several other clades comprising some 300 species of unicellular eukaryotes. Amoebozoa and Opisthokonta are sometimes grouped together in a high-level taxon, named Amorphea.Amoebozoa includes many of the best-known amoeboid organisms, such as Chaos, Entamoeba, Pelomyxa and the genus Amoeba itself. Species of Amoebozoa may be either shelled (testate) or naked, and cells may possess flagella. Free-living species are common in both salt and freshwater as well as soil, moss and leaf litter. Some live as parasites or symbionts of other organisms, and some are known to cause disease in humans and other organisms.

While the majority of amoebozoan species are unicellular, the group also includes several clades of slime molds, which have a macroscopic, multicellular stage of life during which individual amoeboid cells remain together after multiple cell division to form a macroscopic plasmodium or, in cellular slime molds, aggregate to form one.

Amoebiasis, or amoebic dysentery, is an infection of the intestines caused by a parasitic amoeba Entamoeba histolytica. Amoebiasis can be present with no, mild, or severe symptoms. Symptoms may include lethargy, loss of weight, colonic ulcerations, abdominal pain, diarrhea, or bloody diarrhea. Complications can include inflammation and ulceration of the colon with tissue death or perforation, which may result in peritonitis. Anemia may develop if there is prolonged gastric bleeding.

Cysts of Entamoeba can survive for up to a month in soil or for up to 45 minutes under fingernails. Invasion of the intestinal lining may result in bloody diarrhea. If the parasite reaches the bloodstream it can spread through the body, most frequently ending up in the liver where it can cause amoebic liver abscesses. Liver abscesses can occur without previous diarrhea. Diagnosis is made by stool examination using microscopy, but it can be difficult to distinguish E. histolytica from other harmless entamoeba species. An increased white blood cell count may be present in severe cases. The most accurate test is finding specific antibodies in the blood, though the antibody test may remain positive following treatment. Bacterial colitis can result in similar symptoms.

Antiparasitics are a class of medications which are indicated for the treatment of parasitic diseases, such as those caused by helminths, amoeba, ectoparasites, parasitic fungi, and protozoa, among others. Antiparasitics target the parasitic agents of the infections by destroying them or inhibiting their growth; they are usually effective against a limited number of parasites within a particular class. Antiparasitics are one of the antimicrobial drugs which include antibiotics that target bacteria, and antifungals that target fungi. They may be administered orally, intravenously or topically. Overuse or misuse of antiparasitics can lead to the development of antimicrobial resistance.

Broad-spectrum antiparasitics, analogous to broad-spectrum antibiotics for bacteria, are antiparasitic drugs with efficacy in treating a wide range of parasitic infections caused by parasites from different classes.