Percolozoa

Percolozoa is a taxonomic phylum of protozoans best known for their species that can transform between amoeboid, flagellate, and encysted stages. They also include the acrasids, a group of social amoebae that aggregate to form sporangia. Most Percolozoa are found as bacterivores in soil, freshwater, and on feces. There are a few marine and parasitic forms, including the species Naegleria fowleri, which can become pathogenic in humans and is often fatal.

Anatomy
Naegleria fowleri has three stages in its life cycle, cysts, trophozoites, and flagellate forms. Cysts do not form in human tissue, but in the environment and culture are spherical, 7-15 µm in diameter and have a smooth, single-layered wall, and have a single nucleus.

There are two forms of trophozoites in Naegleria fowleri: amoeboid and ameboflagellate, only the former of which is found in humans. The ameboid trophozoites measure 10-35 µm but when rounded are usually 10-15 µm in diameter. In culture, trophozoites may get over 40 µm. The cytoplasm is granular and contains many vacuoles. The single nucleus is large and has a large, dense karyosome and lacks peripheral chromatin.

Reproduction


The image at left shows the life cycle of Naegleria fowleri and the three forms that are involved: cysts (1), trophozoites (2), and flagellate (3). They are found in fresh water, soil, thermal discharges of power plants, heated swimming pools, hydrotherapy and medicinal pools, aquariums, and sewage. The Trophozoite stage infect humans or animals by penetrating the nasal mucosa and migrating to the brain via the olfactory nerves causing primary amebic meningoencephalitis (PAM).

The trophozoites replicate by promitosis (nuclear membrane remains intact). They often turn into temporary non-feeding flagellated forms which usually revert back to the trophozoite stage. N. fowleri trophozoites are found in cerebrospinal fluid (CSF) and tissue, while flagellated forms are occasionally found in CSF.