Naegleria fowleri

Description and significance
Naegleria fowleri (commonly known as the “brain-eating amoeba) is a thermophilic free-living amoeba found in moist environments (25-35 degrees Celsius) including warm fresh water, soil, and sewage. N. fowleri is the causative agent of primary amoebic meningoencephalitis (PAM), a rare but nearly always fatal disease of the central nervous system. Cases of PAM resulting from infection with Naegleria fowleri have been found in countries all around the world including India, England, Czech Republic, China, and southern US states. Naegleria fowleri belongs to the Percolozoa phylum and can exist in different forms including trophozoite, flagellate, and encysted.

Genome structure
Although the genome of N. fowleri has yet not been sequenced, several ways of distinguishing between N. fowleri and non-pathogenic species of Naegleria by PCR have been developed. Since timely diagnosis of primary amoebic meningoencephalitis is imperative for survival, rapid detection of N. fowleri is crucial. A real-time, multiplex PCR assay was recently developed to identify the DNA of N. fowleri from samples of cerebrospinal fluid. The identification can usually be done within five hours of entering the laboratory.

Life Cycle
Naegleria fowleri is an amoeba with a three-stage life cycle.

The trophozoite is the reproductive and feeding stage of the N. fowleri lifecycle. During this stage N. fowleri reproduces via binary fission. This is the only stage of its life cycle during which it reproduces. In this form it measures 10-20 µm in diameter and moves by way of its lobopodium which project from the surface of the bacteria. The lobopodium allow the trophozoites to ingest their food, mostly other bacteria or yeast. Once N. fowleri infects the human nervous system, it enters the trophozoite stage and feeds on red and white blood cells causing tissue damage and eventually necrosis.

When the trophozoite is exposed to water, it may enter the temporary flagellate stage. The flagellate stage of Naegleria fowleri is temporary and allows for rapid movement or swimming. During this stage, N. fowleri is pear-shaped and has two flagellum attached at its broader end. The flagellate stage is when the ameba can infect an individual by swimming up the nasal cavity, where it enters the trophozoite form. The amoeba does not feed or divide during its flagellate stage.

When Naegleria fowleri is exposed to harsh conditions such as aridity or food shortage it enters the cystic, or resistant, stage. The encysted amoeba is round and measures 7-10 µm in diameter. It is protected by a smooth double layered wall that is 1 µm thick.

Ecology
Naegleria fowleri is a thermophilic protist and can be found in many moist or aqueous environments, including freshwater lakes, rivers, ponds, sewage, hot springs, moist soil, and thermally polluted bodies of water. It can also be found in swimming pools or stagnant water that has not been cared for by chlorination. It grows fastest at 42 degrees Celsius, and is able to survive and multiply in mammalian body temperatures. Most N. fowleri infections in the United States occur in the summertime in the Southern tier states, when its growth and reproduction is favored by the warm temperatures. N. fowleri is sensitive to environmental conditions such as desiccation, pH extremes, and high salinity.

Pathology
Naegleria fowleri can cause the disease primary meningoencephalitis (PAM) in humans or animals. Initial symptoms of PAM are closely related to those of bacterial meningitis. Initial symptoms include headache, fever, changes in taste or smell, stiff neck, nausea and vomiting. Later symptoms include confusion, lethargy, seizures and hallucinations. These symptoms are almost always accompanied by rapid onset of coma and death within 3-5 days after symptoms first occur. N. fowleri infects its victims by entering the CNS through the nose, usually while the victims are swimming in warm freshwater lakes or hot springs. The amoebas, which are in flagellate form in the water, swim into nasal cavitiy where they enter the trophozoite (“feeding”) form. They crawl up the olfactory mucosa, along nerve fibers on the floor of the cranium, and through the cribiform plate into the brain. The glucose and protein present in the cerebrospinal fluid of the brain support the growth and rapid multiplication of the amoebas. N. fowleri feeds on red blood cells and white blood cells in the brain resulting in hemorrhaging and necrosis. The mortality rate of PAM is over 95% and in the rare cases of survival early detection is key. One of the few survivors of PAM was treated early and aggressively with intravenous and intrathecal amphotericin B, miconazole, and oral rifampin.

Application to Biotechnology
The use of Naegleria fowleri in current biotechnology is limited to the development of tests for its presence in infected individuals and in hazardous bodies of water or water supplies.