Naegleria fowleri is a thermophilic (heat-loving), free-living amoeba. It is found in warm and hot freshwater ponds, lakes and rivers, and in the very warm water of hot springs. As the water temperature rises, its numbers increase. The amoeba was identified in the 1960s in Australia but appears to have evolved in the United States.
N. fowleri occurs in three forms – as a cyst, a trophozoite (ameboid), and a biflagellate (it has two flagella). It does not form a cyst in human tissue, where only the amoeboid trophozoite stage exists. The flagellate form can exist in the cerebrospinal fluid.
Biotic phases: cyst, trophozoite, flagellate
The cyst form is spherical and about 7–15 µm in diameter. They are smooth, and have a single-layered wall with a single nucleus. Cysts are naturally resistant to environmental factors, so as to increase the chances of survival until better conditions occur. Trophozoites encyst due to unfavorable conditions. Factors that induce cyst formation include a lack of food, overcrowding, desiccation, accumulation of waste products, and cold temperatures. When conditions improve, the amoeba can escape through the pore, or ostiole, seen in the middle of the cyst. N. fowleri has been found to encyst at temperatures below 10 °C (50 °F).
The trophozoite is the feeding, dividing, and infective stage for humans. The trophozoite attaches to olfactory epithelium, where it follows the olfactory cell axon through the cribriform plate (in the nasal cavity) to the brain. This reproductive stage of the protozoan organism, which transforms near 25 °C (77 °F) and grows fastest around 42 °C (106.7 °F), proliferates by binary fission. The trophozoites are characterized by a nucleus and a surrounding halo. They travel by pseudopodia, which means that they extend parts of their body's cell membrane (the pseudopods) and then fill them with plasma to force locomotion. The pseudopods form at different points along the cell, thus allowing the trophozoite to change directions. In their free-living state, trophozoites feed on bacteria. In tissues, they phagocytize (consume by enclosing and then digesting prey) red blood cells and white blood cells, destroying tissue.
The flagellate is pear-shaped and biflagellate: this means that it has two flagella. This stage can be inhaled into the nasal cavity during swimming or diving. This biflagellate form occurs when trophozoites are exposed to a change in ionic concentration, such as placement in distilled water. The flagellate form does not exist in human tissue, but can exist in the cerebrospinal fluid. Once inside the nasal cavity, the flagellated form transforms into a trophozoite. The transformation of flagellate to trophozoite occurs within a few hours.
Naegleria fowleri are
amoeboflagellates that inhabit soil and water. N. fowleri is more sensitive to drying and pH (acid levels) than other amoeboflagellates. It cannot survive in sea water. This amoeba is able to grow best at moderately elevated temperatures making summer month cases more likely. N. fowleri is
thermotolerant and able to survive 46 °C (115 °F). Warm, fresh water with a sufficient supply of bacterial food provides a habitat for amoebae. Man-made bodies of water, disturbed natural habitats, or areas with soil and unchlorinated/unfiltered water are locations where many amoebic infections have occurred.
N. fowleri seems to thrive during periods of disturbance; the flagellate-empty hypothesis explains that Nagleria's success may be due to decreased competition from a depleted population of the normal, thermosensitive protozoal fauna.
In other words, N. fowleri thrives in the absence of other predators consuming its food supply. This hypothesis suggests that human disturbances such as thermal pollution increase N. fowleri abundance by removing their resource competitors. Ameoboflagellates have a motile flagellate stage that is designed for dispersal, which is advantageous when an environment has been cleared of competing organisms.