LIFE CYCLE AND MORPHOLOGY


Life cycle

courtesy of Markell and Voge's Medical Parasitology

The life cycle of N. fowleri has three stages: trophozite (ameba), flagellate, and cyst stage. In the ameboid trophozite stage, the organism feeds on bacteria and replicates through promitotisis, a type of binary fission where the nuclear membrane remains intact. When there is a chance in ionic concentration, for instance if the organism is placed in distilled water, or when nutrients are low, the trophozite can temporarily change into the flagellate form. The trophozite will generally revert back to the trophozite form when conditions are restored. When conditions are very unfavorable, for instance if the surrounding temperature is too cold, the trophozite will encyst.

In primary amebic meningoencephalitis, usually only the trophozite is found in the tissue and cerebral spinal fluid, although sometimes the flagellate form can be found in the CSF. While trophozites feed on bacteria in the external environment, in the body they feed on red and white blood cells as well as tissue.

Note that the life cycle of N. fowleri differs from that of the other pathogenic, free-living amoeba, Acanthamoeba and Balamuthia. Both Acanthamoeba and Balamuthia only have trophozite and cyst stages--they do not have flagellate stages. This is useful in distinguishing N. fowleri, since placing the organism in distilled water will reveal the flagellate form only in the case of N. fowleri.

Morphology

In all three stages, there is one nucleus with a large karyosome and no peripheral chromatin.

Trophozite/Ameba:

Trophozites are about 10-20 micrometers in diameter, have a "limax," or sluglike, shape, and are mobile. Movement occurs by the organism extending broad, blunt pseudopodia (called lobodium), and then having the intracellular fluid and contents flow into the projection. Trophozites have mitochondria and demonstrate aerobic metabolism.

 

Courtesy of Medical Protozology at http://www.tulane.edu/~wiser/protozoology

Flagellate:

The flagellate is pear-shaped and has two flagellum. This form can be used to distinguish N. fowleri from other pathogenic free-living amoeba, which do not have a flagellate form. The sample in question can be placed in distilled water, inducing the trophozite to convert to the flagellate form if the organism is N. fowleri.

Courtesy of Markell and Voge's Medical Parasitology

Cyst:

The spherical cyst is single-walled and 8-12 micrometers in diameter. The ameba encysts under harsh conditions. When conditions improve, the ameba can escape the cyst through the pore, or ostiole, seen in the middle of the cyst.

 

 

Courtesy of Markell and Voge's Medical Parasitology