Microscopy

Acanthamoeba spp.

The cysts of Acanthamoeba spp. are typically 10-25 µm in diameter.  The cysts have two walls: a wrinkled fibrous outer wall (exocyst) and an inner wall (endocyst) that may be hexagonal, spherical, star-shaped or polygonal.  Cysts contain only one nucleus with a large karyosome.  Cysts may be found in the brain, eyes, skin, lungs and other organs.

A	B

A, B: Cysts of Acanthamoeba sp. from brain tissue, stained with hematoxylin and eosin (H&E).

Trophozoites of Acanthamoeba spp. are pleomorphic and measure approximately 15-45 µm.  They often produce many spine-like processes called acanthapodia.  Trophozoites contain a large nucleus with a large, centrally-located karyosome but no peripheral chromatin.  There is no flagellated trophozoite stage in Acanthamoeba spp.

C	D

C: Trophozoite of Acanthamoeba sp. in tissue, stained with H&E.
D: Trophozoites of Acanthamoeba sp. in a corneal scraping, stained with H&E.

Balamuthia mandrillaris

The cysts of Balamuthia mandrillaris are typically 10-25 µm in diameter.  The cysts have two walls: a wrinkled fibrous outer wall (exocyst) and an inner wall (endocyst) that may be hexagonal, spherical, star-shaped or polygonal.  Cysts contain only one nucleus with a large karyosome.  Cysts may be found in the brain, eyes, skin, lungs and other organs.

E	F

E, F: Cysts of B. mandrillaris in brain tissue, stained with H&E.  Images courtesy of the University of Kentucky Hospital, Lexington, Kentucky.

G	H

G, H: Cysts of B. mandrillaris in brain tissue, stained with H&E.  Images courtesy of Cook Children’s Hospital, Fort Worth, Texas.

Trophozoites of Balamuthia mandrillaris are pleomorphic and measure approximately 15-60 µm.  They often produce long, slender pseudopodia. Trophozoites contain a large nucleus with a large, centrally-located karyosome but no peripheral chromatin.  There is no flagellated trophozoite stage as in Naegleria spp.

I	J

I, J: Trophozoites of B. mandrillaris in brain tissue, stained with H&E.

Naegleria fowleri

Naegleria fowleri does not form cysts in human tissues.  There are two forms of trophozoites: ameboid 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.

K	L

K: Trophozoite of N. fowleri in CSF, stained with H&E.
L: Trophozoite of N. fowleri in CSF, stained with trichrome.  Image courtesy of the Texas State Health Department.

Sappinia spp.

Sappinia is a genus of free-living amebae rarely isolated from human specimens.  The genus is found worldwide and has been isolated in the feces of many animals, including mammals and reptiles.  Cysts and trophozoites both possess two nuclei.

M

M: Four trophozoites (yellow arrows) of S. diploidea in brain tissue, stained with H&E.  In three of the amebae, the two nuclei can easily be seen.

Indirect Immunofluorescence (IIF) assay for the detection of free-living amebic infections.

N	O	P

N: IIF assay for Acanthamoeba sp., 400x magnification
O: IIF assay for Balamuthia mandrillaris.
P: IIF for Naegleria fowleri, 1000x oil magnification.