Case 125. Acanthamebic meningoencephalitis (60 y-o M)
    Brain biopsy from highly edematous right hemisphere in case of alcoholic liver cirrhosis manifesting left hemiparesis (HIV: negative)    
Key words : 
Acanthamoeba culbertsonii, granulomatous meningoencephalitis    
opportunistic infection
Multiple low density areas and marked brain edema are seen in the right hemisphere (CT scan). The patient died of brain hemorrhage two months later.   Infiltration of lymphocytes and macrophages is observed, in association with hemorrhage and marked edematous change (HE, low power).
Some of the large round cells around small vessels, resembling macrophages, possess round nuclei with central karyosome, a characteristic microscopic feature of protozoa (HE, high power).   The perivascular macrophage-like cells with centrally situated karyosome, embedded in the chronic inflammatory background, correspond to the trophozoites of free-living ameba (HE, oil immersion).
Another view demonstrates a smaller, round-shaped cell with eosinophilic cytoplasm, a round nucleus and wrinkled, double-layered cyst wall (HE, oil immersion). A cyst form of free-living ameba is indicated.   Another mature cyst found in the edematous brain tissue is presented (HE, oil immersion). Based upon the appearance of both trophozoites and cysts in the tissue, the diagnosis should be Acanthamoeba infection. Cysts are not formed in case of Naegleria infection.
The 1:1,000 diluted patient's own serum stains the amebic cells and the cytoplasm of macrophages phagocytizing the amebic antigens (indirect immunoperoxidase). Serologic study confirmed a high titer against A. culbertsoni.   A mouse antiserum against A. culbertsoni demonstrates ameba bodies clustered around vessels (immunostaining after heating pretreatment). Antibodies against A. polyphaga, A. castellani and Entamoeba histolytica failed to identify the pathogens. Opportunistic infection by A. culbertsoni was thus confirmed.
Electron micrograph of cultured Acanthamoeba castellanii
Ultrastructurally, targeted round nucleus with central karyosome and vacuolated cytoplasm are noted. The ameba contains electron-dense mitochondria.  Acanthamoeba sp. and Naegleria sp. belong to free-living ameba seen widespread in the environmental warm water, so that they are aerobic, contain mitochondria, and lack glycogen particles. In contrast, Entamoeba sp. is oblilgately anerobic, lack mitochondria, and are rich in glycogen.
  Electron micrograph of cultured Acanthamoeba castellanii
At the periphery of the cytoplasm just beneath the plasma membrane, bundles of fine filaments are seen. Acanthamoeba sp. and Naegleria sp. are a reservoir of Legionella pneumophila in water. L. pneumophila belongs to be an obligate intracellular microbe. Both ameba species, particularly Naegleria sp., preferentially grows at 42℃, the temperature of hot springs. Hot spiring bathing is thus a risk factor of legionnaire's disease.