The ameba Balamuthia mandrillaris feeds by entering into mammalian cells in culture

Microscopic observations of live cultures of the pathogenic ameba Balamuthia mandrillaris and mammalian cells showed that amebic feeding involved the invasion of the pseudopodia, and/or the whole ameba into the cells. The ameba, recognized by their size and flow of organelles in the cytosol, was seen to extend the tip of a pseudopodium into the cytoplasm of a cell where it moved about leaving visible damage when retracted. In rounded cells, whole amebas were seen to enter into and move around before exiting a cell and then remain quiescent for hours. The invaded mammalian cells retained their turgidity and excluded vital dyes until only their denuded nuclei remained. The cytoplasm of the cells was consumed first, then the nuclei, but not their mitotic chromosomes. The feeding pattern of four isolates of B. mandrillaris, two from humans and two from soil samples, was by amebic invasion into the mammalian cells. The resulting ameba population included cysts, amebas on the surface, and free-floating amebas as individuals or in dense-packed clusters. There was no morphologic indication of a cytopathic change in the mammalian cells before their invasion by the amebas. Feeding by cell invasion is a distinctive feature of B. mandrillaris.