Observations by light microscopy on the cytopathogenicity of Naegleria fowleri in mouse embryo-cell cultures.

Naegleria fowleri, strain HB-1, caused a destructive cytopathic effect (CPE) in secondary mouse-embryo (ME) cells. No evidence was found to suggest that cell-free cytotoxic factors secreted by the amoebae play a part in ME-cell destruction. In culture systems designed for the study of cytopathic factors, mammalian-cell damage seemed to occur only as a result of direct contact with active amoebae. This was confirmed when the progressive destruction of individual ME cells was observed continuously by direct microscopy and time-lapse cinemicrography. The cytoplasmic shrinkage characteristic of naegleria-induced CPE appeared to be associated with phagocytic activity of trophozoites. Adjacent ME cells remained undamaged until they themselves were physically attacked. The apparently intracellular location of amoebae seen in fixed and stained preparations was considered to be an artefact created when trophozoites and ME cells were superimposed.