Influence of culture medium pH on internalization, growth and phenotypic plasticity of Neospora caninum

Neospora caninum, a strictly intracellular protozoan, is a major leading cause of parasite-induced abortion in cattle. A widely held view of N. caninum infection is that both cellular proliferation and stage interconversion (tachyzoite-bradyzoite transformation) are triggered, perhaps even modulated by, changes in cultural conditions. This study tested the hypothesis that exposure of N. caninum tachyzoites to different pH culture media affects the parasite's entry, proliferation and cyst formation in cultured cells. The endocytic pathway for N. caninum entry into the K562 cell line was found to be mediated by low pH of culture medium. Internalization of N. caninum by host cells was significantly increased in acidic and alkaline culture medium compared to cells maintained in neutral medium as revealed by transmission electron microscopy. Parasite proliferation within Vero cells was assessed by plaque formation assay and was found to be highest when pH level was optimum, paralleled by a decrease in the number of cysts. In contrast, parasite encystation increased when the pH level was alkaline or acidic, as evaluated by indirect immunofluorescence and immunocytochemical analyses. Acidic pH regardless of state of host cell infection suppressed the rate of host cell division. These findings suggest that culture medium pH has a determinable effect on the host cell-N. caninum interaction and support the hypothesis that pH of culture medium influence the entry, growth, and phenotypic plasticity of N. caninum in mammalian cells.