Scanning electron microscopy of Toxoplasma gondii: parasite torsion and host-cell responses during invasion.

Scanning electron microscopy confirmed our previous finding that toxoplasmas actively invade mouse peritoneal cells that are inhibited from phagocytosis. The parasites entered cells with the conoid end first and sometimes showed a counter-clockwise torsion of the body during invasion. Counter-clockwise torsion was also noted in free toxoplasmas. Host-cell responses to active invasion varied with experimental conditions and with the type of host cell. Under adverse culture conditions for phagocytosis, normal macrophages formed rudimentary filopodia or lamellipodia around the tips of invading toxoplasmas; macrophages subjected to hyperthermia before similar incubation with toxoplasmas showed little or no response to invasion. Normal and heat-treated lymphocytes showed little surface reaction to invasion, but occasionally a flocculent collar was seen around the tip of an invading toxoplasma. Scanning electron microscopy provides clues to possible mechanisms of toxoplasma locomotion and host-cell invasion.