Trophozoites of P. hominis (Figures 11.1, 11.4) are pear- or teardrop-shaped and, in fixed and stained preparations, 6-14 ^ long x 4-6.5 ^ wide. They possess five anterior flagella and an undulating membrane whose recurrent flagellum extends well beyond the length of the body. A moderately thick axostyle extends through the body from the region of the nucleus and appears to project some way beyond the posterior end. In fresh preparations or in culture, the flagella and undulating membrane beat rapidly (and are thus impossible to observe), propelling the organism with a characteristic jerky motion.
The ultrastructure of P. hominis has been described (Honigberg et al., 1968) and is basically similar to that of other trichomonads; interestingly, the basal body of the fifth anterior flagellum is perpendicular to the other four. Also surprisingly, the number of anterior flagella is not invariable, at least in culture, a minority of organisms having a lesser number (Flick, 1954). Microbodies (presumably hydrogenosomes) are visible and many enzyme activities have been detected during studies on isoenzyme characterization, including superoxide dismutase, but few detailed biochemical studies appear to have been undertaken, although the purine salvage pathway is known and some cysteine proteases have been detected. Axenic culture is possible (Linstead, 1989).
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