Causative Agent

Giardia lamblia, a member of the Mastigophora, is a flagellated protozoan shaped like a pear cut lengthwise, with two side-by-side nuclei that resemble eyes. These features along with an adhesive disc on its undersurface give the organism an unmistakable appearance (figure 24.22). Giardia lamblia can exist in two forms: as a vegetative trophozoite or as a resting form called a cyst. The trophozoite is the actively feeding form, and it colonizes the upper part of the small intestine. Generally, only trophozoites are present in the feces when an infected person is having diarrhea. When trophozoites are carried slowly by intestinal contents toward the large intestine, they develop into cysts. Giardia cysts have thick walls composed of chitin, a tough, flexible, nitrogen-containing polysaccharide. The cyst wall protects the organism from harsh environmental conditions. Curiously, Giardia lacks mitochondria, and its ribosomal RNA resembles that of prokaryotes. ■ chitin, p. 70

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