E. histolytica possesses both trophozoite and cyst forms. The trophozoites are microaerophilic, dwell in the lumen or wall of the colon, feed on bacteria and tissue cells, and multiply rapidly in the anaerobic environment of the gut. When diarrhea occurs, the trophozoites are passed unchanged in the liquid stool. Here they can be recognized by their size (12 to 20 μm in diameter); directional motility; granular, vacuolated endoplasm; and sharply demarcated, clear ectoplasm with finger-like pseudopods.
Inva-sive strains tend to be larger and may contain ingested erythrocytes within their cyto-plasm (Fig 53–1). Appropriate stains reveal a 3- to 5-μm nucleus with a small central karyosome or nucleolus and fine regular granules evenly distributed around the nuclear membrane (peripheral chromatin). Electron microscopic studies demonstrate microfila-ments, an external glycocalyx, and cytoplasmic projections thought to be important for attachment.
With normal stool transit time, trophozoites usually encyst before leaving the gut. Ini-tially, a cyst contains a single nucleus, a glycogen vacuole, and one or more large, cigar-shaped ribosomal clusters known as chromatoid bodies. With maturation, the cyst becomes quadrinucleate, and the cytoplasmic inclusions are absorbed. In contrast to the fragile trophozoite, mature cysts can survive environmental temperatures up to 55°C, chlorine concentrations normally found in municipal water supplies, and normal levels of gastric acid. E. histolytica can be differentiated from the other amebas of the gut by its size, nuclear detail, and cytoplasmic inclusions (Table 53–1).
Humans are the principal hosts and reservoirs of E. histolytica. Transmission from person to person occurs when a parasite passed in the stool of one host is ingested by another. Because the trophozoites die rapidly in the external environment, successful passage is achieved only by the cyst. Human hosts may pass up to 45 million cysts daily. Although the average infective dose exceeds 1000 organisms, ingestion of a single cyst has been known to produce infection. After passage through the stomach, the cyst eventually reaches the distal small bowel. Here the cyst wall disintegrates, releasing the quadrinucle-ate parasite, which divides to form eight small trophozoites that are carried to the colon. Colonization is most intense in areas of fecal stasis such as the cecum and rectosigmoid but may be found throughout the large bowel.
Trophozoites are facultative anaerobes that require complex media for growth. Most re-quire the addition of live bacteria for successful isolation. Sterile culture techniques (ax- enic) have been developed, however, and are essential for the preparation of the purified antigens required for serologic testing, zymodeme typing, and characterization of viru- lence factors. Such techniques are generally available only in research laboratories.