It is well recognized that Entamoeba histolytica lives in the colon and that trophozoites represent the multiplying stage. But where the trophozoites live is seldom addressed in textbooks and reviews. It had been speculated in early years that they live in the colonic tissue. Necropsy examination of accident victims had demonstrated E. histolytica trophozoites in the mucosa, but whether the invasion occurred during the events leading to death or afterwards cannot be established beyond doubt. Fecal occult blood testing of individuals with “intestinal amebiasis” (stool microscopy positive, without distinguishing E. histolytica and E. dispar) was negative, implying lack of intestinal ulceration in such individuals. Based on the observation that axenically cultivated trophozoites attach to purified rat colonic mucin, it has been recently proposed that the trophozoites live in the mucus blanket. A review in a leading journal even included a diagram, without any qualifications, showing the trophozoites in the mucus blanket. However, there are reasons to question the validity of this interpretation. First, experiments in rat intestinal loops show a lack of continued mucus layer adherence. Second, in the presence of glycosidases produced by intestinal bacteria at physiologically relevant concentrations trophozoites do not exhibit gal-galNAc-inhibitable lectin on their surface and bacterial glycosidases have been shown to degrade this lectin. Only one published study has until now examined the cecal mucosa of patients with noninvasive amebiasis. Specimens obtained at colonoscopy were examined by scanning electron microscopy (with care to preserve the mucus layer), light microscopy and immunofluorescence microscopy. Trophozoites adherent to the mucus layer or invading the tissue were rare to absent. Although this study too did not distinguish E. histolytica and E. dispar infections, a study one year later in a similar group of patients at the same center showed approximately half of similar infections to be due to E. histolytica. The appearance of trophozoites in the stool after purgation or during colonic lavage is consistent with a luminal location. Thus, although there is no convincing human study clearly establishing the location of E. histolytica trophozoites in intestinal E. histolytica infection without dysentery, available evidence favors a luminal site. This is compatible with data on luminal oxygen gradients and with known aspects of colonic motility.
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