PATHOGENESIS OF INFECTION BY ENTAMOEBA-HISTOLYTICA

Entamoeba histolytica, a protozoan parasite, is the etiologic agent of amoebiasis in humans. It exists in two forms--the trophozoite which i s the active, dividing form, and the cyst which is dormant and can sur vive for prolonged periods outside the host. In most infected individu als the trophozoites exist as commensals. In a small percentage of inf ections, the trophozoites become invasive and penetrate the intestinal mucosa, causing ulcers. The trophozoites may reach other parts of the body--mainly liver, where they cause tissue necrosis, leading to life -threatening abscesses. It is thought that pathogenesis of infection b y Entamoeba histolytica is governed at several levels, chief among the m are (i) adherence of trophozoite to the target cell, (ii) lysis of t arget cell, and (iii) phagocytosis of target cell. Several molecules w hich may be involved in these processes have been identified. A lectin inhibitable by galactose and N-acetyl-D-galactosamine is present on t he trophozoite surface. This is implicated in adherence of trophozoite to the target cell. Various amoebic pore-forming proteins are known, of which 5 kDa protein (amoebapore) has been extensively studied. Thes e can insert into the lipid bilayers of target cells, forming ion-chan nels. The phagocytic potential of trophozoites is directly linked to v irulence as measured in animal models. Factors like association of bac teria with trophozoites also influence virulence. Thus, pathogenesis i s determined by multiple factors and a unifying picture taking into ac count the relative contributions of each factor is sought. Recent tech nical advances, which includes the development of a transfection syste m to introduce genes into trophozoites, should help to understand the mechanism of pathogenesis in amoebiasis.