[Recent findings on the pathogenicity of Entamoeba histolytica]

Entamoeba histolytica is the causative agent of human amoebiasis. During recent years, research in amoebiasis has concentrated on two subjects: 1. the dual manifestation of the infection as harmless colonization of the intestinal cavity or pathogenic tissue invasion and 2. the molecular analysis of functions of E. histolytica that are considered essential for pathogenicity. Besides epidemiological studies and isoenzyme analyses, molecular genetics have revealed additional evidence that two genetically distinct forms of E. histolytica do exist, named "pathogenic" and "nonpathogenic" forms, respectively. Both can infect humans but only the "pathogenic" form is able to invade the tissue and cause disease whereas the "nonpathogenic" is not. Questions remain open about the mechanism that triggers "pathogenic" E. histolytica to become invasive and about the molecules that are involved. Current data indicate that at least three functions of the amoebae are considered essential for pathogenic tissue invasion. Pathogenicity is viewed as a result of 1. adherence of the amoeba to host cells, predominantly mediated by a galactose- and N-acetylgalactosamine-inhibitable lectin, 2. killing of host cells by a pore-forming peptide known as amoebapore, and 3. proteolysis of the host's extracellular matrix mediated by cysteine proteinases. Structural detailed molecular analysis including cloning of the corresponding genes have led to a better understanding of the function of these proteins.