Ameba-bacterium relationship in amebiasis.

Human infection with the protozoan intestinal parasite Entamoeba histolytica, which causes amebiasis, usually begins with ingestion of the dormant form of the organism, the cyst, which is present in water or vegetables polluted with human fecal material (54). Studies of the life cycle of this parasite have shown that motile trophozoites emerge after digestion of the tough cyst wall (excystation) in the small intestine and migrate to the colon region, where they can proliferate in the microaerophilic conditions together with the resident bacterial flora (Fig. 1). In most cases, trophozoites in the intestine live as commensals, without causing any noticeable damage or discomfort to the host. In some instances, however, trophozoites display an aggressive behavior, attacking and invading the intestinal mucosa and causing dysentery. The parasites can also progress through blood vessels to the liver, brain, or lungs, where they may form life-threatening abscesses (extraintestinal lesions). Trophozoites which remain in the intestine exit the body together with the feces. Before leaving, however, many of them encyst and produce a quadrinucleated cell which is surrounded by a protective wall that can withstand the hostile environmental conditions encountered outside the host and enable propagation. Epidemiological studies on the prevalence of E. histolytica infection indicate that over 480 million people in the world harbor the parasite, and every year about 10% of those infected show some sort of clinical symptoms (dysentery, intestinal invasion) (114). One of the theories for the disparity between the number of asymptomatic carriers and diseased individuals is that carriers are infected with nonpathogenic strains which are incapable of causing disease. On the other hand, in the absence of clear genetic evidence for strain differences, we should also examine other factors which may determine the virulence of E. histolytica in the human host and its occasional conversion from a harmless intestinal commensal to an aggressive invader. The study of microbial interrelationships as a factor in microbial pathogenicity is an obscure area of microbiology which has lent itself to only limited scientific investigation. The possible role of symbiosis of associated microorganisms in the production of disease is largely unknown. The possibility that the indigenous or autochthonous flora of a host protects, and may in some instances help combat invasions by various microbes, is generally accepted (23), but the possibility that it may, in other instances, actually render the host susceptible to certain pathogens has received less consideration. It is generally recognized that pathogenicity of E. histolytica is related in part to the association of amebae with suitable bacterial species. The definitive influence exerted by bacteria in the production of amebic dysentery was suspected already in 1891 (4), and since that time it has been frequently observed (14, 24, 83-85, 117, 119). The aim of this article is to try to critically review the available information on this subject and to discuss this poorly understood, yet intriguing, phenomenon.