Feeding Mechanisms of Pathogenic Protozoa with a Focus on Endocytosis and the Digestive Vacuole

Abstract

Endocytosis is a quintessential feature of eukaryotes, and the emergence of endocytosis played a major role in the origin and evolution of eukaryotes. During the early evolution of eukaryotes, phagocytosis and the digestion of prey (i.e., bacteria) combined with the endocytosis of macromolecules opened a new source of nutrients beyond osmotrophy. Pathogenic and commensal protozoa have retained endocytosis as a major mechanism of nutrient acquisition even though, in theory, nutrients could be obtained from the host through osmotrophy. Nearly all pathogenic protozoa exhibit endocytosis and have lysosomal-like compartments that function as digestive vacuoles, and endocytosis appears to play a major role in the acquisition of nutrients. Cryptosporidium is a possible exception that may not exhibit endocytosis. Phagotrophy, however, is only observed in parasites of the intestinal lumen and appears to have been lost in blood and tissue parasites. Overall, the basic features of endocytosis and lysosomes are similar to other eukaryotes. Nonetheless, adaptation to the host has generated some novel features that are specific to certain protozoan lineages.