Physiology of Parasitic Protozoa

Physiology Of Parasitic Protozoa

Parasitic protozoa, like their freeliving counterparts, are singlecelled eukaryotic organisms that utilize flagella, cilia or amoeboid movement for motility. The complexity of some parasite life cycles means that some species may exhibit, at different times, more than one form of motility. All pathogenic protozoa are heterotrophs, using carbohydrates or amino acids as their major source of carbon and energy. Some parasitic protozoa utilize oxygen to generate energy through oxidative phosphorylation, but many protozoan parasites lack functional or ‘typical’ mitochondria, or have mitochondria that do not function like those in mammalian cells. As a result of this adaptation many parasites exhibit a fermentative metabolism that functions even in the presence of oxygen. The reason for the utilization of less efficient fermentative pathways is not clear, but it is presumably due in part to the fact that such parasites survive in environments where oxygen is only present occasionally or at low levels. For some parasites oxygen is toxic, and they appear to utilize it possibly in an effort to remove it and thus maintain an anaerobic metabolism.

The metabolism of parasites is highly adapted, with many possessing unique organelles such as kienetoplasts and hydrogenosomes. Many synthetic pathways that are found in other eukaryotes are absent because many metabolic intermediates or precursors such as lipids, amino acids and nucleotides are actively scavenged from their environment. This minimizes energy expenditure, which is finely balanced in parasites and means that the membrane of parasitic protozoa is rich in transporters. Secretion of haemolysins, cytolysins, proteolytic enzymes, toxins, antigenic and immuno-modulatory molecules that reduce host immune response also occurs in pathogenic protozoa.

Survival of parasites is partly due to their high rate of reproduction, which may be either sexual or asexual; some organisms such as Plasmodium exhibit both forms of reproduction in their life cycle. Simple fission is characteristic of many amoeba, but some species also undergo nuclear division in the cystic state (cysts are forms required for survival outside the host) with each nucleus giving rise to new trophozoites (the growing, motile and pathogenic form).