Rhizopods, which belong to the protozoa, do not form a single species or class of unicellular organisms with a defined nucleus (eukaryotes); they are all united only by the ability to form pseudopodia. Rhizopods embody such diverse unicellular organisms as amoebae, radiolarians, solarians, foraminifera, and others. For humans, only a few species of amoebae have any significance as a harmless component of the intestinal flora and also as pathogens.
What are rhizopods?
Rhizopods, also called root-feet, are classified as protozoa, which are single-celled organisms with a cell nucleus. They are not a uniform species, family, or class of protozoa, but rather eukaryotic single-celled organisms from completely different and independent evolutionary-biological lineages. The only common feature that links them is their ability to form rapidly changing pseudopodia (false feet). These are protrusions of cytoplasm that enable them to actively move, feed, and “stick” to a substrate, among other things. In evolutionary biology, these are very early earth-historical creatures that have existed for more than a billion years. Most rhizopod species inhabit the world’s oceans, although a few species also prefer the freshwater of local lakes and rivers or live in the soil. Almost all rhizopods feed heterotrophically, that is, on organic decomposition and waste products. Except for some species of amoebae that are part of the healthy intestinal flora and a few pathogens that can cause amebic dysentery, primary amebic meningoencephalitis, or amebic creatitis, rhizopods have no direct health significance to humans.
Occurrence, distribution, and characteristics
Rhizopods, which include orders as diverse as amoebae, foraminifera, sundarians, and radiolarians with thousands of species and subspecies, are native to all of the world’s oceans. Some species are also known to be freshwater inhabitants. As mostly free-living protozoans, they play no role in human health, with the exception of a few amoeba species. Most amoeba species with health relevance usually live as commensals in the large intestine and feed heterotrophically on waste products that the body metabolism can no longer utilize. They are part of the healthy intestinal flora and occur worldwide. Amoebae reproduce asexually by division. First, the nucleus divides so that there are temporarily two nuclei in the amoeba, before the subsequent division of the cytoplasm completes the division process and two equal new amoebae have emerged from one amoeba, which can divide again under favorable growth conditions. If amoebae living in the intestine are excreted with the stool and find very unfavorable living conditions, they form permanent forms (cysts). They shrink into a small ball by excretion of excess water and surround themselves with a thick capsule. The cysts are very hardy and survive adverse conditions such as cold, heat, and drought for long periods of time. Cysts of amoebae are almost ubiquitous and, after oral ingestion, survive the gastrointestinal passage before leaving the cyst stage in the colon. This is problematic when the ingested cysts are derived from one of the few pathogenic amoeba species.
Significance and function
The health significance of the amoeba strains that live as commensals in the human colon has not (yet) been adequately researched. What appears to be certain is that they do not parasitize and cause no discernible harm if the immune system is intact. A positive effect is that they utilize degradation products, which the body metabolism can no longer catabolize, by phagocytosis and thus contribute to keeping the colon “clean”. Whether the amoebae supply the body with useful substances in the process is not known. Known non-pathogenic strains of amoebae include Entamoeba hartmanni, Entamoeba coli, and three other species, of which Dientamoeba fragilis is also a pathogen, especially when the amoeba encounters a weakened immune system. Dientamoeba fragilis is morphologically very similar to the species Entamoeba histolytica, which is known as the causative agent of amoebic dysentery.
Diseases and ailments
Hazards and risks to humans associated with rhizopods are largely limited to a few pathogenic strains of amoebae and to those described as facultative pathogens when appropriate conditions, such as an immune system weakened by disease or by artificial immunosuppression, are present. The most important and most common pathogenic amoeba species is Entamoeba histolytica. It is the causative agent of amebic dysentery, also called amebiasis. Amoebic dysentery occurs predominantly in the tropics. Infection usually occurs by oral ingestion of the cysts, the resistant permanent form of Entamoeba histolytica. Strictly speaking, Entamoeba histolytica is also a facultative pathogen, since only about 10 percent of infected people develop symptoms, although these can take a serious course if left untreated. If symptoms are confined to the gastrointestinal tract, it is intestinal amebiasis. In rare cases, the amoebae enter the bloodstream and can infect other organs. This is then an extraintestinal amebiasis. A very rare infectious disease is primary amoebic meningoencephalitis (PAM). It is caused by the amoeba Naegleria fowleri, an amoeba found in freshwater worldwide, predominantly in the tropics and subtropics and in warm springs. In very rare cases, after entering the nose, Naegleria fowleri can penetrate the olfactory epithelium and neural pathways to the brain, causing a PAM that can be fatal in a very short time. Acanthamoeba is also a free-living amoeba with a worldwide distribution, living in freshwater lakes and rivers as well as in soil. However, it is also commonly detected in drinking water and swimming pools. In very rare cases, the amoeba occurs as the causative agent of Acanthamoeba keratitis, a corneal inflammation of the eye. It usually affects contact lens wearers, whose contact lenses absorb the amoebae in the infected cleaning fluid and infect the cornea of the eye when inserted. In extremely rare cases, this can result in meningitis, granulomatous amoebic encephalitis.
