Are Parasites a Natural Part of the Microbiome?

Many people are under the impression that parasites are a necessary part of the microbiome. Often they are confused between bacteria and other microorganisms that are beneficial to the microbiome and actual parasitic microorganisms and helminths.

There are symbiotic relationships in nature, in which symbiosis or harmony exists between two life forms. There are also antagonistic relationships in nature, where harm is exchanged between two species.

Just like insects can be harmonious or detrimental, microorganisms can either help or be harmful. A bee is a harmonious insect because it helps to pollinate flowers which create fruit. Locusts are harmful because they swarm and destroy plant life without any benefit. Microorganisms can either aid in digestion and nutrition as well as help fight off invasive pathogens, or steal nutrition and cause illness by creating acidity and inviting in more pathogens.

The common classification of symbiotic relationships (or lack thereof) includes the categories of commensalism, mutualism and parasitism.

In Commensalism, one species benefits while the other is unaffected. In the world of microorganisms, one good example is nitrification — the oxidation of ammonium ions to nitrate. Nitrification occurs in two steps: first, bacteria such as Nitrosomonas spp. and certain crenarchaeotes oxidize ammonium to nitrite; and second, nitrite is oxidized to nitrate by Nitrobacter spp. and similar bacteria. Nitrobacter spp. benefit from their association with Nitrosomonas spp. because they use nitrite to obtain energy for growth.

In Mutualism, both species are benefitting from each other. A mutualistic relationship with humans and microorganisms includes the relationship with Bifidobacterium. This bacteria helps digest dietary fiber, prevents infections and produces b vitamins and fatty acids for the body.

In Parasitism, one species benefits while the other is harmed. The bacteria Escherichia coli, commonly known as E. coli has a particular strain, O157:H7, that produces a powerful toxin that damages the lining of the small intestine. While it is gaining nutrition from the host, it causes damage to the host in the form of bloody diarrhea, stomach cramps, nausea, and vomiting.