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What Is Commensalism?

Commensalism is a symbiotic relationship between two organisms where one organism is benefited from the other, whereas the other is neither harmed nor benefited. It is a biological interaction that organisms form, in the quest of food and shelter.

Commensalism is in contrast with mutualism, where both organisms benefit from each other; ammensalism, where one organism is harmed and the other is unaffected; and parasitism, where one organism is harmed and the other is benefited.

The organism that benefits from this biological interaction is known as the commensal and the other is known as the host species. The commensal, in such cases, is usually a small organism and the host species is often larger in size. The commensal benefits in transportation, food, shelter and support from the host species.

The host species remains unmodified in this relationship; however, the commensal shows great adaptations related to its habit and habitat.

Examples of Commensalism

Staphylococcus and Aspergillus: Staphylococcus is a bacteria with numerous pathogenic species that cause infections and illnesses in humans. However, some of them are metabolic commensals that reside as a part of skin flora. S. aureus is found living in ambient conditions in nasal and oral cavities.

Aspergillus is one of the few microbial species that can survive in the highly changing (alkaline and acidic) environment of the gastrointestinal tract. This fungal species does not produce any illness in normal conditions; however, if the person is immunocompromised or suffering from conditions such as tuberculosis, it can cause aspergillosis.

Nitrobacter and Nitrosomonas

Nitrobacter and Nitrosomomas are two bacterial species that take part in nitrification (oxidation of ammonium to nitrate). Commensal relationships also take place when waste produced by one organism serves as the substrate for another species. In the first step of nitrification, Nitrosomonas sp. converts ammonia to nitrite. This nitrite serves as the substrate of growth for Nitrobacter that later converts it into nitrate.

Similarly, commensalistic interaction can also occur when one organism modifies the environment to make it more hospitable for the other organism to survive in. During fermentation, the release of acidic waste increases the proliferation of acid-tolerant bacteria.

Similar examples include proliferation of the microbe during milk spoilage and production of biofilm.

Types

Phoresy

Phoresis or phoresy is a commensalistic interaction where one organism attaches to another organism solely for the purpose of locomotion. It is a non-permanent type of interaction. Examples of phoresy include mites on insects, false scorpions on mammals and millipedes on birds.

Inquilinism

Inquilinism is a type of commensalism where one organism lives permanently on the second organism. The organism that uses the second organism for housing is known as inquiline. Examples include orchids that grow as epiphytes on other trees, birds that live on trees, and insects that live in the burrows of rodents.

Metabiosis

Metabiosis is a type of commensalistic interaction where indirect dependency is observed, one organism produces a suitable environment for the other organism. Examples include maggots that feed on corpses and hermit crabs that use shells of gastropods to protect their bodies.

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