commensali

Commensali, in biology, refers to organisms that engage in commensalism, a type of symbiotic relationship in which the commensal benefits from the association while the host is neither harmed nor helped in a significant way. The term describes the relationship itself and the organisms that participate in it, known as commensals.

Commensalism is one of the traditional categories of symbiosis, alongside mutualism and parasitism. In practice, the boundary between these relationships is often context-dependent and debated; what appears commensal under one set of conditions may shift toward mutualism or parasitism under others.

Common examples include certain gut microorganisms that utilize nutrients or byproducts produced by the host without causing obvious harm, and microbes on the skin or mucous membranes that exploit secretions or available resources without damaging the host. Ecological instances also include small organisms that benefit from resources created by a larger animal, such as epiphytes taking advantage of moisture on plant surfaces, or remoras that attach to larger marine animals to gain transport and scraps of food.

In humans and other animals, many members of the normal microbiota are considered commensals: they gain nutrients from the host environment while having little impact on health under normal conditions. However, commensals can become opportunistic pathogens if the host’s immune defenses are compromised or environmental conditions change.

Etymology derives from Latin com mensalis, meaning sharing a table, reflecting the idea of one organism “dining” at the host’s expense without harming it.