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(kəmĕn`səlĭz'əm), relationship between members of two different species of organisms in which one individual is usually only slightly benefited, while the other member is not affected at all by the relationship. For example, some flatworms live attached to the gills of the horseshoe crab, obtaining bits of food from the crab's meals; the crab is apparently unaffected. In many cases commensalism cannot be distinguished from parasitism (see parasiteparasite,
plant or animal that at some stage of its existence obtains its nourishment from another living organism called the host. Parasites may or may not harm the host, but they never benefit it.
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). See also competitioncompetition,
in biology, relationship between members of the same or different species in which individuals are adversely affected by those having the same living requirements, such as food or space. Intraspecific competition, i.e.
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; symbiosissymbiosis
, the habitual living together of organisms of different species. The term is usually restricted to a dependent relationship that is beneficial to both participants (also called mutualism) but may be extended to include parasitism, in which the parasite depends upon
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inquilinism, the cohabitation of animals of different species, whereby one (the commensal) benefits permanently or temporarily from the other without causing the latter any damage.

A commensal may use the other organism for protection or transportation, or it may take advantage of the latter’s food supply. Characteristically commensal animals are divided into three groups, according to the character of the interrelationship:

(1) The commensal limits itself to the use of the food of the organism of the other species; for example, the annelid Nereis lives in the coils of the shell occupied by the hermit crab and eats the crab’s leftover food.

(2) The commensal attaches itself, temporarily or permanently, to the body of the other species, in this case called the host; for example, the remora attaches itself by the dorsal fin, which has developed into a sucker, to sharks and other large fishes and uses them for transportation; certain marine hydroids settle on the skin of fish and feed on their excrement; and certain crustaceans (such as acorn barnacles) live on the skin of whales and the shells of mollusks.

(3) The commensal settles in the viscera of the host; for example, the infusorians of subclass Protociliata live in the rectum of frogs, and certain flagellates live in the intestines of mammals.

Parasitism may have originated evolutionarily from commensalism in which the commensal settles in the organs of the host. Symbiosis may also have been an elaboration of commensalism. However, commensalism is not an obligatory transitional step to parasitism or symbiosis; these forms of interrelationship may have originated independently of commensalism in the historical development of the given organisms.


An interspecific, symbiotic relationship in which two different species are associated, wherein one is benefited and the other neither benefited nor harmed.
References in periodicals archive ?
After birth, colonization by commensal bacteria occurs immediately and continues throughout the first year of life.
Regardless of the microbial source, the feeding of crude antimicrobial preparations to animals is plausible as a delivery process for transferring the cognate antimicrobial resistance genes between producing strains and the commensal bacteria of animals (21); the concomitant selection for resistance would ensure the survival of rare resistant strains.
One of the most important functions of commensal bacteria is boosting the immune system.
The Company's four product programs focus on: mucosal vaccines against strep throat and periodontal disease, commensal bacteria for the delivery of vaccines, new broad-spectrum antibiotics, and Surface Protein Expression System for the cost-effective production of proteins.
Tuning of skin immunity by skin commensal bacteria.
Change in commensal bacteria is key to the inflammatory processes and are also altered in the presence of inflammation.
Current pilot study is an attempt to formulate fluoride containing HA graft and evaluate its antibacterial property on commensal bacteria.
During vaginal birth, the newborn is exposed to commensal bacteria that modulate immune development, Joseph Weimels, Ph.
Researchers are now exploring the possibility that certain changes to the gut microbiome can prime an abnormal autoimmune response to these commensal bacteria in genetically susceptible individuals.
Population genetics studies found clarithromycin-resistant commensal bacteria persist in gastrointestinal (GI) flora years after completion of antibiotic therapy (Andersson & Hughes, 2010; Sjolund, Wreiber, Andersson, Blaser, & Engstra nd, 2003).
Commensal bacteria typically found around the ocular adnexa include species of Haemophilus, Staphylococcus, Corynebacterium, Pneumococcus and Streptococcus.
Under homeostasis of the intestinal microbiota, IAP acts suppressing inflammatory responses from the host that may be induced by lipopolysaccharide (LPS) from commensal bacteria (Vaishnava and Hooper, 2007).