Symbiont

Definition

An organism that lives in close, long-term association with another organism of a different species, forming a symbiotic relationship. The term applies regardless of the nature of the interaction—mutualistic, commensal, or parasitic—and describes either partner in the association. In arthropod biology, symbionts may be intracellular bacteria housed in specialized host structures (bacteriomes), gut-dwelling microbes that aid digestion of refractory plant materials, or ectosymbiotic fungi cultivated by attine ants.

Etymology

From Greek symbiosis 'living together', from syn- 'together' + bios 'life'.

Example

Buchnera aphidicola, an obligate intracellular bacterium, is a mutualistic symbiont of aphids that synthesizes essential amino acids the aphid cannot obtain from its phloem sap diet; the aphid in turn provides the bacterium with a protected cellular environment and steady nutrient supply.

Synonyms

• symbiote

Related Terms

• Symbiosis
• mutualism
• commensalism
• Parasitism
• holobiont
• bacteriome
• microbiome
• endosymbiont
• ectosymbiont
• obligate symbiont
• facultative symbiont

Usage Notes

Specialists distinguish between 'symbiont' (the organism) and 'symbiosis' (the relationship). The term carries no implication of benefit or harm to either party; for mutual benefit, specify 'mutualistic symbiont', for harm to host, 'parasitic symbiont'. In entomology, 'symbiont' often implies microbial associates, though the term technically includes macroscopic partners such as mites phoretic on beetles. Contrast with 'pathogen', which implies disease causation, and 'contaminant', which implies incidental, non-persistent association.