Symbiont
A symbiont is an organism that lives in close and long-term association with another organism, the host. Symbiosis covers interactions where the partners live in direct contact or within each other. Relationships are commonly described by their effect on the host: mutualism (both partners benefit), commensalism (the host is unaffected), and parasitism (the symbiont benefits at the host’s expense). Symbiotic partnerships can be obligatory or facultative for one or both parties, and symbionts may reside inside host cells (endosymbionts) or on external surfaces or within body compartments (ectosymbionts).
Mutualistic associations include Buchnera bacteria providing essential amino acids to aphids in exchange for plant sugars,
Some symbionts are intracellular, others are extracellular. Endosymbiosis has played a major role in evolution; the