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exotoxin ex·o·tox·in (ěk'sō-tŏk'sĭn)
A toxin secreted by a microorganism and released into the environment in which it grows. Also called extracellular toxin.
a poisonous substance secreted by certain bacteria. In their purest form they are the most potent poisons known and are the active agents in diphtheria, tetanus, and botulism. The term is now sometimes restricted to poisonous proteins that are antigenic-i.e., that stimulate the formation of antibodies-and formed by gram-positive bacteria. Compare endotoxin.