word-forming element meaning "small in size or extent, microscopic; magnifying;" in science indicating a unit one millionth of the unit it is prefixed to; from Latinized form of mikros, Attic form of Greek smikros "small, little, petty, trivial, slight," perhaps from PIE *smika, from root *smik- "small" (source also of Old High German smahi "littleness"), but Beekes thinks it a Pre-Greek word.
1660s, "organic structure, organization" (a sense now rare or obsolete), from organize + -ism. Sense of "living animal or plant, body exhibiting organic life" is by 1842. Related: Organismic; organismal.
<a href="https://www.etymonline.com/word/micro-organism">Etymology of micro-organism by etymonline</a>
Harper, D. (n.d.). Etymology of micro-organism. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/micro-organism