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English definition of “microorganism”

microorganism

noun [C]     /ˌmaɪ.krəʊˈɔː.ɡən.ɪ.zəm/ US  /-kroʊˈɔːr-/
a living thing that on its own is too small to be seen without a microscopeBacteria, moulds, germs and viruses
(Definition of microorganism noun from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)
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