microorganism has definitions from the field of biology
1
[ noun ] (biology) any organism (animal or plant) of microscopic size

Used in print

(Walter H. Buchsbaum, "Advances in Medical Electronics"...)

Many cells , bacteria , and other microorganisms are transparent to visible_light and must be stained for microscopic investigation .

Many_of the cells and microorganisms which are transparent to visible_light , absorb or reflect the much shorter wavelengths of the ultraviolet_spectrum .

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