micro-organism(n = noun.animal) microorganism - any organism of microscopic size;
is a kind of being, organism
has parts: pilus
has particulars: monad, intestinal flora, virus, moneran, moneron, bacteria, bacterium, bug, germ, microbe, pathogen, protoctist, protist, protistan, microflora

micro-organism, n. any of various microscopic organisms, including algae, bacteria, fungi, protozoa, and viruses.