any microbiological entity, cellular or non-cellular, capable of replication or of transferring genetic material, very small organism which can only be seen with a microscope (viruses, bacteria and fungi are forms of microorganism), väikseimad, inimesele mikroskoobita nähtamatud organismid. Mikroobide hulka kuuluvad bakterid, mikroseened ..., pms. üherakulised vetikad ja ainuraksed.