Is it ensure, insure, or assure?
microorganism mi·cro·or·gan·ism (mī'krō-ôr'gə-nĭz'əm)
An organism of microscopic or submicroscopic size, especially a bacterium or protozoan.
An organism that can be seen only with the aid of a microscope and that typically consists of only a single cell. Microorganisms include bacteria, protozoans, and certain algae and fungi. See Note at germ.