<P> An anaerobic organism or anaerobe is any organism that does not require oxygen for growth . It may react negatively or even die if oxygen is present . (In contrast, an aerobic organism (aerobe) is an organism that can survive and grow in an oxygenated environment .) </P> <P> An anaerobic organism may be unicellular (e.g. protozoans, bacteria) or multicellular . For practical purposes, there are three categories of anaerobe: obligate anaerobes, which are harmed by the presence of oxygen; aerotolerant organisms, which cannot use oxygen for growth but tolerate its presence; and facultative anaerobes, which can grow without oxygen but use oxygen if it is present . Human muscle cells function as facultative anaerobes during vigorous exercise and this creates a build - up of the byproduct lactic acid in the muscles, resulting in the characteristic burning and aching sensations . </P>

An organism which is completely dependent on oxygen for growth is