obligate

(ob'li-gat) [L. ob, to, ligare, to bind.] Of nece ity, compulsory; distinguished from facultative. 0. a'Srobe, a microorganism which can grow only in the presence of oxygen, o. ana'iSrobe, a microorganism which can grow only in the absence of oxygen, o. par'asite, an organism to which a parasitic existence is nece ary for development, one which cannot live as a saprophyte. •