Obligate

1. Describes a microbe (bacterium, fungus) that must have a certain element (such as oxygen) or compound present or absent in their environment; for instance, an obligate anaerobe cannot survive in an oxygen-containing environment, whereas an obligate aerobe must have oxygen or it will die.
2. Describes a parasite that cannot survive without a host.

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.

Ob"li*gate (?), v. t. [imp. & p. p. Obligated (?); p. pr. & vb. n. Obligating.] [L. obligatus, p.p. of obligare. See Oblige.]

1.

To bring or place under obligation, moral or legal; to hold by a constraining motive.

Proudfit.

That's your true plan -- to obligate The present ministers of state. Churchill.

2.

To bind or firmly hold to an act; to compel; to constrain; to bind to any act of duty or courtesy by a formal pledge.

That they may not incline or be obligated to any vile or lowly occupations. Landor.

© Webster 1913.