- 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.
- 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.