1. A microorganism that requires nutrients from its environment in order to exist; for example, a mutant strain of an organism, e.g. a bacterium, that has lost the ability to synthesize a specific nutrient growth factor and must obtain it from its environment to survive.
2. A mutant organism, especially a microorganism, that has a nutritional requirement not shared by the parent organism.
3. A mutant microorganism that requires some nutrient that is not required by the organism (prototroph) from which the mutant was derived.