A bradytroph is a strain of an organism that exhibits slow growth in the absence of an external source of a particular metabolite.
This is usually due to a defect in an enzyme required in the metabolic pathway producing this chemical.
As the organism can still produce small amounts of the chemical, the mutation is not lethal.
In these bradytroph strains, rapid growth occurs when the chemical is present in the cell's growth media and the missing metabolite can be transported into the cell from the external environment.
The first usage of "bradytroph" was to describe Escherichia coli mutants partially defective in arginine biosynthesis.