Final answer:
A mutant strain (auxotroph) is "cured" by a substance if that substance provides the specific nutrient or compound the auxotroph can't synthesize due to a genetic defect. Auxotrophs are identified via techniques like replica plating, where their inability to grow on nutrient-deficient media can be observed and corrected with supplementation.
Step-by-step explanation:
Whether the mutant strain (auxotroph) is "cured" by a particular substance is determined by the substance's ability to compensate for the genetic defect of the auxotroph. Auxotrophs are nutritional mutants unable to synthesize a specific organic compound required for their growth. They are identified using techniques like replica plating, where they fail to grow on media lacking the specific nutrient they cannot produce. For example, arginine auxotrophs require supplementation with arginine or intermediates in the arginine biosynthesis pathway in order to grow. Similarly, the ability of a histidine auxotroph to grow in the absence of histidine when exposed to a potential mutagen is the basis for the Ames test, which is a method used to measure the mutagenicity of a chemical compound and its potential carcinogenicity.
When an auxotroph is supplemented with the nutrient it cannot synthesize, and it resumes growth, it essentially indicates that the added substance successfully 'cures' the defect. Identifying the specific gene involved in the defect, such as in the arginine biosynthesis pathway, allows researchers to understand metabolic pathways on a more granular level, as illustrated by the pioneering work of Beadle and Tatum, which laid the foundation for the one gene-one enzyme hypothesis.