Final answer:
Replica plating is used in both positive and negative selection to identify bacterial mutants, particularly auxotrophs. The Ames test, a form of positive selection, employs this method to determine the mutagenic potential of chemicals.
Step-by-step explanation:
The procedure known as "replica plating" is used in both positive and negative selection. This technique plays a crucial role in identifying bacterial mutants, especially nutritional mutants known as auxotrophs. These auxotrophs have mutations in genes that are responsible for synthesizing enzymes needed in nutrient biosynthesis pathways. Replica plating allows for the growth of bacterial colonies on various media, including a complete medium and several nutrient-deficient media, to detect growth patterns corresponding to the presence or absence of specific nutrients. The process involves transferring cells from a master plate to another plate with different nutritional content using sterile velvet. This helps in detecting mutant strains that can grow on a complete medium but fail to do so on a medium lacking a certain nutrient.
The Ames test, on the other hand, is a form of positive selection that is used to identify mutagenic chemicals that could potentially cause cancer by inducing mutations in DNA. The test specifically looks for reversion mutations that restore the ability of histidine auxotrophs of Salmonella typhimurium to synthesize histidine in a medium lacking this amino acid. This is a critical test as it helps to gauge the carcinogenic potential of a compound, which can be further validated through more comprehensive animal testing.