Devise an experiment to identify bacteria that are auxotrophic and unable to produce two amino acids, lysine (lys) and valine (val). The auxotrophic bacteria are in a pool of bacteria in which all the other bacteria are prototrophic. The genotype of the auxotrophs is lys⁻ val⁻. Describe each step in the experiment, identify the constituents in any growth medium or growth plates you propose, and identify the results that will conclusively identify bacteria that are lys⁻ val⁻.

Key Concepts

Auxotrophy vs. Prototrophy

Auxotrophy refers to the inability of an organism to synthesize a particular compound required for its growth, such as certain amino acids. In contrast, prototrophic organisms can produce all necessary compounds from basic nutrients. Understanding these terms is crucial for designing an experiment to differentiate between auxotrophic bacteria, which cannot produce lysine and valine, and prototrophic bacteria, which can.

Selective Media

Selective media are growth media formulated to support the growth of specific types of organisms while inhibiting others. In this experiment, a selective medium lacking lysine and valine will be used to isolate auxotrophic bacteria. Only those bacteria that cannot synthesize these amino acids will grow, allowing for the identification of lys⁻ val⁻ mutants among the prototrophic population.

Genotype and Phenotype

The genotype refers to the genetic constitution of an organism, while the phenotype is the observable characteristics resulting from the genotype. In this case, the auxotrophic bacteria have the genotype lys⁻ val⁻, which leads to the phenotype of inability to grow on media lacking lysine and valine. Recognizing the relationship between genotype and phenotype is essential for interpreting the results of the experiment.