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4 8.4 Mutant bacterial strains can be isolated on the basis of their nutritional requirements.

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tected by simple chemical tests. Nutritional requirements of the bacteria are used to detect some commonly studied phenotypes. Suppose we want to detect auxotrophic mutants that cannot synthesize leucine (leu~ mutants). We first spread the bacteria on a petri plate containing complete medium that includes leucine; so both prototrophs that have the leu+ allele and auxotrophs that have leu~ alleles will grow on it ( FIGURE 8.4). Next, using a technique called replica plating, we transfer a few cells from each of the colonies on the original plate to two new replica plates, one containing complete medium and the other containing selective medium that lacks leucine. The leu+ bacteria will grow on both media, but the leu~ mutants will grow only on the complete medium, because they cannot synthesize the leucine that is absent from the selective medium. Any colony that grows on complete medium but not on this selective medium consists of leu~ bacteria. The auxotrophic mutants growing on complete medium can then be cultured for further study.

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