Transformation is not thought to occur naturally in E. coli, but, if subjected to certain treatments in the laboratory, cells of this species can be made to take up DNA, even from a completely unrelated source. This is done by effecting a state of induced or artificial competence, and by introducing the foreign DNA in a self-replicating vector molecule, which does not depend on integration into the host chromosome. As we shall see in the next chapter, this is of enormous significance in the field of genetic engineering.
A type of mutant that has proved to be of great use to the microbial geneticist is the auxotrophic mutant. Here, the mutation causes the organism to lack a gene product, usually an enzyme, involved in the synthesis of a nutrient such as an amino acid or vitamin. If the nutrient in question is supplied in the culture medium, the auxotroph can survive quite happily, as its 'handicap' is not exposed. If the nutrient is not provided however, as in a minimal medium, the cells would be unable to grow. Thus microbiologists can detect the existence of an auxotrophic mutant by use of selective media.
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