With the use of this method, Lederberg and Tatum demonstrated that genetic recombination between bacterial genotypes is possible. (a) The basic concept: two auxotrophic cultures (A− and B−) are mixed, yielding prototrophic wild types (WT). (b) Cells of type A− or type B− cannot grow on an unsupplemented (minimal) medium (MM) because A− and B− each carry mutations that cause the inability to synthesize constituents needed for cell growth. When A− and B− are mixed for a few hours and then plated, however, a few colonies appear on the agar plate. These colonies derive from single cells in which genetic material has been exchanged; they are therefore capable of synthesizing all the required constituents of metabolism.