The bacteria prior to recombination is lacking a gene that allows it to grow in certain circumstances. Imagine a bacteria like E coli that has a mutation in it that makes it unable to grow using proline as an energy source. If we introduce DNA by a bacteriophage or HFR that contains the proline utilization operon, recombinants can now grow using on minimal media with proline as it's energy source. nonrecombinants can't grow.