The occurrence of sex in bacteria was first described by Joshua Lederberg and Edward Tatum in 1946 (Nature, volume 158, page 558), who were studying mixed cultures of E. coli strains with various nutritional mutations. The mutant strains differed from the wild type strains in lacking the ability to synthesize growth factors such as amino acids and vitamins, similar to the strains in the present problem.
In one experiment, two triple mutants of E. coli, one requiring threonine, leucine, and thiamine, and the second requiring biotin, phenylalanine, and cystine were grown in mixed cultures. At very low frequency, recombinant strains with no growth-factor requirement were obtained. They ruled out spontaneous mutations and transformation by the culture medium as the source of recombinant strains. In the words of Lederberg and Tatum, "These experiments imply the occurrence of a sexual process in the bacterium Escherichia coli."