Conjugation is the transfer of genetic material between bacterial cells that are in direct contact. Joshua Lederberg and Edward Tatum discovered conjugation in 1946 through an experiment where they mixed two auxotrophic E. coli strains that could complement each other's mutations. Bernard Davis provided evidence in 1950 that direct cell contact was required by showing that genetic transfer did not occur when the two strains were separated by a filter. During conjugation, a pilus forms to attach the donor and recipient cells and allows single-stranded DNA to pass between them to make both cells viable donors that can now transfer the genetic material.