In this article we will discuss about the definition of conjugation in bacteria.
The occurrence of recombination by sexual union was first shown experimentally in 1946 by Joshua Lederberg and E.L. Tatum in E. coli. They took two auxotrophic strains neither of which could grow on minimal medium due to mutation in genes controlling synthesis of vitamins thiamine and biotin, and amino acids methonine, threonine and leucine.
For simplicity strain I can be designated as a– b– c– d+ e+ and strain II as a+ b+ c+ d– e–. A mixture of the two auxotrophic strains was cultured together on a complete medium and samples taken and plated on minimal medium. Surprisingly prototrophic colonies in a frequency of 1 per 106 or 107 cells plated were found growing on the minimal medium.
Lederberg argued that the prototrophs would have a genotype of the wild type a+ b+ c+ d+ e+. The question arose on the origin of the prototroph colonies whether from mutation, transformation or recombination by some form of sexual union.
Mutation was ruled out because it is improbable for so many gene loci in each strain to undergo mutation simultaneously. Transformation was negated experimentally when broken DNA fragments from either strain failed to produce recombinants. It appeared therefore that some form of sexual union between living cells had produced the wild type genotype.
Further experiment by Hayes, a British geneticist, and by Jacob and Wollman, two French geneticists gave better insight of this process when they found that genetic recombination takes place in bacteria as a one-way transfer of genetic material from a male type donor to a female type receptor and the process was termed conjugation.