Conjugation is the physical union of bacterial cells. So, two bacterial cells come together; they actually fuse in part, and during this fusion, they exchange genetic material. This phenomenon was first discovered in 1946 by scientists, including Leta Bergan and Tatum. In their experiment, they used E. coli and had two strains of E. coli, referred to as strain A and strain B.
Strain A only grows in media that contains specific components, which we do not need to memorize, just understand the concept. Strain B could only grow in a medium with different materials, requiring three specific amino acids and a chemical that strain A did not need. They mixed these two strains in a tube, placing strain A and strain B, and plated them onto a surface lacking any of the materials necessary for either strain to grow. Logically, no growth should occur because the essential nutrients were absent.
However, against expectations of a completely blank plate, some cells did grow. This perplexed the scientists, and they hypothesized that the strains had exchanged DNA. Strain A, being a wild type, could grow without certain chemicals, and strain B gained this ability through the DNA that Strain A donated, allowing it to grow independent of the other's specialized chemical requirements. For example, if strain A didn't need leucine to grow, it transferred the necessary gene to strain B, enabling strain B to also grow without leucine. This DNA exchange between the two strains facilitated their growth under otherwise inhibitory conditions.
I'll discuss further how this exchange occurred and the specifics of what was exchanged. However, I also want to mention some structures involved in this process. The structures that enable the bacteria to come together for conjugation are known as sex pili or f pili, depending on the textbook reference. The conjugation bridge, which is formed between the cells, acts as a passageway resembling a physical bridge, allowing the DNA to cross from one bacterium to another. Future videos will include images of these structures for better visualization. Thus, the existence and function of bacterial conjugation were illuminated through this pivotal experiment.
Here is a simple recap of the experiment: You have bacterial strain A, which grows in condition A, and bacterial strain B, which grows in condition B. Without conjugation, there would be no growth in a medium lacking both A and B. But with conjugation, growth still occurs because the DNA is transferred between the two strains. This was the core experiment that unveiled the mechanism of bacterial conjugation.