• Replication: Transcription and Translation • Operons • Mechanism of Transduction, Conjugation and transposition

Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells usually through the pili of the microbial cells.  It is a mechanism of horizontal gene transfer just as are transformation and transduction although these two other mechanisms do not involve cell-to-cell contact.

Conjugation occurs in four major steps:

1. Donor cell produces pilus
2. Pilus attaches to recipient cell and brings the two cells together.
3. The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipient cell.
4. Both cells synthesize a complementary strand to produce a double stranded circular plasmid and also reproduce pili; both cells are now viable donor for the F-factor.^[1]

Conjugation diagram

Transduction is the process by which foreign DNA is introduced into a cell by a virus or viral vector.  An example is the viral transfer of DNA from one bacterium to another. Transduction does not require physical contact between the cell donating the DNA and the cell receiving the DNA Transduction is a common tool used to introduce a foreign gene into a host cell’s genome (both bacterial and mammalian cells).

Griffith’s experiment was an experiment done in 1928 by Frederick Griffith. It was one of the first experiments showing that bacteria can get DNA through a process called transformation.

Griffith used two strains of Streptococcus pneumoniae. These bacteria infect mice. He used a type III-S (smooth) and type II-R (rough) strain. The III-S strain covers itself with a polysaccharide capsule that protects it from the host’s immune system. This means that the host will die. The II-R strain does not have that protective shield around it and is killed by the host’s immune system.

In this experiment, bacteria from the III-S strain were killed by heat, and their remains were added to II-R strain bacteria. While neither harmed the mice on their own, the blend of the two was able to kill mice.

Griffith was also able to get both live II-R and live III-S strains of S. pneumoniae from the blood of these dead mice. He concluded that the type II-R had been “transformed” into the lethal III-S strain by a “transforming principle” that was somehow part of the dead III-S strain bacteria.

The “transforming principle” Griffith saw was the DNA of the III-S strain bacteria. While the bacteria had been killed, the DNA had survived the heating process and was taken up by the II-R strain bacteria. The III-S strain DNA contains the genes that form the shielding polysaccharide part from attack. Armed with this gene, the former II-R strain bacteria were now protected from the host’s immune system and could kill the host.

Griffith’s Experiment on Transformation