Bacterial Genetics: Plasmid DNA & Conjugation Gene Transfer

Sep 24, 2010
Tami Port

Conjugation Is How Bacteria Share Plasmid DNA - Adenosine Wiki, Creative Commons Share Alike

A plasmid is a molecule of DNA, independent of the bacterial nucleoid chromosome, which often has "bonus DNA" with instructions for weapons of infection.

Both the bacterial nucleoid and bacterial plasmids are circular molecules of DNA - genetic information. Plasmids are, however, much smaller than the bacterial chromosome, usually only containing a few genes.

Unlike the bacterial chromosome, plasmid traits are not normally required for the cell to function, but do help a bacterium survive and cause disease. Not all bacteria have plasmids, but those that do can share these advantageous molecules with other bacteria in a process called conjugation.

Conjugation: Transferring Plasmids

Conjugation is a type of horizontal gene transfer, meaning a way for organisms to share genetic information with each other. Horizontal gene transfer can be contrasted with vertical gene transfer - the transmission of genetic traits from generation to generation, parent to offspring. Conjugation is how a bacterium can share the information on a plasmid with its neighbors.

In order to transfer a plasmid, the bacterium with the plasmid must have the genetic instructions required to construct a sex pilus—an extension that can hook up to a recipient bacterium, connecting the two bacteria and physically pulling them towards each other. The genetic information for making a pilus is called the F (fertility) factor. The donor bacterium, which is considered F+, makes a copy of the plasmid and extends the pilus to the recipient (the F- cell). The newly copied plasmid is then transferred to the recipient. In the end, both the donor and the recipient bacteria have a copy.

Although plasmids are most often exchanged between closely related bacteria, some plasmids can be shared by very dissimilar microbes. A donor and a recipient do not always need to be of the same species in order to share a plasmid. Plasmids can even be shared between different cell types, such as when a bacterium (prokaryotic cell) shares a plasmid with a eukaryotic microbe, such as yeast.

Types of Bacterial Plasmids

There are many different types of plasmids, in addition to F factors mentioned above. Plasmids called R factors carry genes for antibiotic resistance, or resistance to chemicals that, without the plasmid, would be toxic to the bacterium. MRSA (methicillin-resistant Staphylococcus aureus) is a disease-causing type of bacteria that, due to the acquisition of R factors, is resistant to a wide range of antibiotics, and is able to cause dangerous, difficult-to-treat infections.

Bacteriocin factors carry genes for making proteins that kill other bacteria that do not have this factor, reducing competition for the bacteria that have it. Other plasmids, called virulence factors have genes that help bacteria cause disease, such as the productions exotoxins that make those infected with the bacterium ill, or genes for adhesins, that allow the bacterium to adhere to and infect certain host cell types.

For more information on microbiology, see the SPO Virtual Microbiology Classroom or Todar's Online Textbook of Bacteriology.