Genetic Recombination: Conjugation

I. MICROBIAL GENETICS

G. Genetic Recombination in Bacteria

4. Conjugation

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Genetic Recombination in Bacteria (def)

Genetic recombination is the transfer of DNA from one organism to another. The transferred donor DNA may then be integrated into the recipient's nucleoid by various mechanisms.

Natural mechanisms of genetic recombination in bacteria include:

a. transformation
b. transduction
c. conjungation

We will now look at conjugation.

Bacterial Conjugation (def)

Conjugation is the transferof DNA from a living donor bacterium to a recipient bacterium. In Gram-negative bacteria, a sex pilus (def) produced by the donor bacterium binds to the recipient. The sex pilus then retracts, bringing the two bacteria in contact. In Gram-positive bacteria sticky surface molecules are produced which bring the two bacteria into contact. DNA is then transferred from the donor to the recipient.

1. F⁺ conjugation (def).

This results in the transfer of an F⁺ plasmid (def) (coding only for a sex pilus) but not chromosomal DNA from a male donor bacterium to a female recipient bacterium. One plasmid strand enters the recipient bacterium while one strand remains in the donor. Each strand then makes a complementary copy. The recipient then becomes an F⁺ male and can make a sex pilus (see Fig. 1 through 4). Other plasmids present in the cytoplasm of the bacterium, such as those coding for antibiotic resistance, may also be transferred during this process.

GIF animation illustrating F+ conjugation.

2. Hfr (high frequency recombinant) conjugation (def).

An F⁺plasmid inserts or integrates into the nucleoid to form an Hfr male. The nucleoid then breaks in the middle of the inserted F⁺ plasmid and one DNA strand begins to enter the recipient bacterium. The bacterial connection usually breaks before the transfer of the entire chromosome is completed so the remainder of the F⁺ plasmid seldom enters the recipient. As a result, there is a transfer of some chromosomal DNA, which may be exchanged for a piece of the recipient's DNA, but not maleness (see Fig. 5 through 9).

GIF animation illustrating Hfr conjugation.

3. Resistance plasmid conjugation (def).

This results in the transfer of a resistance plasmid (R-plasmid) (def)from a donor bacterium to a recipient. One plasmid strand enters the recipient bacterium while one strand remains in the donor. Each strand then makes a complementary copy (see Fig. 10 through 13). As seen in Unit 1, the R-plasmid has genes coding for multiple antibiotic resistance and sex pilus formation. The recipient becomes multiple antibiotic resistant and male, and is now able to transfer R-plasmids to other bacteria.

GIF animation illustrating R plasmid conjugation.

This is a big problem in treating opportunistic gram-negative infections such as urinary tract infections, wound infections, pneumonia, septicemia by such organisms as E. coli, Proteus, Klebsiella, Enterobacter, Serratia, and Pseudomonas, as well as with intestinal infections by organisms like Salmonella and Shigella.

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