Bacterial Transformation: Mechanism

Bacterial transformation is one of the three horizontal gene transfer mechanisms found in bacterial cells.  The other two horizontal gene transfer mechanisms are conjugation (Transfer of F Plasmid & Transfer of Chromosomal DNA)  and transduction (generalized transduction & specialized transduction).

Key information regarding the bacterial transformation

1. DNA is released into the environment when bacterial cells (i.e. donor) die and undergo lysis
2. Recipient cells uptake free DNA released into the environment.
3. Not all bacteria are capable of taking free DNA from the environment, only competent bacteria can do.

Those bacteria that are able to take free DNA and are able to go transformation are called competent bacteria.

Some examples of pathogenic bacteria showing competence:

• Haemophilus spp
• Streptococcus spp
• Neisseria spp

Mechanism of Bacterial Transformation:

In this particular example, we are going to observe how ampicillin-sensitive bacterial cells turn to ampicillin-resistant via uptake of ampicillin resistance gene through transformation. 

1.  Competent bacteria have DNA receptors on the surface of its cell. Free DNA which is available in the vicinity of those bacteria can bind to the DNA receptors.
2. Double-stranded donor DNA binds to specific receptors on the surface of a competent cell.

3. Only a single strand of donor DNA reaches the interior of the recipient cell, the other strand gets degraded by nuclease.

4. Recombination with the recipient’s homologous DNA occurs.

5. Mismatch repair system of DNA comes into play and it either removes donor DNA or recipient DNA.

• A: If the donor DNA is removed, the cell will carry ampicillin sensitive gene, but
• B: if the donor DNA is not removed, the cell will now have an ampicillin resistance gene.

so in a mixture of cells, some cells contain the new donor DNA and others have the original DNA sequences.
Bacterial cells which are successfully transformed can be selected in the laboratory by growing these cells in the medium containing ampicillin. Only resistance cells will grow.

References

1. Lorenz, M. G., & Wackernagel, W. (1994). Bacterial gene transfer by natural genetic transformation in the environment. Microbiological reviews, 58(3), 563–602. https://doi.org/10.1128/mr.58.3.563-602.1994 
2. Gingold E. B. (1985). Bacterial transformation. Methods in molecular biology (Clifton, N.J.), 2, 237–240. https://doi.org/10.1385/0-89603-064-4:237