16.6: Bacterial Transformation

In 1928, bacteriologist Frederick Griffith worked on a vaccine for pneumonia, which is caused by Streptococcus pneumoniae bacteria. Griffith studied two pneumonia strains in mice: one pathogenic and one non-pathogenic. Only the pathogenic strain killed host mice.

Griffith made an unexpected discovery when he killed the pathogenic strain and mixed its remains with the live, non-pathogenic strain. Not only did the mixture kill host mice, but it also contained living pathogenic bacteria that produced pathogenic offspring. Griffith concluded that the non-pathogenic strain received something from the dead pathogenic strain that transformed it into the pathogenic strain; he called this the transforming principle.

At the time of Griffith’s studies, there was heated debate surrounding the identity of the genetic material. Much early evidence implicated proteins as the hereditary molecules. Griffith’s experiments on bacterial transformation provided some of the earliest data demonstrating that DNA is the genetic material.

Bacteria incorporate external DNA through transformation. Transformation occurs naturally but is also induced in laboratories—often to clone DNA. To clone a specific gene, scientists can insert the gene into a plasmid, a circular DNA molecule that can independently replicate. The plasmid often contains an antibiotic resistance gene. Bacteria take up the plasmid through transformation. Scientists then expose the bacteria to antibiotics. Surviving bacterial colonies should contain the plasmid because the plasmid contains an antibiotic resistance gene. DNA analysis can confirm the gene’s presence in the plasmid. Bacterial colonies with the desired gene propagate and can be used to make more plasmids or proteins.

Why would bacteria take in foreign DNA? Unlike sexually reproducing organisms, bacteria essentially clone themselves. This reproductive method, called binary fission, offers few opportunities for genetic variation. Although mutations introduce some diversity, many mutations are harmful. Sharing genes through transformation, as well as conjugation and transduction, allows prokaryotes to evolve.

Bacterial transformation is a process by which bacteria take up exogenous DNA  that comes from outside the cell. 

Some bacteria can transform naturally or be induced in the lab as part of the DNA cloning process, which is useful for studying the sequences and functions of genes and the proteins they encode.

For transformation to occur, bacteria must either be naturally competent to take up extracellular DNA from the environment or are chemically treated in labs to make their cell walls permeable to DNA. 

In the laboratory, the DNA sequence of interest is inserted into a circular piece of DNA called a plasmid. The plasmid typically also contains a sequence encoding an antibiotic resistance gene that allows screening of transformants, or in other words, selection of the bacteria that took up the plasmid.

Several copies of the plasmid are then added to the liquid media containing competent bacteria, followed by a Heat shock treatment to enable the bacteria to take up the DNA. 

The transformed bacteria are then plated on selective media containing specific antibiotics that allow the survival and growth only of resistant cells that have taken up the cloned plasmid. 

These transformants further multiply to form colonies, which are visible spots of bacterial growth derived from a single cell.