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Bacterial Transformation

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Bacterial transformation is the process by which bacteria take up exogenous DNA from the environment.

Some bacteria can naturally take up this exogenous DNA, while others are chemically induced in the lab to make their cell membrane permeable to DNA. This step is part of DNA cloning, which helps scientists study gene sequences, their functions, and the proteins they encode.

In the lab, the DNA sequence of interest is inserted into a circular piece of DNA called a plasmid. This DNA of interest is the exogenous DNA that will be introduced into the bacteria.

The plasmid usually also carries an antibiotic resistance gene, which helps scientists to identify bacteria that have taken up the plasmid.

A large number of copies of the plasmid are added to the liquid medium containing the competent bacteria, followed by a brief heat shock. The sudden temperature change from ice to 42°C increases membrane permeability, allowing plasmid DNA to enter the cell.

The bacteria are then grown on selective media containing a specific antibiotic, which allows only resistant cells that have taken up the cloned plasmid to survive and grow.

Only transformed cells multiply to form colonies, which are visible spots of bacterial growth derived from a single cell.

Overview

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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.

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Bacterial Transformation

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