Bacterial Transformation

Back in the early 20th century, a British bacteriologist named Frederick Griffith was working with the pneumonia-causing bacteria Streptococcus pneumoniae. He performed a simple experiment using two different strains. One strain is known as the S strain because of a protective capsule that makes the colonies or clumps it forms appear smooth, and also makes it virulent or harmful. The second was the R strain, a version of the bacteria lacking the protective capsule, giving the colonies a rough appearance and rendering it non-virulent.

First, Griffith took some of the S strain bacteria and heated it, producing a heat-killed version of the S strain. Then, he gathered some mice and divided them into four groups. He injected the first group with the virulent S strain, and the second with the non-virulent R strain. He dosed the third group with the heat-killed S strain, and finally, be combined the heat-killed S strain and the R strain together, and injected this mix into the fourth group. As expected, the mice in the first group died, and the ones in the second and third group lived. But to Griffith's surprise, the mice in the last group also died.

When he published his study in 1928, he called this mysterious process transformation, as he speculated the presence of an underlying transforming principle, which allowed the previously non-virulent strain to become deadly. Later, in 1943, Avert, MacLeod, and McCarty reported that this transforming principle was likely desoxyribonucleic acid, or DNA, which we now know is the heredity material. Essentially what happened in Griffith's experiment was that when the bacteria were combined, some DNA leaked from the heat-killed S strain and into the R strain cells, transforming these non-virulent bacteria and passing on the information to make the protective capsule, turning the R strain into a virulent one, now able to kill the animals in the fourth group.

Fast forward to today, and scientists have developed a much simpler way to study bacterial transformation, using the bacteria E. coli and small, circular loops of DNA called plasmids. Usually, the plasmid used in transformation experiments includes a gene for a special function, like antibiotic resistance. E. coli are an excellent subject for transformation because they can display a property called competence, the ability to take up DNA from the environment. Essentially, this means that under certain environmental conditions, like a chemical or an electrical or heat shock, the cell wall of the E. coli can become temporarily permeable and allow the uptake of DNA from the environment. Once the plasmid is inside the E. coli, it can either hang out in the cytoplasm of its new host cell and be replicated and expressed over generations alongside the genome, or it may fully incorporate itself into the genome of the host. If the bacteria lose the plasmid at any point, the cell will also lose its antibiotic resistance, and so scientists grow the bacteria in a media containing the antibiotic to ensure that the only survivors are those containing the plasmid of interest.

In this lab, you will learn how to transform E. coli cells with a plasmid containing an antibiotic resistance gene, while practicing sterile microbiological techniques.