Bacterial transformation is the process of transfer of foreign genetic materials from the external environment through the cell membrane.
It’s a horizontal form of gene transfer first discovered in Streptococcus pneumoniae by British bacteriologist Frederick Griffith in 1928. He tested whether or not heat-killed bacteria might be used to vaccinate mice against pneumonia and found that a non-virulent strain of Streptococcus pneumoniae could be rendered virulent by exposing it to heat-killed virulent strains. He theorized the transforming principle based on this experiment, claiming that the heat-killed strain was responsible for making the innocuous strain infectious. Morton Mandel and Akiko Higo found in 1970 that E. coli may take up DNA from bacteriophage.
Even when a donor cell is not living, the gene transfer process in transformation necessitates the presence of persistent DNA in the environment. Some bacterial genera that have been exposed to extreme and harsh environmental conditions leak DNA into the environment. The bacteria’s major goal is to change its ability to take in extracellular genetic material from the environment. Competent cells are bacteria that have this ability. These capable cells are also able to adapt to changes in the environment and acquire genes through the natural transformation process.
But not all competent bacterial cells are capable of taking up DNA from the environment. It needs to be artificially competent with the use of chemicals or electric pulses. Chemicals like calcium phosphate are used with heat shock treatment that causes the DNA to enter into the cell. Electric pulse methods like electroporation make the bacterial cell-permeable that allows DNA to enter into the cell.
Factors that affect transformation efficiency
- The concentration of DNA (<10ng of DNA is used for efficacy).
- Supercoiled DNA is efficient than linear DNA
- The high concentration of salt lowers the transformation efficiency during electroporation.
- Ligase must be heat-inactivated as it inhibits the electroporation of cells.
- The time between transformation and plating.
- SOB medium is most suitable for transformation.
- Freezing and thawing of the cell decrease transformation efficiency.
Four steps are involved during bacterial transformation
- Preparation of competent cells
- Cell recovery period
- Cell plating
Calculation of transformation efficiency
The colonies need to be screened further to detect the presence of desired plasmid or not. After conformation, the colonies are employed in downstream applications such as plasmid isolation, sub-cloning, transfection and protein expression.