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Bacterial transformation is a process of horizontal gene transfer by which some bacteria take up foreign genetic material (naked DNA) from the environment. It was first reported in Streptococcus pneumoniae by Griffith in 1928.1 DNA as the transforming principle was demonstrated by Avery et al in 1944.2
The process of gene transfer by transformation does not require a living donor cell but only requires the presence of persistent DNA in the environment. The prerequisite for bacteria to undergo transformation is its ability to take up free, extracellular genetic material. Such bacteria are termed as competent cells.
The factors that regulate natural competence vary between various genera. Once the transforming factor (DNA) enters the cytoplasm, it may be degraded by nucleases if it is different from the bacterial DNA. If the exogenous genetic material is similar to bacterial DNA, it may integrate into the chromosome. Sometimes the exogenous genetic material may co-exist as a plasmid with chromosomal DNA.
The phenomenon of natural transformation has enabled bacterial populations to overcome great fluctuations in population dynamics and overcome the challenge of maintaining the population numbers during harsh and extreme environmental changes. During such conditions some bacterial genera spontaneously release DNA from the cells into the environment free to be taken up by the competent cells. The competent cells also respond to the changes in the environment and control the level of gene acquisition through natural transformation process.
Figure 1.Schematic representation of transformation in bacteria
Not all bacteria are capable of taking up exogenous DNA from their environment. The practical approach to acquire competent cells is to make the bacterial cells artificially competent using chemicals or electrical pulses.
Note: To endure the heat shock treatment, it is important the cells used are in the log phase of growth
Sigma-Aldrich offers a wide range of chemically competent cells and electrocompetent cells. Choose the right product for you with our Selection Guide.
The phenomenon of transformation has been widely used in molecular biology. As they are easily grown in large numbers, transformed bacteria may be used as host cells for the following:
The materials required and the detailed protocol of transformation can be found here.
The transformation efficiency is defined as the number of transformants generated per µg of supercoiled plasmid DNA used in the transformation reaction.
Transformation efficiency is calculated using the formula below:
Number of Colonies on Plate (df)
|X 1000 ng/µg|
|Amount of DNA plated (ng)|
Comparison of Transformation and Transfection