How-To: Calculate Transformation Efficiency

Background:

The basic unit of all living organisms, from bacteria to humans, is the cell. Most cells contain DNA, which is the genetic blueprint used to build an organism. In nature, bacteria pass small pieces of DNA back and forth through transformation. In the laboratory, we can force cells to take up DNA using the "heat shock" technique, where the combination of charged ions and a rapid change in temperature force bacteria to take up DNA from the surrounding environment.

In practice, transformation is highly inefficient - only one in every 10,000 cells successfully incorporates the plasmid DNA. However, since many cells are used in a transformation experiment (about a billion cells), only a few cells must be transformed to achieve a positive outcome.

We can use the data from our experiment to determine how well our transformation worked by calculating the transformation efficiency. This is a quantitative determination of the number of cells transformed by per 1µg of plasmid DNA. In essence, it is an indicator of the success of the transformation experiment.

To calculate the transformation efficiency:

To calculate the transformation efficiency, we need a few pieces of data:

1. The amount of the plasmid that is used in the transformation, in micrograms. A common amount of plasmid to use for transformation is 10 nanograms, or 0.01 micrograms.
2. The final volume of the cell suspension at recovery. This includes the volume of the competent cells and the recovery broth.
3. The volume of cells plated on the nutrient agar. Since most of the time we will not plate the entire volume of transformed cells, this will allow us to determine the proportion of cells that were plated.
4. The number of colonies on the plate. Each colony represents one transformed cell.

Count the number of colonies on the transformation plate. A convenient method to keep track of counted colonies is to mark each colony with a lab marking pen on the outside of the plate.

Determine the transformation efficiency using the following formula:

Plug the data into the above equation to determine the number of transformants per microgram of DNA.

Transformation efficiency generally ranges from 1 x 10⁴ to 1 x 10⁸ cells transformed per µg plasmid. A fun way to explore this concept in class would be to change the heat shock conditions and to analyze the results. Can you make your transformation more efficient by adding more DNA or changing the duration or temperature of the heat shock? Try it and find out!