Transformation efficiency

Transformation efficiency is the efficiency by which cells can take up extracellular DNA and express genes encoded by it. This is based on the competence of the cells. It can be calculated by dividing the number of successful transformants by the amount of DNA used during a transformation procedure. Transformants are cells that have taken up DNA (foreign, artificial or modified) and which can express genes on the introduced DNA.

Transformation efficiency should be determined under conditions of cell excess. The number of viable cells in a preparation for a transformation reaction may range from 2×10⁸ to 10¹¹; most common methods of E. coli preparation yield around 10¹⁰ viable cells per reaction. The standard plasmids used for determination of transformation efficiency in Escherichia coli are pBR322 or other similarly-sized or smaller vectors, such as the pUC series of vectors. Different vectors however may be used to determine their transformation efficiency. 10–100 pg of DNA may be used for transformation, more DNA may be necessary for low-efficiency transformation (generally saturation level is reached at over 10 ng).

After transformation, 1% and 10% of the cells are plated separately, the cells may be diluted in media as necessary for ease of plating. Further dilution may be used for high efficiency transformation.

Transformation efficiency can be measured in transformants or colony forming unit (cfu) per μg DNA used. A transformation efficiency of 1×10⁸ cfu/μg for a small plasmid like pUC19 is roughly equivalent to 1 in 2000 molecules of the plasmid used being transformed. In E. coli, the theoretical limit of transformation efficiency for most commonly used plasmids would be over 1×10¹¹ cfu/μg. In practice the best achievable result may be around 2–4×10¹⁰ cfu/μg for a small plasmid like pUC19, and considerably lower for large plasmids.

Individual cells are capable of taking up many DNA molecules, but the presence of multiple plasmids does not significantly affect the occurrence of successful transformation events. A number of factors may affect the transformation efficiency:

Plasmid size — A study done in E. coli found that transformation efficiency declines linearly with increasing plasmid size, i.e. larger plasmids transform less well than smaller plasmids.

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