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Transformation Efficiency Calculator

Calculate transformation efficiency in cfu per microgram of DNA from the colonies counted, the DNA added, the recovery volume and the fraction plated.

Formula

efficiency=coloniesμg DNA plated\text{efficiency} = \dfrac{\text{colonies}}{\mu\mathrm{g}\ \mathrm{DNA\ plated}}
mplated=madded×VplatedVrecovery×1DFm_{\text{plated}} = m_{\text{added}} \times \dfrac{V_{\text{plated}}}{V_{\text{recovery}}} \times \dfrac{1}{\mathrm{DF}}
cfu\text{cfu}
colony-forming units, the colonies counted
DNA plated\text{DNA plated}
the part of the DNA added that was in the volume put on the plate

How it works

Transformation efficiency measures how many cells in a preparation take up and express a plasmid. It is the number of colonies divided by the mass of DNA they came from. Only part of the recovered culture is plated, so the DNA on the plate is the DNA added multiplied by the fraction plated.

Efficiency is quoted per microgram of a control plasmid such as pUC19, because it depends on the DNA. It is highest for small, supercoiled plasmids, and lower for large ones and for ligation products. Competent cells made by careful chemical or electroporation methods can reach 10⁸ to 10⁹ cfu/µg; a value well below what the supplier claims points to a problem with the cells or the procedure.

Worked example

100 pg of control plasmid in cells recovered in 1,000 µL; 100 µL plated; 150 colonies.

  1. Fraction plated = 100 / 1000 = 0.1.
  2. DNA on the plate = 100 pg × 0.1 = 10 pg = 10⁻⁵ µg.
  3. Efficiency = 150 / 10⁻⁵ µg = 1.5 × 10⁷ cfu/µg.

1.5 × 10⁷ cfu/µg, adequate for routine subcloning, though below high-efficiency cells.

These are the values the calculator opens with, so you can check its output against this example.

Assumptions

  • Every colony came from one transformed cell.
  • The DNA added was the amount stated, and the control plasmid is of a size typical for its use.
  • Plating volume and dilution are accurate.

Common mistakes

  • Counting a plate with hundreds of merging colonies. Plate a smaller volume or dilute.
  • Comparing the efficiency of a ligation with that of a control plasmid. They are different DNA and will differ by orders of magnitude.
  • Forgetting to divide by the fraction plated, which understates efficiency.

Related tools

CFU Calculator (Colony-Forming Units per mL)Viable count from a plate count and dilutionOpen calculatorLigation Calculator (Vector:Insert Ratio)Insert mass for a molar ratio, or the ratio from the masses you haveOpen calculatorPlasmid Size CalculatorSize of a recombinant plasmid after cloningOpen calculatorSerial Dilution CalculatorTransfer volumes and the concentration in every tubeOpen calculator

Related equipment

Service documentation, failure modes and parts for the instruments this calculation is used with.