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Short answer
Competent cells are bacteria, most often E. coli, that have been prepared so they take up foreign DNA far more readily than untreated cells. Labs use them to introduce plasmids during cloning. The two common preparations are chemically competent cells (calcium chloride treatment, then a brief heat shock) and electrocompetent cells (taken up by electroporation). The figure that separates products is transformation efficiency, reported in colony-forming units per microgram of control DNA.
What "competent" means: natural versus artificial competence
Thermo Fisher defines cell competence as "the ability of a cell to take up exogenous DNA." Some bacteria do this on their own, but natural bacterial transformation occurs at a low frequency, quoted as 10-2 to 10-10 and highly variable between species. Competence can also be introduced artificially, chemically or by electroporation, and that is what the phrase "competent cells" means on a product label: bacteria treated to enhance uptake of foreign DNA compared with normal cells.
Competent cells in E. coli
E. coli is a hardy gram-negative bacterium and a standard model organism, which is why most commercial competent cells are E. coli strains. Two preparations dominate.
Chemically competent cells
These are treated with calcium chloride (CaCl2), described by Thermo Fisher as facilitating attachment of plasmid DNA to the cell membrane, followed by a heat shock at 42°C that the vendor says opens pores in the membrane. Thermo Fisher describes them as best suited to general cloning and subcloning. Addgene notes they are fast and easy to use but less efficient at taking up larger plasmids.
Electrocompetent cells
These are cells with highly permeable membranes; an electrical pulse during electroporation creates pores through which DNA enters. For large plasmids, Addgene advises that electrocompetent cells are a good idea. For the pulse settings and buffers involved, see the CASRAI guide to electroporation parameters, buffers and cell-type optimization.
Chemical versus electrocompetent cells compared
| Feature | Chemically competent | Electrocompetent |
|---|---|---|
| How DNA gets in | CaCl2 treatment plus 42°C heat shock (Thermo Fisher) | Electrical pulses create pores in highly permeable cells (Thermo Fisher) |
| Efficiency range listed by one vendor | 1 x 106 to more than 5 x 109 transformants per µg DNA (Thermo Fisher) | Listed products from more than 5 x 109 up to more than 3 x 1010 cfu per µg (Thermo Fisher) |
| Uses the vendor lists | General cloning and subcloning | Library construction, small amounts of DNA, cloning unstable DNA (Thermo Fisher) |
| Large plasmids | Less efficient uptake (Addgene) | Recommended option (Addgene) |
The efficiency figures are one vendor’s catalogue values and are measured under that vendor’s own conditions, so they indicate tiers rather than a head-to-head ranking.
Reading transformation efficiency
Efficiency is stated as cfu (colony-forming units) or transformants per microgram of DNA. Addgene points out that commercial competent cells range significantly in efficiency. Two cautions follow from the sources:
- The label number comes from a control plasmid. Addgene notes that transformation of a ligation reaction is approximately 10-fold lower than transformation of an intact control plasmid, so expect fewer colonies from a cloning experiment than the label suggests.
- A positive control is recommended, and many vendors include one, which lets you tell a failed batch of cells from a failed ligation.
Where it matters for labs and supply decisions
A short checklist for reading a competent-cell listing or its instructions for use:
- Preparation type: chemical or electro.
- Efficiency tier against the application: routine cloning and subcloning sits with chemically competent cells; library construction and small DNA amounts are the uses listed for the highest-efficiency electrocompetent cells.
- Strain traits: Thermo Fisher’s strain table lists whether a strain carries an F′ episome, T1 resistance, supports blue-white screening, or permits cloning of methylated DNA.
- Control plasmid included: useful for the check described above.
- Handling: follow the instructions that came with the cells; Addgene advises the same and notes that competent cells are kept at −80°C and thawed on ice.
Once transformed, colonies move on to the steps in the gene cloning workflow. Related bench guides cover site-directed mutagenesis and restriction enzyme digest setup.
Common misunderstandings
- "Competent" does not mean the cells are healthier or more capable overall. It refers only to the ability to take up exogenous DNA.
- Label efficiency is not your expected yield. It is measured on an intact control plasmid; ligations run roughly 10-fold lower.
- Higher efficiency is not always the goal. Chemically competent cells are described as suited to general cloning and subcloning, so the highest tier is not required for routine work.
- Efficiency numbers from different vendors are not directly comparable because each is measured under that vendor’s conditions.
FAQ
What is the meaning of competent cells?
Cells able to take up exogenous DNA. In the lab the term usually means bacteria treated, chemically or for electroporation, to take up DNA far more efficiently than normal cells.
What are competent cells in E. coli?
E. coli strains prepared for transformation, either chemically competent (CaCl2 and heat shock) or electrocompetent (electroporation), commonly used for cloning and subcloning plasmid DNA.
Sources
- Thermo Fisher Scientific, "What are chemically competent cells?" (checked 2026-10-29): https://www.thermofisher.com/us/en/home/life-science/cloning/competent-cells-for-transformation/chemically-competent.html
- Thermo Fisher Scientific, "What are electrocompetent cells?" (checked 2026-10-29): https://www.thermofisher.com/us/en/home/life-science/cloning/competent-cells-for-transformation/electrocompetent-cells.html
- Thermo Fisher Scientific, "Overview of transformation" (checked 2026-10-29): https://www.thermofisher.com/us/en/home/life-science/cloning/competent-cells-for-transformation/competent-cells-resources/overview-of-transformation.html
- Addgene, "Bacterial Transformation" protocol (checked 2026-10-29): https://www.addgene.org/protocols/bacterial-transformation/
