Bacterial Transformation

Competent Cells for Transformation

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What is Bacterial Transformation?

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.

Reasons for Transformation

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.

Schematic representation of bacterial transformation

Figure 1: Schematic representation of transformation in bacteria

Competence of 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.

Chemical induction of competence involves the following steps:
- chilling the cells in the presence of calcium phosphate (Catalog Number 50552) to make them permeable
- incubation with DNA
- heat shock treatment at 42°C for 60-120 seconds that causes the DNA to enter the cells

Note: To endure the heat shock treatment, it is important the cells used are in the log phase of growth

Alternatively, the bacterial cells are made permeable by subjecting them to electrical pulses, a process known as electroporation.

Sigma-Aldrich offers a wide range of chemically competent cells and electrocompetent cells. Choose the right product for you with our Selection Guide.

What Are Applications of Transformation?

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:

to make multiple copies of the DNA
in cloning procedures
to express large amounts of proteins and enzymes
in the generation of cDNA libraries
in DNA linkage studies

What is Required in a Typical Transformation Reaction?

Competent cells
Supercoiled plasmid DNA
Transformation medium
Selection marker (antibiotic and/or chromogenic substrate)

The materials required and the detailed protocol of transformation can be found here.

Calculation of Transformation Efficiency

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)

What Factors Affect Transformation Efficiency?

DNA used for transformation reaction
- The concentration of DNA must be carefully quantified and the same DNA must be used for all transformations. The pUC19 DNA (Catalog Number D3404) offered by Sigma-Aldrich is accurately quantitated and is suitable to maintain the amount of plasmid DNA used in transformation reactions.
- The transformation reaction is efficient when <10ng of DNA is used. pUC19 DNA (0.1ng) is suitable as control.
- Supercoiled DNA is most efficient for transformation compared to linear or ssDNA that has the transformation efficiency of <1%.
- During electroporation, the salts present in the preparation mix may lower transformation efficiency. Limit the volume of plasmid DNA to 1µL per transformation.
- Column-purified DNA is most suitable as it is devoid of contaminants that interfere with transformation.
- The volume of miniprep DNA must be limited to not more than 5µL per 50µL reaction to prevent the contaminants from decreasing the transformation efficiency.
- Ligation mixtures inhibit transformation as the ligases inhibit electroporation of cells. The ligases must be heat-inactivated (65°C for 5 minutes) before the mixture is added to the cells.
Heat shock: Optimal heat shock set up is as follows:
- 42°C for 45 seconds for PCR tubes or thin-walled tubes
- 37°C for 60 seconds for microfuge tubes or thick-walled tubes
- General set up: 37°C for 60 seconds
Time between transformation and plating: The transformation efficiency is significantly decreased as the time between the transformation reaction and the plating is increased. This, however, also depends on the strain and the plasmid used.
Microbial medium used: SOB medium (Catalog Number H8032) is most suitable for transformation and gives almost two-fold higher transformation efficiency when compared to LB medium (Catalog Numbers L2542, L3522, L3141) for chemically competent cells.
Selective plates: Commercially prepared agar plates with selection agent (Catalog Numbers L5667, L0168, L0543, L0418, L8795) are most suitable for identifying transformant colonies. Plating large number of cells may give rise to satellite colonies that are not true transformants. Streaking the colonies on selective agar plates is recommended to identify true transformants.
Freeze/thawing of cells: Activity of cells that are refrozen and thawed is significantly reduced resulting in at least two-fold decrease in transformation efficiency.

Comparison of Transformation and Transfection

Transformation	Transfection
Applicable to bacteria	Applicable to eukaryotic cells*
Exogenous genetic material is taken up by competent bacteria	Exogenous genetic material is introduced into the eukaryotic cells
Bacteria can be made competent either chemically or by electroporation	Introduction of exogenous genetic material may be liposome-mediated, by electroporation or by using viral vector
The exogenous genetic material may integrate into the bacterial genome or exist as a plasmid	The exogenous genetic material is either integrated into the genome or is degraded
Transformation enables the expression of multiple copies of DNA resulting in large amounts of protein or enzyme that are not normally expressed by bacteria	Genetic material of transformed bacteria may be used to transfect eukaryotic cells for DNA or protein expression studies

* Eukaryotic cells that undergo cellular changes and become malignant by increased proliferation are also referred to as "transformed" cells. This transformation is due to dysregulation at gene, mRNA and/or protein level and does not resemble transformation in bacteria.

Materials

Application	Product No.	Product Description	Transformation Efficiency	Genotype	Blue White Screening Capable
for protein expression and DNA plasmid production	CMC0001	SIG10 Chemically Competent Cells	≥ 1 × 10⁸	F- mcrA Δ(mrr-hsdRMS-mcrBC) endA1 recA1 Φ80dlacZΔM15 ΔlacX74 araD139 Δ(ara,leu)7697 galU galK rpsL nupG λ- tonA	Y
for protein expression and DNA plasmid production	CMC0002	SIG10 F' Chemically Competent Cells	≥ 5 × 10⁸	[F´ pro A+B+ lacIqZΔM15::Tn10 (TetR)] /mcrA Δ(mrr-hsdRMS-mcrBC) endA1 recA1 Φ80dlacZΔM15 ΔlacX74 araD139 Δ(ara, leu)7697 galU galK rpsL nupGλ tonA	Y
for protein expression and DNA plasmid production	CMC0003	SIG10 HIGH Electrocompetent Cells	≥ 5 × 10⁹	F- mcrA Δ(mrr-hsdRMS-mcrBC) endA1 recA1 Φ80dlacZΔM15 ΔlacX74 araD139 Δ(ara,leu)7697galU galK rpsL nupG λ- tonA (StrR)	Y
for protein expression and DNA plasmid production	CMC0004	SIG10 MAX Electrocompetent Cells	≥ 2 × 10¹⁰	F- mcrA Δ(mrr-hsdRMS-mcrBC) endA1 recA1 Φ80dlacZΔM15 ΔlacX74 araD139 Δ(ara,leu)7697galU galK rpsL nupG λ- tonA (StrR)	Y
for general cloning & library production	CMC0005	SIG10 F' MAX Electrocompetent Cells	≥ 2 × 10¹⁰	[F´ pro A+B+ lacIqZΔM15::Tn10 (TetR)] /mcrA Δ(mrr-hsdRMS-mcrBC) endA1 recA1 Φ80dlacZΔM15 ΔlacX74 araD139 Δ(ara, leu)7697 galU galK rpsL nupG λ- tonA (StrR)	Y
for general cloning & library production	CMC0006	SIG10 ULTRA Electrocompetent Cells	≥ 4 × 10¹⁰	F- mcrA Δ(mrr-hsdRMS-mcrBC) endA1 recA1 Φ80dlacZΔM15 ΔlacX74 araD139 Δ(ara,leu)7697galU galK rpsL nupG λ- tonA (StrR)	Y
for general cloning & library production	CMC0007	SIG10 5a Chemically Competent Cells	≥ 1 × 10⁸	fhuA2Δ(argF-lacZ)U169 phoA glnV44 Φ80 Δ(lacZ)M15 gyrA96 recA1 relA1 endA1 thi-1 hsdR17	Y
for plasmid production using unstable DNA	CMC0008	STEADY Chemically Competent Cells	> 1 × 10⁷	recA13 supE44 ara-14 galK2 lacY1 proA2 rpsL20(StrR) xyl-5 λ– leu mtl-1 F– mcrB mrr hsdS20(rB–, mB–)	N
for plasmid production using unstable DNA	CMC0009	STEADY Electrocompetent Cells	> 1 × 10⁷	recA13 supE44 ara-14 galK2 lacY1 proA2 rpsL20(StrR) xyl-5 λ– leu mtl-1 F– mcrB mrr hsdS20(rB–, mB–)	N
for making Uracil-containing DNA for mutagenesis	CMC0010	CHANGER Electrocompetent Cells	1 × 10⁹	[F’ Tra+ Pil+ (CamR)] ung-1 relA1 dut-1 thi-1 spoT1 mcrA	N
for BAC & cosmid cloning	CMC0011	XLDNA V2 Electrocompetent cells	≥ 1 × 10¹⁰	F- mcrA Δ(mrr-hsdRMS-mcrBC) endA1 recA1 Φ80dlacZΔM15 ΔlacX74 araD139 Δ(ara,leu)7697 galUgalK rpsL nupG (attL araC-PBAD-trfA250 bla attR) λ-	Y
for BAC & cosmid cloning	CMC0012	XLDNA SIG10 Electrocompetent cells	≥ 1 × 10¹⁰	F - pro A+B+ lacIqZΔM15::Tn10 (TetR)] /mcrA Δ(mrr-hsdRMS-mcrBC) endA1 recA1 Φ80dlacZΔM15 ΔlacX74 araD139 Δ(ara, leu)7697 galU galK rpsL nupG λ- tonA (StrR)	N
for production of biotinylated proteins	CMC0013	BIOTINYLATER F' Electrocompetent Cells	>1 ×10¹⁰	MC1061 [F´ pro A+B+ lacIqZΔM15::Tn10 (TetR)] araD139 ∆(ara-leu)7696 ∆(lac)l74 galU galK hsdR2(rΚ- mΚ+) mcrB1 rpsL (StrR) birA	N
for protein expression	CMC0014	BL21(DE3) Chemically Competent Cells	≥ 1 × 10⁷	F – ompT hsdSB (rB- mB-) gal dcm (DE3)	N
for protein expression	CMC0015	BL21(DE3) pLysE Chemically Competent Cells	≥ 1 × 10⁷	F – ompT hsdSB (rB- mB-) gal dcm (DE3)	N
for protein expression	CMC0016	BL21(DE3) Electrocompetent Cells	≥ 5 x 10⁹	F – ompT hsdSB (rB- mB-) gal dcm (DE3)	N
for the highest protein expression	CMC0017	OverExpress C41(DE3) Chemically Competent Cells	≥ 1 × 10⁶	F – ompT hsdSB (rB- mB-) gal dcm (DE3)	N
for the highest protein expression	CMC0018	OverExpress C41(DE3) pLysS Chemically Competent Cells	≥ 1 × 10⁶	F – ompT hsdSB (rB- mB-) gal dcm (DE3) pLysS (CmR)	N
for the highest protein expression	CMC0019	OverExpress C43(DE3) Chemically Competent Cells	≥ 1 × 10⁶	F – ompT hsdSB (rB- mB-) gal dcm (DE3)	N
for the highest protein expression	CMC0020	OverExpress C43(DE3) pLysS Chemically Competent Cells	≥ 1 × 10⁶	F – ompT hsdSB (rB- mB-) gal dcm (DE3) pLysS (CmR)	N
for the highest protein expression	CMC0021	OverExpress C41(DE3) Electrocompetent Cells	≥ 1 × 10⁹	F – ompT hsdSB (rB- mB-) gal dcm (DE3)	N
for the highest protein expression	CMC0022	OverExpress C43(DE3) Electrocompetent Cells	≥ 1 × 10⁹	F – ompT hsdSB (rB- mB-) gal dcm (DE3)	N
for controlled protein expression	CMC0023	CONTROLLER SIG10 Chemically Competent Cells	> 1 × 10⁹	mcrA Δ(mrr-hsdRMS-mcrBC) endA1 recA1 ɸ80dlacZΔM15 ΔlacX74 araD139 Δ (ara,leu)7697 galU galK rpsL (StrR) nupG λ− tonA Mini-F lacIq1 (GentR)	N
for controlled protein expression	CMC0024	CONTROLLER BL21(DE3) Chemically Competent Cells	> 1 × 10⁷	F- ompT hsdSB (rB- mB-) gal dcm (DE3) Mini-F lacIq1(GentR)	N

References

J Hyg (Lond). 1928 Jan;27(2):113-59. The Significance of Pneumococcal Types. Griffith F.
J Exp Med. 1944 Feb 1;79(2):137-58. Studies on the chemical nature of the substance inducing transformation of Pneumococcal types: induction of transformation by a desoxyribonucleic acid fraction isolated from Pneumococcus type III. Avery OT, Macleod CM and McCarty M.

03/14-1