Transformation of Electrocompetent E. coli with Blue/White Selection

Posted on June 2nd, 2016 by UNM CC

M. Fero — 4/04 NOTE: add links once the posts are up.


  1. Desalt DNA template by EtOH precipitation in NaOAc followed by at least 2x washes with 70% EtOH. Resuspend in 5 - 15 µL of sterile H2O.
  2. Rinse cuvettes (if they have been used before) 5x with deionied H2O, and place them on ice. This is sufficient to avoid background growth in most cases.
  3. Set a BioRad MicroPulser to "Ec1" for 1 mm cuvettes, or "Ec 2" for 2 mm cuvettes.
  4. Electroporate the DNA into the bacteria:
    1. Add 5 µL of DNA to 50 µL of bacteria and mix by pipetting.
    2. Transfer bacteria/DNA mix to a cold cuvette, and immediately pulse in the electroporator.
    3. Quickly add 1 mL of r.t. L.B. to the bacteria in the cuvette. Use a sterile Pasteur pipette to transfer the suspension to a bacteria tube.
    4. Repeat for additional DNA samples and control DNA.
  5. Rotate at 37ºC x30 min. to allow the bacteria to recover.
  6. Meanwhile plate LB/Amp plates with IPTG + X-gal if blue/white selection of colonies will be performed, as follows:
    1. Mix 100 µL X-gal + 20 µL of IPTG per plate.
    2. Spread X-gal/IPTG mix across surface of plate with a sterile glass spreader and a plate spinner.
    3. Allow the mix to infiltrate the media for 20 min.
  7. Perform two 10-fold serial dilutions of the LB/bacteria suspension into fresh sterile L.B. Concentrate the remaining bacteria by spinning 1 min. at 5k rpm in a sterile microcentrifuge tube, and then resuspend pellet in 0.1 mL of L.B.
  8. Plate 100 µL of the various concentrations of bacteria onto LB/Amp (+ X-gal/IPTG as necessary), and spread with a sterile spreader and plate spinner to evenly coat the plate. Record the DNA construct and the Bacteria Dilution Factor (9x, 1x, 1/10x, 1/100x) on each plate.
  9. Incubate the plates inverted at 37ºC o.n. They may need to be placed in a container if the humidity of the incubator is so low that it causes the agar to dry out.
  10. Remove the plates when the colonies are ~1mm in diameter. The color development on X-gal treated plates will continue to occur after the plates are removed from the incubator.
  11. Pick white colonies (also with no central blue coloration) and restreak on L.B./Amp plates to ensure that pure clones are obtained before performing plasmid preps.
  12. Store at 4ºC for at least 1 hr. (to firm up the agar) if colony hybridiation will be performed. Seal edges of plates with parafilm for storage up to 1 week.
  13. Count the numbers of colonies on the plate with control DNA to determine the efficiency of competent cells:
  14. Efficiency (Transformants/µg) = colonies/plate x (Bacterial dilution factor) x 20,000
  15. Highly competent cells should have ~50 colonies on the plate with a 1/10 dilution of the control DNA's bacteria suspension.


Control DNA (e.g. pBS) diluted to 1 pg/µL in sterile H2O, on ice.
Electrocompetent bacteria, frozen (50 - 100 µL aliquots), on ice.
BioRad cuvettes (1 mm gap), on ice.
Sterile L-Broth (L.B.)
Bacteria culture tubes
P1000, P200, P20 Pipetman and sterile tips
Sterile Pasteur pipets and bulb.
37ºC incubator and rotating wheel
L.B./Amp bacterial culture plates (or other suitable selection media)
X-gal 20 mg/mL in DMF (dimethlyformamide), store at -20ºC
IPTG 0.2 g/mL in H2O, store at -20ºC

Tags: Fero-Bacteria

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