McClean:Competent Cells

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  • This protocol is used for preparing DH5 alpha competent cells for transformation with some modifications by Renee.
  • The base protocol is the Zymo Mix & Go E. coli Transformation Kit & Buffer Set Protocol.


  • LB Plate of DH5 alpha cells streaked for single colonies (This needs to be less than a week old. For best results, store plate at room temp, not 4C.)
  • SOB (not pHed!!! The pH should be approx. 6.8-6.9 If the pH is at 7.5, the cells will grow too fast.)
  • LB
  • 2 buckets of Ice
  • Zymo Mix & Go kit

Glassware & Equipment

  • Sterile culture tubes with lids
  • 2- 250 ml flasks with lids (no detergent residue, autoclaved)
  • 50 ml BD Falcon tubes
  • 15 ml BD Falcon tubes
  • Sterile 1.5ml eppie tubes
  • Beckman Coulter Allegra X14R refrigerated centrifuge with adapters


  1. First thing in the morning, pick a single colony from your source plate and inoculate 5 ml of sterile LB medium in a sterile culture tube. Incubate the culture in 37°C for 4-5 hours with rotation.
  2. After the culture is incubated 4-5 hours, check the OD at 600nm. If it is in log phase, use this culture to inoculate two 250ml flasks, each containing 50ml of SOB (not pHed). Calculate how much culture to put in the 50ml SOB to obtain an OD 600 of 0.01. Incubate both flasks at room temperature overnight with shaking at 200rpm.
  3. The next morning, check OD600 of both flasks. Hopefully the flasks will be in log phase. (Depending on the temp of the room, this will usually take 16-19 hours.) If the flasks are between 0.4 and 0.6, put them on ice right away; otherwise, let them keep growing at room temp until they reach log phase. I have been successful with an OD600 of 0.37. You just get fewer cells. I have also been successful with an OD600 of 0.68. Too much more and it is a problem, though.
  4. Incubate the culture on ice for 10 minutes.
  5. Pre-chill the centrifuge to 4 degrees.
  6. Make 10ml each of 1X Wash Buffer and 1X Competent Buffer using the 2X stock solutions and Dilution Buffer. EX. use 5ml of 2X Wash Buffer and 5ml Dilution Buffer. Mix well and keep on ice.
  7. Pour the cultures into 50ml tubes. Keep on ice.
  8. Centrifuge at 2500g for 5 minutes at 4C to gently pellet the cells.
  9. Decant the medium and drain the open tubes on a stack of paper towels for 10 seconds. Get them back on ice.
  10. Gently resuspend each pellet in 5 ml of ice cold 1X Wash Buffer. Resuspend the cells by pipetting gently with a large bore pipet (P1000), NOT vortexing. Keep on ice!!!
  11. Centrifuge at 2000g for 5 minutes at 4C.
  12. Decant the medium as before. Get it back on ice.
  13. Add 5 ml ice-cold 1X Competent Buffer to each tube. Gently resuspend the cell pellets and put the tubes on ice.
  14. Aliquot the cells into 1.5ml eppie tubes on ice. Each 50ml falcon tube is a separate batch of competent cells. So this protocol makes 2 batches. Keep them on ice at all times. When finished aliquoting, put each batch in a labelled freezer box and place at -80 degrees to freeze.
  15. Store at -80 degrees.

Related topics & references

Original protocol from Inoue et al. [1]. Useful comments and speculation about reducing agents in [2].

  1. Inoue H, Nojima H, and Okayama H. High efficiency transformation of Escherichia coli with plasmids. Gene. 1990 Nov 30;96(1):23-8. DOI:10.1016/0378-1119(90)90336-p | PubMed ID:2265755 | HubMed [Inoue90]
  2. Hengen PN. Methods and reagents. preparing ultra-competent Escherichia coli. Trends Biochem Sci. 1996 Feb;21(2):75-6. PubMed ID:8851666 | HubMed [Hengen96]

All Medline abstracts: PubMed | HubMed