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Electrocompetent Cell Preparation

This protocol can be used to make E. coli stocks that transform at high efficiency by BacterialElectroporation.

  1. StreakForIsolation onto an agar plate.
    • do not omit this step
  2. pick a single colony from the isolation plate and innoculate into 3 ml medium. grow overnight
    • use LB or high-nutrient medium such as Circlegrow
  3. add the entire 3 ml culture to 500 ml prewarmed medium and incubate with shaking until OD600 is 0.5 to 0.7
    • do not let the culture overgrow
    • 0.5 to 0.6 works best
      • if using a baffled flask for improved culture aeration, you can use 1000 ml of prewarmed medium instead of 500 ml
  4. chill culture with shaking in a water / ice slurry to stop growth
    • from this point forward, the cells must be kept at 4C or on ice at all times and all tubes, bottles and solutions must be pre-chilled to 4C
  5. centrifuge culture to pellet cells
    • 6000g for 10 minutes works well. eg: 250 ml bottle, Sorvall GSA rotor, 6000 rpm
      • if the size of the culture required use of multiple centrifuge tubes to get the cells to pellet, the pellets should all be combined together into a single tube for all of the subsequent processing steps
  6. resuspend cells in 200 ml ice-cold sterile water and centrifuge to pellet
    • do this step twice
  7. resuspend cells in 50 ml ice-cold sterile 10% glycerol and centrifuge to pellet
    • eg: 50 ml oakridge tube, Sorvall SS34 rotor at 7000 rpm or SA600 rotor at 6500 rpm
  8. remove supernatant and estimate volume of the cell pellet
  9. resuspend cells in an equal volume of ice-cold sterile 10% glycerol
  10. aliquot into prechilled tubes and freeze on crushed dry ice
    • do not freeze in liquid nitrogen
    • do not add ethanol to the crushed dry ice
    • 50 μl is a good aliquot size
  11. store at -80C
  12. important control: take one of the newly prepared compentent cell aliquots and spread untransfected on both ampicillin and kanamycin plates. incubate overnight at 37C. there should be no colonies on these plates. any colonies will indicate contamination with drug resistant bacteria -- discard this compenent cell preparation and try again
    • this kind of contamination really can happen so do not skip this step


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