Team:LMU-Munich/Notebook/Protocols/2 Highly competent cells





Preparing glassware and media

Eliminating detergent

Detergent is a major inhibitor of competent cell growth and transformation. Glass and plastic must be detergent free for these protocols. The easiest way to do this is to avoid washing glassware, and simply rinse it out. Autoclaving glassware filled 3/4 with DI water is an effective way to remove most detergent residue. Media and buffers should be prepared in detergent free glassware and cultures grown up in detergent free glassware.

Prechill plasticware and glassware

Prechill 250mL centrifuge tubes and screw cap tubes before use.

Preparing seed stocks

  • Streak TOP10 cells on an SOB plate and grow for single colonies at 23°C
    • room temperature works well
  • Pick single colonies into 2 ml of SOB medium and shake overnight at 23°C
    • room temperature works well
  • Add glycerol to 15%
  • Aliquot 1 ml samples to Nunc cryotubes
  • Place tubes into a zip lock bag, immerse bag into a dry ice/ethanol bath for 5 minutes
    • This step may not be necessary
  • Place in -80°C freezer indefinitely.

Preparing competent cells

  • Inoculate 250 ml of SOB medium with 1 ml vial of seed stock and grow at 20°C to an OD600nm of 0.3
    • This takes approximately 16 hours.
    • Controlling the temperature makes this a more reproducible process, but is not essential.
    • Room temperature will work. You can adjust this temperature somewhat to fit your schedule
    • Aim for lower, not higher OD if you can't hit this mark
  • Centrifuge at 3000g at 4°C for 10 minutes in a flat bottom centrifuge bottle.
    • Flat bottom centrifuge tubes make the fragile cells much easier to resuspend
    • It is often easier to resuspend pellets by mixing before adding large amounts of buffer
  • Gently resuspend in 80 ml of ice cold CCMB80 buffer
    • sometimes this is less than completely gentle. It still works.
  • Incubate on ice 20 minutes
  • Centrifuge again at 4°C and resuspend in 10 ml of ice cold CCMB80 buffer.
  • Test OD of a mixture of 200 μl SOC and 50 μl of the resuspended cells.
  • Add chilled CCMB80 to yield a final OD of 1.0-1.5 in this test.
  • Incubate on ice for 20 minutes
  • Aliquot to chilled screw top 2 ml vials or 50 μl into chilled microtiter plates
  • Store at -80°C indefinitely.
    • Flash freezing does not appear to be necessary
  • Test competence
  • Thawing and refreezing partially used cell aliquots dramatically reduces transformation efficiency by about 3x the first time, and about 6x total after several freeze/thaw cycles.