Lidstrom:Competent Cell Preparation

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Chemically Competent E. Coli

Notes:

• You need fresh cells.
  • Often people inoculate a few mL of the culture for overnight growth, then use 200 uL to inoculate the ~50 mL of culture they will make competent.
  • Some people in our lab believe you want to start with a fresh plate and don't even use one that is a few days old. Other people (Mila) are mostly concerned about the OD being right at the time of harvest.
• You need to flash freeze the cells at the end of the procedure. You can do this by pouring liquid nitrogen over them, or you can freeze your tubes at -80oC the night before, put your aliquots in, and stick them back at -80oC.

Electrocompetent Cells

• You can make your own electro-competent cells for electroporation.
  • From Amada's past mentor in undergrad:
    • Grow the cells overnight
    • Inoculate from this the next morning: generally use 200 uL/50 mL
    • Let grow for about 3-4 h (OD 0.4-0.6 or so), then centrifuge and wash 3 times with 10% glycerol (everything on ice).
    • Resuspend the pellet in a small volume (Example, if I do a 5 ml culture I try to resuspend the final pellet in no more than 0.2 or 0.3 ml of 10% glycerol). Then split in 1.5 ml centrifuge tubes (0.1 ml in each), freeze on dry ice-isopropanol and store at -80C.
      • Note: you can dilute your re-suspension, measure OD, and calculate the cell concentration if you care. See this page. Would allow you to calculate efficiency.