Notebook

Protocols

Competent Cells (iGEM Protocol)

This protocol is a variant of the Hanahan protocol using CCMB80 buffer for DH10B, TOP10 and MachI strains. It builds on Example 2 of the Bloom05 patent as well. This protocol has been tested on NEB10, TOP10, MachI and BL21(DE3) cells. See OWW Bacterial Transformation page for a more general discussion of other techniques. The Jesse '464 patent describes using this buffer for DH5α cells. The Bloom04 patent describes the use of essentially the same protocol for the Invitrogen Mach 1 cells. This is the chemical transformation protocol used by Tom Knight and the Registry of Standard Biological Parts.

Materials
· Detergent-free, sterile glassware and plasticware (see procedure)
· Table-top OD600nm spectrophotometer
· SOB

CCMB80 buffer
· 10 mM KOAc pH 7.0 (10 ml of a 1M stock/L)
· 80 mM CaCl2.2H2O (11.8 g/L)
· 20 mM MnCl2.4H2O (4.0 g/L)
· 10 mM MgCl2.6H2O (2.0 g/L)
· 10% glycerol (100 ml/L)
· adjust pH DOWN to 6.4 with 0.1N HCl if necessary
➔ adjusting pH up will precipitate manganese dioxide from Mn containing solutions.
· sterile filter and store at 4°C
· slight dark precipitate appears not to affect its function

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
· Ethanol treat all working areas for sterility.
· Inoculate 250 ml of SOB medium with 1 ml vial of seed stock and grow at 20°C to an OD600nm of 0.3. Use the "cell culture" function on the Nanodrop to determine OD value. OD value = 600nm Abs reading x 10
➔ 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
· Fill an ice bucket halfway with ice. Use the ice to pre-chill as many flat bottom centrifuge bottles as needed.
· Transfer the culture to the flat bottom centrifuge tubes. Weigh and balance the tubes using a scale
➔ Try to get the weights as close as possible, within 1 gram.
· 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 · Decant supernatant into waste receptacle, bleach before pouring down the drain.
· Gently resuspend in 80 ml of ice cold CCMB80 buffer
➔ Pro tip: add 40ml first to resuspend the cells. When cells are in suspension, add another 40ml CCMB80 buffer for a total of 80ml
➔ Pipet buffer against the wall of the centrifuge bottle to resuspend cells. Do not pipet directly into cell pellet!
➔ After pipetting, there will still be some residual cells stuck to the bottom. Swirl the bottles gently to resuspend these remaining cells
· Incubate on ice for 20 minutes
· Centrifuge again at 3000G at 4°C. Decant supernatant into waste receptacle, and bleach before pouring down the drain.
· Resuspend cell pellet in 10 ml of ice cold CCMB80 buffer.
➔ If using multiple flat bottom centrifuge bottles, combine the cells post-resuspension
· Use Nanodrop to measure OD of a mixture of 200 μl SOC and 50 μl of the resuspended cells
➔ Use a mixture of 200 μl SOC and 50 μl CCMB80 buffer as the blank
· Add chilled CCMB80 to yield a final OD of 1.0-1.5 in this test.
· Incubate on ice for 20 minutes. Prepare for aliquoting
➔ Make labels for aliquots. Use these to label storage microcentrifuge tubes/microtiter plates
➔ Prepare dry ice in a separate ice bucket. Pre-chill tubes/plates on dry ice.
· Aliquot into chilled 2ml microcentrifuge tubes or 50 μl into chilled micro titer plates
· Store at -80°C indefinitely.
➔ Flash freezing does not appear to be necessary
· Test competence (see below)
· 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.