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Protocols
Media
LB
Ingredients:
10 g Bacto-tryptone
5 g yeast extract
10 g NaCl
SOB
Ingredients
0.5% (w/v) yeast extract
2% (w/v) tryptone
10 mM NaCl
2.5 mM KCl
20 mM MgSO4
Per liter
5 g yeast extract
20 g tryptone
0.584 g NaCl
0.186 g KCl
2.4 g MgSO4
SOC
Ingredients
SOB
20 mM glucose
CCMB80 buffer for preparation of chemically competent cells
Materials
Detergent-free, sterile glassware and plasticware
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
Filter sterilize and store at 4°C
Slight dark precipitate appears not to affect its function
Procedures
Preparation of chemically competent cells
Preparing glassware and media
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 works well. Pick single colonies into 2 ml of SOB medium and shake overnight at 23°C. Add glycerol to 15% and aliquot 1 ml samples into cryotubes. Place tubes into a zip lock bag, immerse bag into a dry ice/ethanol bath for 5 minutes. 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.
Preparing competent cells
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.Gently resuspend in 80 ml of ice cold CCMB80 buffer. 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
Test competence (see below)
Do not thaw and refreeze, as this dramatically decreases the transformation efficiency.
Transform 50 μl of cells with 1 μl of standard pUC19 plasmid. Use a 10 pg/μl stock. Hold on ice 0.5 hours. Heat shock 60 sec at 42C and add 250 μl SOC.
Incubate at 37 C for 1 hour in 2 ml centrifuge tubes under rotation.
To increase yields, an incubation time of 2 hours can be used. Plate 20 μl on AMP plates. Good cells should yield around 100 - 400 colonies. Transformation efficiency is (dilution factor=15) x colony count x 105/µg DNA. The transformation efficiency should be between 5x10^8 and 5x10^9 cfu/µg DNA.
Transformation of chemically competent cells
1. Thaw 25 - 200 μL chemically competent cells on ice. Do not use glass tubes, which adsorb DNA.
2. Add DNA (1-2 μL), pipette gently to mix
3. Incubate on ice for 30 minutes
4. Incubate cells for 30 seconds at 42°C.
5. Incubate cells on ice for 2 min.
6. Add 4 volumes of room temperature SOC (not critical)
7. Incubate for 1 hour at 37°C on shaker.
8. Spread 100-300 μl onto a plate made with appropriate antibiotic.
9. Grow overnight at 37°C.
