Team:Amsterdam/Notebook/Protocols/Making Competent Cells

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Preparation of chemically competent cells

Overview

We use a slightly modified version of the chemical transformation protocol used by the [http://partsregistry.org/Help:Protocols/Competent_Cells Registry of Standard Biological Parts].
This protocol is a variant of the [http://www.ncbi.nlm.nih.gov/pubmed/1943786?dopt=Abstract Hanahan protocol] using CCMB80 buffer for DH10B, TOP10 and MachI strains. It builds on Example 2 of the [http://openwetware.org/images/b/bd/Pat6855494.pdf Bloom05 patent] as well. This protocol has been tested on NEB10, TOP10, MachI and [http://openwetware.org/wiki/Talk:TOP10_chemically_competent_cells BL21(DE3)] cells. See [http://openwetware.org/wiki/Bacterial_Transformation OWW Bacterial Transformation page] for a more general discussion of other techniques. The [http://openwetware.org/images/c/c2/Pat6709852.pdf Bloom04] patent describes the use of essentially the same protocol for the Invitrogen Mach 1 cells. The [http://openwetware.org/images/0/0c/Pat6960464.pdf Jesse '464 patent] describes using this buffer for DH5α cells. Unfortunately, we have found the competence of DH5α to be far lower when compared to TOP10.

Materials

• Prechilled, detergent-free, sterile glassware and plasticware. We use dedicated centrifuge tubes that are only used for preparation of competent cells
• Table-top OD600nm spectrophotometer
• SOB
• CCMB80 buffer (see below)

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

Procedure

Prechill plasticware and glassware

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

Preparing seed stocks

• Prepare TOP10 preculture by scraping a -80d°C TOP10 glycerol stock into 3ml of LB medium and shake overnight at 37°C

Preparing competent cells

• Prepare TOP10 preculture by inoculating 3ml of LB medium with a -80°C TOP10 glycerol stock into and shake overnight at 37°C
• Inoculate 250 ml of SOB medium with 3 ml of preculture and grow in a 37°C shaker to an OD600nm of 0.3
  • This takes approximately 2.5 hours.
  • Aim for an OD600 of slightly below 0.3
  • Prewarmed medium can shorten preparation time by up to one hour
• 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
• Note that from this point on the cells, media, bottles and glassware will have to be kept on ice at all times!! If possible it is recommended to work in a cold room.
• Gently resuspend in 80 ml of ice cold CCMB80 buffer
  • This is is often 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. Usually this additional step is not necessary.
• Incubate on ice for 20 minutes
• Aliquot 50 μl samples to sterile, prechilled, 1.5 ml or 2 ml eppendorf tubes and flash freeze by immersion in liquid nitrogen.
• Store at -80°C indefinitely.
• Test competence (see below)

Measurement of competence

• Transform 50 μl of cells with 10 pg of standard pUC19
  • This is 1 μl of standard pUC19 Invitrogen plasmid
• Hold on ice for 30 minutes
• Heat shock 60 sec at 42°C
• Add 200 μl SOC
• shake at 37°C for 2 hours in 14 ml aerated tubes
• Plate 20 μl and 200 on AMP plates using a sterile plate spreader
  • Transformation efficiency is number of colonies / (ng of DNA x partition of total volume plated) x 1000
  • Competence should be at least 5x10⁶. The higher the better.

References

1. freely adapted from [http://partsregistry.org/Help:Protocols/Competent_Cells the standard chemical transformation protocol used by the Registry of Standard Biological Parts]

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