Preparing chemically competent cells (Inoue)

Materials

• Plate of cells streaked for single colonies
• SOB
• Ice
• TB buffer
• DMSO
• Dry Ice (or liquid nitrogen)

Glassware & equipment

• 2 liter erlenmeyer flask (no detergent residue, rinse with 70% ethanol and DI water)
• 220 ml conical centrifuge tubes BD 35 2075
• Eppendorf 5410R refrigerated centrifuge with conical adapters

Preparation

1. Pick a 10 - 12 large single colonies (2-3 mm dia) from your source plate and inoculate 500 ml of sterile SOB medium (do not use LB) in a 2 liter flask. Save some medium as an OD blank.
2. Grow to an OD of 0.6 with vigorous shaking (200-250 rpm) at 18 degrees (important). This is slow -- approximately 35-40 hours.
3. Prechill the centrifuge to 4 degrees
4. Remove from the incubator and place on ice for 10 minutes
5. Transfer to two 220 ml centrifuge tubes and spin at 3220 x g for 10 minutes at 4 degrees
6. Drain the medium and resuspend each pellet first in 5 ml of ice cold TB. Add an additional 75 ml of cold TB buffer and resuspend.
7. Place on ice for 10 minutes
8. Spin down as above.
9. While spinning, add 1.4 ml of DMSO to 18.6 ml of TB (7% DMSO mixture)
10. Resuspend each pellet in 20 ml of cold TB-DMSO mixture
11. Incubate on ice for 10 minutes
12. Dispense cells into pre-chilled tubes
13. Freeze cells in a dry ice / ethanol bath and store at -80 degrees indefinitely

Related topics & references

• Preparing chemically competent cells
• Preparing TSS buffer
• Transforming chemically competent cells
• Preparing electrocompetent cells
• Electroporation
• TB buffer
• Transforming chemically competent cells (Inoue)
• Bacterial cell culture

Original protocol from Inoue et al. [1].

1. Inoue H, Nojima H, and Okayama H. High efficiency transformation of Escherichia coli with plasmids. Gene. 1990 Nov 30;96(1):23-8. DOI:10.1016/0378-1119(90)90336-p |
2. Hengen PN. Methods and reagents. preparing ultra-competent Escherichia coli. Trends Biochem Sci. 1996 Feb;21(2):75-6.