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Competent Cells Medium Formulations

SOB is originally developed by Hanahan as a medium to prepare competent cells prior to transformation for maximizing transformation efficiency.  Transformation requires making perforations in the bacterium (i.e. making the cells ‘Competent’) to allow the introduction of foreign DNA into the cell.  To survive the process of perforating the cell, competent Escherichia Coli cells need a nutritionally rich and well balanced isotonic medium.

Tryptone and Yeast Extract are sources of nitrogen, vitamins, growth factors and carbohydrates supporting the cell to recover from the stress of transformation and subsequently grow well.

To set a well balanced isotonic environment Sodium Chloride and Potassium Chloride are present in the medium.

Magnesium ions required in a variety of enzymatic reactions including DNA replication are provided by Magnesium Sulfate.

SOC medium as used in the final stage of transformation and is prepared by adding glucose in a final concentration of 20 mM to SOB medium. The glucose added is a readily available carbon and energy source for Escherichia Coli cells in mending the perforations and for replication.

Hanahan, D., Studies on transformation of Escherichia coli with plasmids, J. Mol., Biol., 166, 557, 1983.
Sambrook, J,., E. F. Fritsch, and T. Maniatis, 1989, Molecular cloning: a laboratory manual, 2nd edition ed., Cold Spring Harbour laboratory, Cold Spring Harbour, N.Y.
Difco manual 11th ed., Sparks, MD (1998), 22-23.

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