When making competent cells I've been using a medium called "Psi broth", which per liter consists of 20 g tryptone, 5 g yeast extract and 5 g MgSO4. Today I got curious about why it's called "psi". Is it an acronym like SOC and LB?


Psi is the Greek letter ψ. Psi broth is sometimes referred to as ψ-broth.

Here's an example:

L-Broth or ψ-broth can be used instead of SOC medium, but efficiency may be reduced.

  • L-broth: 10 g Bacto tryptone, 5 g Bacto yeast extract, 5 g NaCl, pH to 7.5 with 1 M NaOH, bring to a final volume of 1 L, and autoclave.
  • ψ b-broth: 5 g Bacto yeast extract, 20 g Bacto tryptone, 5 g MaSO4・7H2O, pH to 7.5 with 1M KOH, bring to a final volume of 1 L and autoclave.
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    $\begingroup$ This only partly answers the question. The key thing which is missing is where the "psi" came from. (e.g. why "psi" and not "zeta" or "epsilon" or even "gimel"?) $\endgroup$ – R.M. Jun 26 '18 at 21:09

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