MacConkey instead of XGAL ?

Michael Benedik benedik at
Mon Mar 31 11:09:40 EST 1997

In article <gshearer-2803971614000001 at>
gshearer at (Glen Shearer) writes:

> Since MacConkey plates can distinguish lac+ from lac- colonies,
> why not use MacConkey + amp plates for blue/white (in this case red/white)
> selection?
> Anybody try this?
> Glen

Because Xgal is much more sensitive than MacConkey. Blue white
screening with 
plasmids/M13 relies on alpha complementation, and the resultant enzyme
activity is not very high. Depending upon the plasmid/strain pair
macConkey won't always
work (in fact I believe it usually doesn't). 

Also, you can just spread Xgal on plates you already have prepared.

Michael Benedik
Department of Biochemical Sciences
University of Houston
benedik at

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