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MacConkeys instead of Xgal/IPTG selection?

Greg Denomme denomme at FHS.CSU.MCMASTER.CA
Thu Aug 26 02:56:05 EST 1993

On 26 Aug 1993, Robert Rumpf wrote:

> I've heard that the alpha-complementation-based color selection using 
> lacZ-phagemids, lacZ deletion host cells, and Xgal/IPTG plates can also be done
> using the same phagemids/host cells but plated onto MacConkey's agar.  Is this 
> true, how is it done, and what's the mechanism for color selection?  I 
> appreciate any help you can give...

  MacConkey plates are commonly used in Microbiology for plating out stool
samples, E. coli grow well because of a few goodies like bile salts and
some sodium chloride.  MacConkey agar also contains lactose and methyl red.
  If your E coli strain has a phagemid with a proper construct for
lactose utilization then lactose is consumed, anaerobic glycolysis
produces pyruvate and lactate, the colony is acidic and therefore turns
red on MacConkey agar.  If the phagemid has a cloned gene that interupts the
lac gene then lactose is not utilized and is less acidic and appears white.

I use MacConkey agar routinely and is available ready-to-use from Difco
(dehydrated).  The only problems are that 1) the colonies grow a little
slower so you may need to incubate the plates longer than with LB agar and
2) the plates are a slight pink so the red colonies are hard to see when
they are very small.  BTW you may see colonies that are white with
red centres as has been discussed just recently with X-gal selection (white
with blue centres).  For about $50.00 you can make 9 liters of MacConkey agar.

Hope this is sufficient for your needs,
denomme at FHS.McMaster.ca

I have no affiliation with Difco Laboratories whatsoever.

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