Unreliable blue/white screening

Jarkko Kortesmaa jkortes at paju.oulu.fi
Fri Sep 1 06:46:49 EST 1995

Dear netters,

I have experienced frustrating problems with ordinary blue/white screening of transformed 
bacteria. Last week, it worked fine as usual. Yesterday, all colonies were white, including 
undigested transformation control. I made a streak plate with about 50 colonies, and most of 
them turned blue. I asked around, and this had happened recently to some of my colleagues, too. 
The strange thing is that I (we) used IPTG and X-Gal which both had worked before, and even the 
LB-AMP plates were from the same batch. (Two weeks old) And it worked again the next day. BTW I 
would bet my life on that I didn't forget to add either reagent on the plates.

I can't think any explanation - there were different workers, different solutions, different 
bugs, although the plates were same, they seem to work sometimes and sometimes not.

If you can suggest any explanation, it would be most welcome. And if you have had similar 
experiences it would be relieving to hear of them.

Jarkko Kortesmaa    

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