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Have you ever add IPTG to TOP10 cell (Invitrogen) - competent cell, IPTG (Aug/04/2005 )

Is there anybody using TOP10 cell (Invitrogen) for white and blue screening using X-gal?

According to company guide, IPTG is not necessary to be added to plate because TOP10 competent cell has not lacI, thus, it does not need inducer for expression of LacZ.

However, It was really hard to differentiate blue and white colony after overnight culture. They were a litte faint blue and white.
If anybody has exprience with TOP10 cell for white/blue screening, please give me some advice.


Yes you shouldn't need to add IPTG since the TOP10 cells don't need an inducer. I've used them plenty of time without adding IPTG. Sometimes it takes a while for the blue colour to develop though. Putting the plates in the fridge for a few hours usually helps darken the blue colour.