Top10 and DH10B need IPTG for induction? - (Jun/16/2014 )

Anyone have ideas that genes driven by lac promoter in Top10 and DH10B needs IPTG to induce?

Neither Top10 nor DH10B require IPTG for induction of genes driven by the LacZ-promoter.  DH10B and Top10 are very closely related (if not almost identical) and lack the Lac inhibitor.  As such, IPTG (which binds to and inactivates the inhibitor) is not necessary.  When our lab makes electrocompetent Top10s, we test efficiency with pUC19 and simply plate on LB+Amp+X-gal and no IPTG and we always get good LacZ expression.