when using E. coli BL21 pLys, grown in Terrific Broth, for toxic/non-toxic protein expression I normally induce protein expression at ~0.5 OD using IPTG, and the resulting induced cultures double ~2-3 times and then stop growing during the ~3 hr induction period. But Terrific Broth with 10-20 mM MgCl2/MgSO4 and good aeration has been reported to support cell growth up to 18 OD. Has anyone tried inducing protein expression with IPTG at higher ODs, like 5 such that the final culture is around 15 OD when harvested? Maybe the higher OD is only good for plasmid production?
UPDATE, for pET T7 expression plasmids and BL21 pLys cells, inducing at OD 1-2 for 3 hr results in a final OD of 14!, unfortunately the protein expression was not nearly as good (on a per cell basis) as when inducing at OD 0.5 and harvesting 2hr later with an OD of 3.5.