growing the bacteria(LB broth) for protein expression: the optical density(1.2) , is this good for protein expression(before IPTG).
Sorry, I meant 0.6 to 0.9 for LB.
What culture method are you using? I think that will influence what OD you want as well.
We do 1 liter in large flasks. Have you tried induction yet?
I am just in the trial step that I am growing in 50 ml LB broth and no I have not injected the IPTG yet
I got best results with starting induction at OD 0.4 - 0.6. then adding from 0.1 to 1.0 mM of IPTG.
You can strat with OD 0.5 and 0.4 mM IPTG and check your protein after 4 hours.
I also have good results when growing bacterial cells to O.D 0.4-0.6 then start induction with IPTG.
I think the 1.2 OD is still fine, as long as they are in the Log phase of their growth. As a trial experiment, you can see where the maximum OD for your bacterial growth is with respect to your Media and get an idea of when you have to induce your IPTG. As others suggested, it is usually between 0.6-0.9. But I have had some cultures where I induced at a higher OD, typically 1.0-1.3. It really depends on your media and cell line. But, you should still be fine, just make sure you continuously monitor the progress of your cells growing before and after the addition of IPTG. All the best!
As people are suggesting here, I think 0.6-0.8 is good for inducing protein expression. You don't want a low cell density which may be toxic by IPTG. You definitely want the cells grow in the fastest condition to produce more protein, mid-log phase can meet all the requirements. I agree that some cells may grow okay at OD over 1.0 but it could reach to late-log phase quickly and you cannot get more protein production. I usually do small scale pilot experiments in IPTG-dose dependent or induction-time dependent manner. Some of my proteins can be induced best at 0.1mM IPTG, some prefer 0.5. Good luck!
I definitely agree with Truc here, you want to find the right balance between the number of cells and the amount of protein yield you get at the end after IPTG induction. You can sometimes get a higher protein yield when induced at a slightly lower OD. It all just requires some optimisation.
Generally it would be best to induce with IPTG before your culture was at 1.2. At that cell density the culture is probably already entering stationary phase. Most protocols recommend inducing at OD600 of 0.3-0.6 range.
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