There might be papers or others out there who already know an answer to your question but this would be an interesting and relatively easy experiment to preform. You could plate out a lawn of control bacteria and then use Whatman filter paper to make antibiotic diffusion disks with different concentrations of amp. Or you could overlay entire plates with the different concentrations and plate out lawns of the control on them and see what concentration is the minimal inhibitory. But when using Amp at lower concentrations you may find an increased issue of non-resistant satellite colonies being allowed to grow because of beta lactamase production from your positive colonies.

Dear Patrick,

in my opinion the best way is to optimize the proper antibiotic concentration (50 ug/ml, 25 ug/ml and less). But even this plasmid is low copy, I think that culture these bacteria in a standard concentration (100 ug/ml) of amp should be safe.

Good luck!

Chris

Yes you can use normal ampicillin concentrations, even with a single copy of the resistance gene on the chromosome it works fine. I routinely use 50ug/ml but 100 is fine.

But the answer to your question regarding concentration is non-trivial, you can go quite low (10ug/ml or so) and you will get selection. But you will also see a lot of satellite colonies arising with just a bit of time. So your plates will be a bit cleaner at higher concentrations. Also ampicillin stock solutions decompose fairly quickly with time so at higher concentrations you can use your stocks and your plates reliably after a few weeks whereas you can't at the lower concentrations.