LB Amp Agar Shelf life - LB Plate Shelf Life (Sep/13/2005 )
How long can you keep LB Plates with antibiotic at 4C before they are bad to use?
depends upon the amount and type of antibiotic.
Adding 100 ug/mL Amp I have used and seen selection after 2-3 months. Usually after that I just re-spread with an additonal 50 ug/mL Amp before using. As long as the plates themselves haven't dried out, you can just spread more (freshly made) antibiotic on and still use them. I usually assume 30mL volume per 100mm plate. Only possible problem is the concentration being too high, and this is why I only use 50 ug/mL to spread on plates that already contained amp. You don't want to get so much that even resistant colonies are affected...
I also usually add a higher concentration of amp to plates 150 ug/ml. The plates last longer and reduces the formation fo satellite colonies. They last about 3-4 months.