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Various people in our lab will prepare a liter or so of LB, add kanamycin to 25-37 mg/L for selection, and store it at 4 °C for minipreps or other small cultures (where dosing straight LB with a 1000X stock is troublesome). Some think using it after more than a week is dubious, but we routinely use kan plates that are 1-2 months old with no ill effect.

How long can LB with antibiotic such as kanamycin, chloramphenicol, or ampicillin be stored at 4 °C and maintain selection?

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This Bitesize Bio article is very informative on this issue. The 1970 study they cite found negligible reduction in the the efficacy of non-beta-lactam antibiotics (kanamycin, chloramphenicol) over spans of 4 weeks or 60 days. Ampicillin loses about 10% activity at 4 weeks.

In my lab, we only take care not to use LB/Amp plates that are older then 4 weeks, for LB/Km, we store them at 4 °C for more then a month, sealed in a plastic bag.

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As far as I know, that is fine. Thats the temperature that Amp is regularly stored at. I always do a negative control anyways to ensure that my antibiotics are still lethal. Just don't stick it in the microwave to make plates

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  • $\begingroup$ Ampicillin degrades by hydrolysis (albeit very slowly if not catalyzed), difficult when in dry, much easier when in solution. $\endgroup$ – Nick T Dec 15 '11 at 23:36
  • $\begingroup$ "That's the temperature that Amp is regularly stored at" - and in fact, Amp does lose activity (a few % per month?) as a consequence. $\endgroup$ – Superbest Oct 25 '14 at 4:55
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I don't know about solutions, but we keep LB-Amp/Carb/Gm plates in 4°C for two-three weeks at a time, and the antibiotics seem to work.

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