Ye Olde Antibiotic Plates
You are working late in the lab and you need to do a transformation, but sod it, you don’t have any antibiotic plates on hand. So you go on the hunt to see if there are any secret stashes anywhere in the lab (you can find secret stashes in every lab if you look hard enough).
And lo and behold, come across the ampicillin plates you poured 4 weeks ago then forgot about. But they are old, so how do you know if they will give adequate selection? Should you use them?
Luckily, someone way back in 1970 determined the lifespan of a whole host of antibiotics in plates, and the good news is that they last a lot longer than you probably thought.
Plates containing methicillin, erythromycin, cephalothin, tetracycline, chloramphenicol, kanamycin, streptomycin, polymyxin B, or nalidixic acid should be a-ok even if stored for one month at 4degC.
Ampicillin was shown to have no loss of activity after 1 week but 10% lower activity after 4 weeks at 4°C, which could result in things like satellite colonies, but should be fine for practical purposes. Other antibiotics with reduced activity after 4 weeks included Penicillin G (23% reduced) and nitrofurantoin (17% reduced).
So even ampicillin plates should be fine for 4 weeks - certainly longer than I would have thought - meaning that your late night experiment can be saved by using your stash of old plates after all.
molecular biology
Liam, _only_ 1 month? Gah! Our branch maintains a cold room where a core facility makes loads of plates. As in… crates of them. You put an order in, and voila, they appear in the cold room a few days later. Now, its my job to keep the cold room clean (i was volunteered for the duty by my PI…) and we have DOZENS of crates of plates that are 1 year old or older. I want to throw them all out, but often times the labs who “own them” usually say something “there fine! just leave them alone!” and then go muttering about how all their clones were negative. Now I have a reference I can use to make my point (and sound like an ‘expert’): “The Sherris lab showed that plates this old blow! I’m tossing them!” =D
What about Gentamycin plates? I used them for Gateway cloning with pDONR 207
Jonathan
I think you can stretch it to 2 months, but beyond that, you’re going to get negative clones. Luckily most labs make their own plates, so smaller batches means less money wasted.
Ariel
I have no idea, but I will keep looking, there does not seem to be much done on the bioactivity of gentamicin over time, will keep you up to date.
Can’t you simply spread 20 ul of ampicillin on a older plate and still use it, even after 2-3 months? I don’t see a reason why this wouldn’t work…
I suppose one could do it, but then how much time are you saving especially if you don’t spread it evenly. Sometimes the corner you cut is one corner too many. Besides which, old plates are like the Sahara, you’re not going to be spreading much on them, and if you’re relying on diffusion of the antibiotic, well then, you go right ahead.