Antibiotic shelf life?

John Brunstein brunstei at UNIXG.UBC.CA
Sat Mar 11 13:37:26 EST 1995

	For what it's worth, I routinely make up ampicillin in 50% EtOH 
and store it at -20 (putative temperature, we also have a frosting 
problem and presumably resulting higher temperatures).  This does not 
freeze, so the problem of repeated freeze-thaw cycles is neatly avoided 
as well as allowing for the ampicillin to be used when it's wanted 
without having to wait for it to thaw.  I have not done any sort of MIC50 
timecourse to estimate the storage life of these stocks, so I don't have 
hard numbers but what I have found is that for ordinary bacterial work 
(ie, in plates and liquid media) this stock is good for at least one 
year.  As the stock is at 200mg/ml, when I dilute it 1/1000 into the media 
the ammount of EtOH added is inconsequential.  

On Sat, 11 Mar 1995, Curt Ashendel wrote:

> "William C. (Bill) Johnson" <ez003195 at> writes:
> >General question - how long do most antibiotics retain their effectiveness?
> >
> >Specifically - how useful is, say, the four year old tube of ampicillin I 
> >found in the -20 freezer?  I've been told that most antibiotics in an 
> >agar plate are only good for a few weeks, and now I wonder about our stocks.
> >
> The life of aqueous solutions of beta-lactams such as ampicillin is the 
> shortest of all antibiotics and is shortend by higher temperatures and 
> higher pHs. The general guideline of 6 months at -20  works only if:
> A. The stocks are in water or pH7.0 buffer (if alkali is used to dissolve 
>    the free acid form the solution pH can be quite high unless this is 
>    done carefully.) 
> B. The stocks are not thawed and refrozen repeatedly. Repeated freezing is 
>    avoided by freezing in single-use aliquots.   
> C. The freezer is really -20.  Our -20 freezers quickly frost up and 
>    actually run about -6 to -10C  most of the time so I recommend using 
>    -60C or colder freezer.
> However, the time needed to make the proper solution, freeze it in 
> aliquots, and the waste involved in tossing unused portions and the 
> ultracold freezer space required caused me to abandon the approach using 
> frozed stocks of Amp.  Ever since then we weigh out the Amp sodium salt 
> (solid) and add it directly to the sterile medium whenever we need it. This 
> only takes a couple of minutes and it is robust (IT ALWAYS WORKS, even in 
> the hands of "lab hazards" and novices).  We have never encountered 
> contamination from the addition of non-sterile solid Amp. The cons to 
> this approach are that it is essential to keep humidity out of the solid 
> stock bottle and that for those who may be highly allergic to pennicillins, 
> this could be a somewhat dangerous task.
> Curt Ashendel
> Purdue University
> West Lafayette, IN
> ashendel at

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