Antibiotic shelf life?
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 peseta.ucdavis.edu> 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 aclcb.purdue.edu
More information about the Methods