Kanamycin Sulfate question - (Oct/11/2011 )
We have Kanamycin sulfate that we aliquoted into 100mg/mL in eppendorf tubes stored in the -20. When we are ready to make our LB+Kan, we thaw one. How long would you recommend leaving it out before having to discard it?
Never worked much with Kanamycin, but usually if I make aliquots of Antibiotics an freeze them, I make sure that the amount of each aliquot is so small that it does not need to be thawed more than two times before the aliquot is used, and do not use aliquots older than 3 month as the activity of many antibiotics decreases when frozen and after freeze/thawing cycles. I do not want to risk re-doing an experiment just because I was to lazy to prepare fresh antibiotics.
this links to paper on the stability of antibiotics in agar plates ...see here!
it seems that they are quite stable (at least for four weeks at 4°C) ...but i would not mess around with that ...since you never know ...and if you use a degraded stock it will ruin your experiment ...therefore i prefere to throw away unsecured stocks of antibiotics that have been on room temperature for an undefined time span.
We keep our Kn stocks in small aliquots at -20C, thawing when needed, then re-freezing whats left. We can do quite a few freeze thaws before a tube is used up (maybe 10 or so) and it has never had any impact on efficacy for me.