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How long Kanamycin can last in broth medium? - (Oct/29/2012 )

I made my Pseudomonas bacteria resistant to kanamycin (kan), However, when I grew the wild type and the recombinant one in LB broth medium containing kanamycin for 48 hours, as expected , nothing grew on my negative control medium (Kan containing medium inoculated with wild type strains) while recombinant bacteria could grew happily. However, on the 3rd day, my negative control media became turbid !!
Anyone has any explanations?
Is it possible that kanamycin had lost its efficiency after 3 days? or my wild type strains could manage to become Kan resistant?
Thank you so much for your replies...

-Morteza Moghadam-

unlike ampicilin, kanamycin doesn't degrade very fast. I have used 9-month old kan plates and they work fine.


Inoculate fresh (unexposed to Kanamycin) mutant and growth of mutant you observed at three days in Kanamycin broth into fresh Kanamycin broth. Use wild type as control as you did before.

-Phil Geis-

Sigma info for Kan sulfate:

Sterile solutions can be prepared by sterile filtration
through a 0.2 μm filter. Solutions are stable at 37 °C
for approximately 5 days. Aqueous stock solutions
can be stored at 2-8 °C for long term storage.

-El Crazy Xabi-