G418 storage and contamination Options

[HSB]

Does anybody know how long a media with geneticin (G418) last at 4ºC? Is the G418 degradeted if it is stored more than three weeks?
I was selecting stable transfectants with a medium containing G418 and everything was contaminated after three weeks of selection. During this period some plates were also contaminated. How can bacteries grow in plates with media containing antibiotic against them?

[milanoj]

It's safest to not store your antibiotic containing medium at 4C. Add it each time from a frozen stock. If you must one month is too long.

[khadley]

I haven't had problems with storing G418 media for a few weeks at a time at 4 degrees. As long as the G418 and original media solutions are sterile to begin with, you shouldn't have problems. Contamination points to a handling problem at some stage in the preparation or use of the media, rather than a problem caused by the G418 itself.

One thing to be wary of with G418 is that it is easily photodegraded. Extended exposure to light may generate toxic degradation products. I try to work with it under dimmed lights, and store G418 and G418 media at 4 degrees inside an opaque container. As another poster has noted, using frozen aliquots of G418 will definitely avoid any concerns about degradation.

Contamination is still quite possible with antibiotic-containing media. The resistance genes you're splicing in mostly came from the wild, after all. Also, even if antibiotics reduce the appearance of overt contamination, more subtle species can slip in under the radar. Contaminants like mycoplasma are highly resistant to most antibiotics, and difficult to detect except through deliberate screening.