Could someone please offer some general advice on the problem of elements precipitating out of solution when autoclaved?
I've noticed this happens with certain solutions and microbial growth medias I am preparing such as M63 minimal medium and Zarrouk's medium for cyanobacteria.
Some protocols will specify things like "prepare the XXX separately and sterilise by 0.22um filtration to avoid precipitation of XXX and then add this to your autoclaved XXX", however, not all do. For the latter circumstances I would very much like to know how to best assess the liklihood of precipitates forming so i can reduce it, or at least some rules of thumb i might be able to go by.
Many thanks
Dear Dominik,
We often have this problem, especially when we are adding CaCl2 and MgSO4 to media, and if you add them in the wrong order (e.g. before adding protease peptone) you can get precipitation occurring. In general, this probably does not matter, so long as you mix up your media well enough before aliquoting for cultures, though sometimes precipitates will spoil agar plates if you are not careful. We don't worry about precipitates in cultures, and bacterial activity is likely to change pH etc. and re-dissolve things. However, if it is a new medium that you are unfamiliar with, or if it is a particularly sensitive bacteria or assay, I'd follow the protocol as much as possible - because it clearly works!
Regards, Andrew.
Kathryn is right. It is the best option. Elements or compounds that are prone to either precipitation or denaturation, can be separately sterilized by ultrafiltration and then added separately.
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