Can I autoclave inorganic salt solutions? - (Sep/03/2015 )

I came across a formula for making 2xSG agar for Bacillus subtilis PY79 sporulation. But there's a sentence in the formula I'm not sure about:

 

This medium is a modification of Schaeffer's sporulation agar containing glucose, which can be used as a solid medium or, by omission of agar, as a liquid medium (Leighton & Doi, 1971). 2x SG is a richer medium, and gives higher cell densities than Schaeffer's medium. 2x SG agar consists of the following:

 

Per litre:

Difco nutrient broth    16.0 g

KCl                               2.0 g

MgS04-7H20               0.5 g

agar                           17.0 g

 

Adjust the pH to 7.0, autoclave, then add the following sterile component solutions to one litre of the cooled (55 °C) medium:

1 M Ca(NO3)2           1.0 ml

0.1 M MnCl2-4H2O    1.0 ml

1 mM FeSO4              1.0 ml

50% (w/v) glucose      2.0 ml

Which is the better way for sterilizing these four solutions in the case of sporulation: autoclave or 0.22 um filter? I've read somewhere that inorganic salts are heat stable and can be autoclaved, so can I assume I could autoclave calcium nitrate, manganese chloride, and ferrous sulfate? I am planning to autoclave all four of these solutions separately (if autoclave is the better option).

you might think that inorganic salts are heat stable but not all are. nitrates and sulfates might not be as stable as you think. you are adding (heat) energy to the system and that may alter the solution.

if you are asked to add the solutions to a cooled solution then it would be better to filter sterilize.

Do as they indicate. Changing the recipe preparation may result in not-so-replicable results.

It's likely that they will react with some other components. The iron can easily be hydrolysed and ending as a mix of oxihydroxides, Mn can be oxidised --> precipitate, Ca will react forming insoluble sulfates, and glucose is generally recommended to be autoclaved on its own.

The 55 °C thing is

• to limit the mix at high temp
• to keep agar liquid

Be careful autoclaving such high concentration of glucose. It'll be very viscous so leave a large empty space in the bottle. You don't want to have that syrup overboiling inside the autoclave ;)