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agars

Chris Fields cjfields at unt.edu.removethisbeforereply
Sat Jul 7 17:18:17 EST 2001


Some agar-based media will not dissolve completely or clump unless
stirred well before autoclaving (Pseudomonas Isolation Agar is one that
we had this problem with).  Also, we have found that, in a few cases,
dissolving the media before autoclaving (especially when agar is
included) prevents some darkening of the media, presumably due to
undissolved chemicals in the media mix.  I also have found that M9 media
(E. coli) and a few other minimal salts media need 3% sterilized agar,
MgSO4, and CaCl2 added AFTER the chemical portion is autoclaved,
supposedly due to the phosphate salts in the media reacting with the
carbon source and agar and calcium phosphate precipitation.  I found
that adding the agar in before autoclaving in these cases turns the
media a dark brown to greyish color.

However, I can attest, as a sometimes lazy and usually hurried grad
student in microbiology, that most dry media can be added to water, have
a stir bar added, and autoclaved.  I do this with LB, nutrient, and TSA
without any problems.

And, I believe that Bacto Agar is a more purified agar used in many
types of minimal media.  Others use Noble Agar or agarose (the latter in
a much lower concentration; you can imagine how expensive this media is
to make).

Austin Reade wrote:
> 
> lamb <L.A.M.Buisman at chello.nl> wrote in message
> news:3B44D7EC.5CD98848 at chello.nl...
> 
> > Do you happen to know why is it that according to the handbooks agar
> always has to be
> > dissolved by boiling before autoclaving?
> 
> Is it possible that commercial agars available today are manufactured to be
> more easily dispersed after direct autoclaving than agars available years
> ago?  In the dim and distant past I seem to remember that it was quite
> difficult to disperse the agar component if you autoclaved it without prior
> mixing and heating.  A lump of agar with a gelatinised outer skin and poorly
> hydrated centre would tend to form in the bottom of the flask.  Once this
> experience is embedded in ones brain it then becomes one of the protocols
> noted by Dr. Evdokimov.
> 
> One lab I worked in routinely used a microwave to prepare selective agar
> media which didn't require sterilization.  With some practice one could set
> the time to obtain a boiling solution without boilover.  However the colour
> of the inside of the microwave attested to many failed experiments before
> this happy state was reached.
> 
> Austin Reade
> Reade BioSciences Inc
> Halifax,NS, Canada

-- 
C. J. Fields
Graduate Student, Dept. of Biological Sciences
The University of North Texas
Denton, TX 

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