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[Microbiology] Re: media preparations

Larry Farrell via microbio%40net.bio.net (by farrlarr from isu.edu)
Tue Jan 8 12:25:56 EST 2008

deepgolwala from gmail.com wrote:
> i am new to the field ,
> i would like to ask while making LB agar(tryptone-10 gms,yeast
> extract-5 gms,NaCl-10 gms-all weight for 1000ml preparations)when
> should i add agar during preparations or after autoclaving.

Agar will survive autoclaving without any problems, and the heat 
involved will melt the agar so it can be easily distributed throughout 
the medium by swirling the container after it is removed from the 
autoclave and cooled for 10-15 minutes.  As the medium cools 
sufficiently for pouring of plates, most/all of the bubbles introduced 
by the mixing will spontaneously pop so there will be few if any left 
when the plates are poured.  If you wait until the agar is cool enough 
to pour to mix, all of those bubbles will be left and you will have to 
deal with them by flaming the surfaces of virtually all the plates you 

A question that you might want to consider is how you would prepare 
sterile agar for addition to medium that has already been autoclaved. 
  There really isn't any convenient way to do that, to my knowledge. 
Even if there is such a method, the agar solution would have to be 
heated sufficiently to melt the agar, which pretty much boils down, 
again, to autoclaving.

Larry D. Farrell, Ph.D.
Professor of Microbiology
Idaho State University

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