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Preparing Agar

Postby mssjs » Mon Apr 16, 2018 10:38 am


I made agar following the recipe and procedures from this website: ... ar-plates/

They solidified but then after we swiped the bacteria growth on them and put them aside in a dark place, the agar melted and is now a sloppy mess.

Any ideas on what could have gone wrong?

Thanks in advance!

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Occupation: Teacher

Re: Preparing Agar

Postby rpeteranderl » Tue Apr 24, 2018 6:34 am

The problem is that you didn't use agar but gelatin. Gelatin is a protein, and many bacteria will gladly digest it with enzymes (proteases) that they release into their environment. I think that there might be some strains of bacteria that do not digest gelatin, but most of them do (as do fresh pineapple, kiwi, ginger, etc.). The bacteria that you were trying to grow started to break down the gelatin right away, and their descendants are now floating in that soupy broth.
What makes agar (or, in the purified form, agarose) so special is that there are very few bacteria that can actually digest it. It stays a nice, solid gel, and the nutrients from the open spaces between the chain molecules slowly diffuse to the surface of the plate where bacteria can eat them.
In the USA you can find agar in health food stores, Asian groceries or through Amazon. It is either called Agar powder or Agar Agar. You need about 15 grams per liter (or roughly 1/2 ounce per quart) to get decent plates, but you will also have to include a protein source in the mix now. Maybe chicken stock?

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