Hi. I have a question about agar used for growing bacteria. The package on my agar powder says to use the entire pack for 1 liter of solution, which will fill about 50 petri dishes. But I however only wish to use a couple of petri dishes at a time. Should I use the entire pack and simply store what's leftover, or measure out what I need?

Depends on when you intend to use the rest. The dishes could get contaminated over time or simply dry out, so you cannot store them indefinitely. If you want to measure it out you have to ensure that the powder is well mixed, which is a bit tricky. For routine cultivation that should not be that big of an issue, but if you want to perform quantitative assays, I would rather use everything.

Thanks for the response! What about storing the extra agar in a sterilized mason jar?

Could work for a certain time, though re-heating agar changes its qualities slightly (especially with very cheap agar pouring plates with re-heated medium can be a bit iffy). Again, for routine cultivation it would work for some time, do not expect quantitative reproducibility.

How soluble is agar?

Could you dissolve the packet into 100 ml and then split that into, for example, 10 lots of 10 ml. Since they are portions of a liquid, they should be pretty much identical to one another.

 

Then each time you need some more agar, take a portion and make it up to 100 ml.

You would probably need to freeze the stock solution and that will mess up the gel, but redissolving it into boiling water should reconstitute it. You would need to autoclave it to be sure it's sterile, but presumably, you would be doing that anyway.

Without cooking, agar pretty much ends up as precipitate. One could try to split suspensions (after vigorous stirring) , and with complex medium it tends not to be that critical. I have never tried to freeze medium though, now that I think about it (fridge would not last long for rich media, though).

Thanks again for the responses guys! I don’t intend to do any intensive research, just basic colonies. I probably will use half of the pack, making about half a liter of solution, and go with the mason jar technique.

Check the total gm in the bottle. gm/liter and find out how much you need to use for 100ml

so that you can reverse calculate the required amount needed for each plate.

For general cultivation, it is routine to make a large batch, put it in a sealable bottle, sterilize it, and use as needed. Yes, over time and with subsequent reheating you will change the composition, but its rarely a big deal for routine use.