Oh, COLO,
I don't know why I didn't think of this earlier -- there is a way to get to a pure culture without solid media if you have a lot of sterile containers. It is called limiting dilutions. The idea is to dilute a culture enough times that only single cells remain.
You will need a bunch of tubes or containers about the size of the tubes you would use to make slants. And you will need something to move your culture/fresh wort around. A pipette with sterile tips is perfect. A bunch of sanitized metal spoons will work too.
The procedure is to take a drop of the most recent mixed wild culture and dilute it into 9 drops of fresh wort. If you're using a pipette, dilute 100 micro Liters into 1 mL. Either way, you have diluted the culture 1 to 10.
Now repeat this 10-fold dilution several times, each time taking a drop of the diluted wort from the previous step and diluting it further into 9 drops of fresh wort. Use a fresh pipette tip / sanitized spoon for each dilution.
Let's say, for example, you started with 1000 cells in your tube. After one dilution, you have 100 cells per tube. After two dilutions, you have 10 per tube. After three dilutions you have just one cell per tube. After four dilutions, you'd have -- either one cell or none at all. This is the level of dilution you want to get to, where most of the tubes don't contain any cells. At this level of dilution, you know that any tube that DOES grow, started from just one cell. This will get you to a pure culture!
But how many dilutions are requried? You're starting with more than 1000 cells, so it will take more than four dilutions, but not too many more. A White Labs tube is extremely dense with yeast, and it is about a billion cells per mL. A billion is 10^9. So, if you do 9, or let's be safe and say 10 of these 10-fold dilutions in a series on your culture, you should already be in the zone.
Let's say you find that all the dilutions starting with the sixth don't grow anything. Now, go back to the undiluted culture and do the 10-fold dilution six times, but make a bunch of tubes of that sixth dilution. Make 20 or so. You already know that most of them will not grow. But a few will grow, and you will know they started with
just one cell.
Cells have a tough time growing by themselves, so make sure you keep the volume small, 1 mL at the most, aerate every day, and give the cultures plenty of time to grow. It might take a few weeks of aerating every day before the most extreme dilutions show signs of growth.
This is a lot of work, and I have only read about it, not tried it. But limiting dilution it is a standard microbiology technique and it will work! You may prefer to just buy some sterile plastic plates and agar, though.