[lisarea]

Flour and water isn't really a hospitable environment for much of anything except yeasts, though, especially after it's gotten started and is developing alcohols. Other ferments can be tricky like that and attract various cooties, but I've never even heard of that happening with a flour and water leaven.

If you do have a little commercial yeast, though, you can use the poolish or biga techniques to stretch it waaay out. You just take a little tiny bit of yeast and a lot of flour and water and then leave it long enough for it to propagate. It takes a lot longer so you have to plan ahead like you do with most sourdoughs, but it might be worth it in case the yeast shortage lasts a while. Commercial yeasts do come out different from wild ones, so if you really like the lighter type crumb you get with commercial yeast, that might be a solution.