What would happen if I put my living yeast in various places?

If you keep feeding it, the colony will continue to grow. You could just raise your own yeast for your next cider batch. Keep it airtight so it doesn't get any funky flavors from wild yeast, or not and you might get lucky and craft some wonderful sour cider. I wouldn't do anything other than ferment with it, though. Modern yeast strains are pretty job-specific; I doubt you could even make a decent loaf of bread with that particular strain.

If you have persistent athlete's foot please see a podiatrist or a dermatologist. Putting one kind of fungus on top of another is not going to do anything for you. As for gastrointestinal distress: I would imagine it depends on the amount consumed. A tiny amount may not be noticeable. As for smelly drains: pour drano down the drain, let it sit for fifteen minutes. Then flush with hot water. Then pour bleach or vinegar down the drain.

The pH of the stomach is 1.5 to 3.5, and it looks like S. bayanus would grow fine in the higher end of the range - and your core temperature is a little higher than it likes but not unbearable, so I think if you eat it with sugar or starch, you'd wish you hadn't. I doubt that it would be totally happy on the skin of your feet because it would be drier than it likes and it doesn't have the right proteases to punch its hyphae into your skin and chow down the way dermatophytic fungi can. I cannot imagine why you would want to replace a bacterial clog in your drain with a fungal one, but I don't think it would work, because cold running water isn't a friendly environment for yeast either. I agree that it's not a good idea to pour your yeast into an open wound. It is unlikely that it would do much of anything, but it could result in further local inflammatory symptoms, or even a fungal infection of the bloodstream, which is not helpful.

Cider yeast, especially in the form of post-brew lees, can be used for baking. You'll get an interesting flavour without the bitterness that beer (with hops) will give. You'll have to wing it with quantities but it sounds like you're up for the experimentation!

When you say "left over," do you mean dry and left over in the packet, or the lees after the cider was finished? If it's the former: save that shit and use it for your next batch. Just fold down the top of the packet and keep it in the fridge. If it's the latter: lots of folks just use that as the yeast for the next batch.

I think I've been reading too much http://what-if.xkcd.com/ and was expecting some sort of mass death or something as one of the answers.

It isn't that clear from the link but the effect is you just get drunker and drunker the more carbohyrates you eat. I only learned about it today from the http://nowiknow.com/ newsletter I learned about here on AskMe.

"Does anyone else think about such unusual applications of yeast as me? Have people studied these things, or tried it without studying and lived to record the consequences?"Oh absolutely, I am a microbiologist, I am also a home brewer, but I am not your microbiologist and this is not microbiological advice. Unfortunately, for the most part, brewers yeast is not all that exciting for anything but brewing or feeding animals you want to get bigger like farmed pigs or shrimp. "My first thought was to put it in my stomach. I was able to find a page that indicated eating live yeast would be extremely uncomfortable--abdominal pain, gaseousness, and alcohol poisoning." To start with, spent yeast doesn't generally really taste that great, except for some strains in some contexts, but yours will taste mostly like the cheapness of cheap champagne mixed with the smell of apple orchards after the fallen apples have all rotted. Its the kind of thing that drives pigs to a wild ravenous bliss that is a sight to behold, but not really appealing to people. Yeast adapted to brewing are sitting ducks in the entropic death match of the human gut and if you eat enough of it you'll unbalance your microbiota by selecting strongly the gut microbes that rip yeast apart most efficiently, this won't hurt you but it'll be really unpleasant. If you're familiar with the 'beer shits' this is what you'd be giving yourself. If you get both really dedicated to it and very unlucky you could spark the auto-brewery syndrome mentioned upthread by just overwhelming your microbiota with enough yeast to just replace it. It might sound cool, but drunkeness really is just flu-like symptoms and a lot of cultural conditioning, and being smashed before breakfast would not be fun at all."Next I started thinking about my persistent athletes foot, which I can never seem to get rid of. What if I gave it some competition? What would happen if I basted my feet with my sticky sugary syrup of living yeast? Would it crowd out and kill the athletes foot yeast? Would the EC-1118 have even worse symptoms than the athletes foot?" Your champagne yeast stands no chance, this would be like sending in a pound of butter to do battle with Paula Deen. The tools that your yeast uses to protect itself in your carboy, like making enough alcohol to poison everything else, are barely relevant there and would not be relevant at all to your foot. The best that could happen is nothing, "Next thought: sometimes my kitchen sink and bathroom drain smell bad and it seems like there is some really gross bacteria living in there in a sort of permanent colony. What would happen if I nourished my yeast colony until I had enough to really fill my entire drain with it? Would it take over the area and get rid of all the weird stuff growing down there?" You'd be unlikely to succeed for very long with this for all the same reasons, but you also really really wouldn't want to, all you could hope to do is add the aroma of cheap stale champagne to the smells you already have. If you dispose of your yeast down a kitchen drain I'd recommend chasing it with a bunch of water and then a strong base like bleach or drano. Trust me, this is not something you want."Perhaps the most insane thought: I have what is basically a permanent open sore caused by inflammation caused by itching caused by inflammation (etc). Part of me wonders if at the end of the chicken/egg cycle of inflammation and itching is some bacterial cause. What would dumping live yeast into an open wound that is constantly seeping plasma do? (PS, yes, I have a medical appointment regarding this issue and no, I'm not actually going to put yeast in my wound. But still. What would happen.)"If your wound were bad enough that brewers years could survive then you would have much much bigger problems than askme could address. Both your immune system and the resident microbiota on your skin, which are each profoundly well armed, would rip your domesticated yeast to shreds. They already do much nastier things to foreign invaders than just poop alcohol, like make much nastier chemicals in more targeted ways along with terrifyingly efficient antimicrobial peptides, and your skin is already adapted to those things while alcohol would hurt you more than anything infecting your wound. What it would more likely do is alarm your immune system into creating more inflammation that would make you itch more, not a good plan.Are there any actual household uses for my yeast, besides the usual food purposes (alcohol/dough/etc)?You have never seen a purer joy than what you can give a pig with your waste yeast, but for the most part, not really.

Awesome answer, Blasdelb! Thanks for your microbiological expertise!