What exactly does live in beer besides yeast? Not what can, but what does. The pH is quite low, alcohol content high enough (up to 15% in some common styles), and yeast makes it rather inhospitable to anything else. I've heard there may be "possible" things living in lambic styles, but not too many (and lambics aren't very common anyway). Homebrew isn't nearly as sophisticated as the megabreweries, and I'd find it hard to think of anything that could possibly live in even homebrew. Not only is everything diligently sterilized (if you're good at it), but the wort is boiled, which kills of anything. Then, even if something airborne landed in your wort while racking to secondary or something, it wouldn't be able to survive in the aforementioned conditions, especially since the yeast population would've exploded by that point.
Posts moved from here. Obviously I created a sort of lame title, so if there are any other suggestions, let me know... -xvart.