FWIW, finished beer--stuff you drink in bottles, kegs, etc. still has active yeast. CO2 is infused into the already fermented, uncarbonated beer (which is called wort) to carbonate it. It only takes a day for the CO2 to be infused enough that when you pour the beer, you get a "head".
If a beer is bottle conditioned, it means that instead of infusing CO2 into the wort, the fermented beer is bottled or kegged after a little bit of new sugars (corn starch or preferably malt extract) is added to the wort. Once it is bottled, the still active yeast will consume the new sugars in the closed container and produce carbonation (CO2). The reason homebrews can explode is that if the first fermentation isn't completed properly, there'll be too much sugar of which the yeast will continue to consume, producing too much carbonation such that the bottle explodes. Been there, done that!
So, the fact is, that all beer still has some amount of live yeast. It will feed off any sugars. There are also all kinds of yeast strains in the air--everywhere. That's why you buy specific types of yeast for the beer you want.If you just let any old yeast "eat" the sugars, you'd get some skunky beer.
Since I'm rambling, that's why it's so difficult to produce Belgian beers and lambic, because often, the yeast which gives a Belgian its distinctive taste(s)are actually found locally in the atmosphere in some areas of the country and even specific to some breweries in the country. Some use "open" fermentation so they don't even add any yeast. They simply allow the yeast in the air to do their job. They don't even clean the brewery walls or ceilings because it would change the environment the yeast have lived in for centuries.
http://www.thecausejams.com
let your life proceed by its own design....
way off topic 
