A bit of interesting information from King Arthur Flour about how an "active baker's" kitchen has enough wild yeast about that bakers can reduce the amount of yeast they add to dough:

Basic flour-water-yeast-salt doughs (which may also contain a bit of oil and/or sugar), such as those for baguettes, focaccia and pizza, are the best candidates for an all-day countertop rise. Keep in mind, however, the vagaries of your own kitchen. If you bake bread all the time, your kitchen is full of wild yeast and any dough you make there will rise vigorously. If you seldom bake bread, or are just beginning, your kitchen will be quite “sterile;” your dough won’t be aided by wild yeast, and will rise more slowly than it would in a more “active” kitchen. We’ve found that here in our King Arthur kitchen, where we bake bread every day, we can cut the yeast back to 1/16 teaspoon in a 3-cup-of-flour recipe and get a good overnight rise. In a kitchen where bread is seldom baked, we needed 1/2 teaspoon of yeast to get the same effect. Use your judgment in rating your own kitchen as to “yeast friendliness.”