Benjamin Wolfe sticks his nose into a Ziploc bag and takes a whiff. "Ooh! That's actually kind of nice," he says. Inside the bag is a pungent, beige goop. It's a sourdough starter — a slurry of water, flour, yeasts and bacteria — from which loaves of delicious bread are born. And it's those microbes that have the attention of Wolfe, a microbiologist at Tufts University.