We may think of bacteria only as something to avoid like the plague. But these single-celled organisms have proven extremely useful to science, medicine, and industry. And with genetic engineering, which enables researchers to transfer traits from one microbe to another or even create entirely new organisms, their usefulness is rapidly mounting. In this slide show, see why you should be thanking these microscopic creatures for everything from the medicines you take to the foods you eat, from the waste you generate to the gold you wear.