Germ free: Microbiologist Jeffrey Gordon reaches into a sterile cage to display one of his germ-free mice. The lack of normal microbes has adversely affected the mice’s development. Credit: David Torrence

Anyone who's ever visited a research lab that studies mice knows how the animals stink. But the mice housed in rows of large plastic bubbles in Jeffrey Gordon's lab at the Washington University School of Medicine smell surprisingly pleasant. They've spent their entire lives in a sterile, protected environment, inhaling purified air. Because of their meticulous upbringing, they harbor none of the microbes that normally give mice their distinctive acrid odor.

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