MicrobeWorld

When John Mayer sang “Your Body is a Wonderland,” he probably wasn’t talking about the trillions of microbes that live all over your skin and inside every orifice you have to offer—but it does pretty much describe things. In the last decade or so, scientists have confirmed that we’re just as much an ecosystem as a rainforest is: full of ecological niches inhabited by countless bacteria, many of which have been evolving with us for millions of years. Our tiny passengers aren’t passive, either. Studies in mice and some in humans have linked these microbial populations, or microbiomes, to the host’s digestion, gut health, behavior, and even mood. A healthy microbiome keeps the host’s systems in good working order and prevents invasion by microbes that mean us harm.

But what is a healthy microbiome, exactly? That’s an important question, since diagnosing and treating illnesses related to microbiome imbalance requires some definition of normal. In the first few studies to try to address this question, scientists have found that there are some patterns: one study suggested that there could be three gut microbiome “types,” similar to blood types. Since proposed treatments for microbiome problems include “transfusions” of bacteria from a healthy microbiome (including “fecal transplants“), this is an attractive analogy. But a new study on the human vaginal microbiome suggests that the real story might be much more complicated.

Studies exploring a healthy micobiome often look at a single sample from each person. But it turns out if you sample someone regularly, at least in the case of the vagina, you can watch the entire microbiome change radically—to the point of becoming unrecognizable—in a matter of days.