The clatter of keyboards. The hum of air conditioning. The smells of coffee and printer ink. The sound of someone, somewhere, saying, "I'll ping you with the details."

They're fixtures of almost every office in the world, and you'd be forgiven for thinking that all offices are bland, sterile and essentially interchangeable. But you'd also be wrong.

Built environments, including our places of work, are actually teeming with microscopic life — bacteria, fungi, not to mention scraps of skin and other material from human inhabitants — that have the potential to affect human health. But we know surprisingly little about the invisible communities that share our office space.

So Greg Caporaso, a microbiologist at Northern Arizona University, decided it was time to take a census.

“It became clear to me that there was a lot of interesting work being done to understand how bacteria or fungi might impact the health of building inhabitants, and how very little was known about what microbes live in the ground, our offices, our cars, hospitals and homes,” he told the American Society for Microbiology blog.

In a study published this week in the microbiology journal mSystems, Caporaso and his colleagues carefully cataloged the microbiomes of offices in three cities across the continent: Flagstaff, Ariz., San Diego, Calif., and Toronto (chosen for their varied climates). They found that each city's microbial inhabitants were as distinctive as their human ones — so much so that they could tell what city a sample was taken from just by looking at its contents.

In fact, where you work seems to be the most important determinant of what tiny organisms surround you. The microbiologists report that community composition didn't seem to change based on the building materials tested or environmental factors like light exposure and humidity. But the spot that a swab was taken from strongly influenced what it contained — floors tended to have the, ahem, "richest" microbial communities (perhaps a good reason to sit in your swivel chair and not on the floor).

The microbiome also changes from office to office, but the variation between cities was by far the most profound effect, suggesting that geography is more important than any other factor — office size, ventilation system, usage patterns, etc. — influencing our tiny, invisible neighbors.

"This is a benchmark study," a senior editor on the paper, Jack Gilbert, a microbial ecologist and professor of surgery at the University of Chicago, told NPR. "Previous studies haven't really controlled for a lot of those variables, so most of the results have been assumptions. This is really a wonderful paper because it's clearing up many of those assumptions."

The way Gilbert and Caporaso describe it, the office microbiome is an intimate reflection of the world around it. People stroll through, shedding skin (human skin bacteria was the source of at least 25 percent of the office's microbial community) and leaving traces of their personalized bacterial cloud on everything they touch. Air wafts through, carrying in traces of the outdoors. Dirt and dust are dropped from muddy shoes and coats.

This isn't gross, or even necessarily bad — the stunning microbial diversity of the environments we inhabit is a sign of their capacity to sustain life. But it does warrant further study, the researchers say. The next step is to test how dramatic environmental events like floods change the microbiome, Caporaso said. But the ultimate goal is to inform construction of spaces that are home to healthier microbiomes — and, hopefully, healthier office workers.

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