What is the dirtiest thing on your desk? If you work in a typical office, it's not actually your computer mouse or your keyboard or even your desk. According to a new study, published online May 30 in PLoS ONE, it's your phone—but your chair's not far behind.

Before you drop that receiver or leap out of your seat, hold that thought for just a second. "Dirty" is a messy word. It doesn't necessarily mean that will make you sick. For this study, researchers measured the abundance of bacteria. Plenty of species are dangerous—take the drug-resistant bug MRSA for example, which can be found in healthcare and community settings. But most are relatively neutral, and many are important members of our own internal and external ecosystems, our microbiomes, keeping our metabolic and immune functions running well.

We, however, are just starting to piece together this invisible landscape. "Humans are spending an increasing amount of time indoors, yet we know little about the diversity of bacteria and viruses where we live, work and play," Scott Kelley, of San Diego State University's biology department and co-author of the new study, said in a prepared statement.

For the study, Kelley and his colleagues used genetic sequencing to sample bacteria from five surfaces in 30 offices in three large cities: New York City, San Francisco and Tucson.

They found bacteria from 549 different genera, most of which hailed from the skin, mouth and nose, but some of which also generally live in the digestive tract. The most common major groups included Proteobacteria (which includes the bacteria that cause cholera as well as salmonella and E. coli poisoning), Firmicutes (which includes staph and listeria bugs) and Actinobacteria (which includes some bacteria that have been the source of antibiotics). With deep genetic sequencing, the group also managed to detect some rare specimens, such as those that are more commonly found in hot springs, which could eventually help researchers study how these hardy bacteria spread across the globe.

New York and San Francisco had similar bacterial makeup, despite having vastly different climates. But San Francisco offices had lower concentrations of bacteria than did New York (which had the most) or Tucson offices. (If you've ever ridden the NYC subway alongside so many of the city's commuters, you might have an inkling of why this is.) And men's offices had higher bacterial concentrations than those belonging to women (the researchers suggest this might be due to less thorough hygiene practices—as well as, perhaps charitably, to men’s bodies having overall larger surface areas, which might shed more bacteria).

The researchers don't elaborate on why the chair seems almost as bacteria-covered as our phones (which presumably get coated from specks of saliva as well as from our skin). The study instead attempts to provide "baseline information about the rich bacterial communities in typical office settings and insight into the sources of these organisms," Kelley said. From here, he and others can start swabbing other surfaces to develop more thorough profiles of "typical" offices, which could be compared to bacterial communities in "sick buildings" (for which the bacteria themselves might not be to blame, but rather for which they could work as an indicator for other environmental forces, such as toxins).

But before you wipe down your whole workspace with antibacterial cleaner, just remember that most of these bugs are harmless now and that killing off the weak ones only makes the survivors stronger.

Read more about how bacteria keep us healthy in the Scientific American June 2012 feature: "How Bacteria in Our Bodies Protect Our Health" by Jennifer Ackerman.