Cities Have Unique Microbial Signatures, New Study Says

It turns out that the world's cities are unique in a way that you might not expect.

Even though they often contain similar indoor environments like offices and apartments, each metropolis appears to have its own unique microbial signature, according to a new study published Tuesday in mSystems, a journal from the American Society for Microbiology.

In other words, every urban environment, from Berlin to Tokyo, could be home to its own special community of microscopic critters.

For the study, researchers from Northern Arizona University collected samples from nine different offices in three North American cities: Flagstaff, Arizona; San Diego, California; and Toronto, Ontario, over the course of a year. The cities were chosen because they each had distinct climates.

In each office, they installed sampling plates made of different kinds of surfaces like painted drywall, ceiling tile, and carpet. They also put in environmental sensors that allowed them to control for factors such as humidity, temperature, and light.

The study's results showed that each city has its own particular group of bacteria, protists, and fungi inhabiting its offices. Using a machine learning technique, the researchers were able to predict the geographic origins of an unmarked microbe sample correctly 85 percent of the time.

"This was especially interesting because even within each city, the offices we studied differed from each other in terms of size, usage patterns, and ventilations," Gregory Caporaso, Ph.D, one of the paper's authors, said in a news release.

What this suggests is that geography is more important than any other feature in determining the bacterial composition of an office. It doesn't so much matter what an office is like, but rather where it's located.

There was no significant association between the microbe communities discovered and indoor environmental factors like temperature or humidity, the researchers discovered.

"The thing that surprised me the most was the lack of community difference based on the building science parameters we looked at. I really thought that we would see differences in offices that were more humid or hotter than others," the paper's lead author, John H Chase told me.

The sample size in the study was relatively small, since it only looked at three cities, but the results mirror what similar work has found.

"Studies like this match what we have also seen in our laboratory, and represent the beginning of 'geospatial genome forensics,' where you can trace when and where people have traveled through their molecular signatures," Christopher Mason, a geneticist at Weill Cornell Medical College, who spent 18 months mapping the microbes found in the New York City subway system told me.

Among the places that the researchers looked at, Flagstaff was the most rich in microbes. The results also showed that San Diego and Toronto shared more similarities to each other than Flagstaff, but it's not clear why.

These kinds of studies can tell scientists a great deal about the history of a place, and as Mason points out, in the future data about the microbial makeup of a city could help track where people have traveled, without looking at their passport.