Every city is unique—defined by its inhabitants, industries, cultural institutions, and, apparently, its microbes. According to a recent study published in the journal mSystems [PDF], each city may have its own distinct microbial fingerprint, made up of microscopic elements specific to its environment. These microbes don’t just populate outdoor areas, but blanket the interiors of our cars, homes, and offices. We carry them with us on our clothes, and even track them into work on our shoes.

NPR reports that researcher Gregory Caporaso of Northern Arizona University realized each urban microbiome was a true individual while studying offices in three cities. While his initial interest was to study how the microbial environments in work settings can impact health, Caporaso was surprised to find that the biggest differences between office microbiomes came down to city rather than office type.

Over the course of a year, Caporaso and his team monitored nine offices in Flagstaff, Arizona, San Diego, California, and Toronto, Canada. They left collection plates in different parts of each office; installing them on floors, walls, and ceilings, in order to study the distribution of different microbes. They found that floors had the richest array of microbes (likely because office workers tracked them in on the bottoms of their shoes) and that more than a quarter of each office’s microbial community consisted of human skin bacteria. They also found that the human nasal microbiome was consistently present across offices, though in smaller quantities.

The majority of microbes, however, were from non-human sources—in particular the environment outside the office. Caporaso and his team found that the non-human elements of each office’s microbiome made it distinct from offices in other cities. “We can accurately predict which city an office microbiome sample is derived from, but office-specific bacterial communities are less apparent,” the authors write in the study. “Offices within cities look more similar to each other than to offices in different cities.”

Coporaso told the microbiology blog mBiosphere that he initially wanted to study how built environments—like office buildings—affected people’s health, but he quickly realized geography was also extremely important: “... even within each city, the offices we studied differed from each other in terms of size, usage patterns, and ventilation systems, suggesting that geography is more important than any of these features in driving the bacterial community composition of the offices within the ranges that we studied."

Aside from being an interesting look at how the outdoor world affects indoor life, the study could pave the way for future application. “As we continue to expand our understanding of the microbiology of the built environment, possibly including routine monitoring of microbial communities to track changes that may impact human health, our results will help inform future research efforts," Coporaso told mBiosphere. 

[h/t NPR]