AILSA CHANG, HOST:
If you've ever waited for a late subway train, you most certainly have stared into dark railways, maybe wondering what on earth might live in there.
MARY LOUISE KELLY, HOST:
Well, more than 900 scientists and volunteers around the world found out. A study published in the journal Cell last month shows their findings - a catalog of microorganisms in the subways of 60 cities.
CHRISTOPHER MASON: My favorite, really, response was actually, one of our students was out, and she was thanked by just a straphanger who said, thank you so much for cleaning the subway.
KELLY: That is Christopher Mason, one of the lead researchers. Now, the volunteers were not cleaning exactly, but they did put on masks and latex gloves when they scrubbed surfaces for DNA samples. They were told, focus on the turnstiles, benches, floors. Stay away from sources that were not permanent.
DAVID DANKO: Not to say that that never happens - I'm sure that they do. I mean, who couldn't resist sampling, like, a weird puddle or something?
CHANG: That's David Danko, another researcher in the study. He found some trends in the data.
DANKO: So with microbes, I can distinguish if a city is on the coast, and I can distinguish whether a city has high population density or low population density.
CHANG: Danko says each city has a unique signature of microorganisms. But the biggest surprise for the researchers was that all of their cities had a common core of microbes - a set of 31 species found in 97% of samples.
DANKO: And that's surprising in that cities are geographically diverse. They're culturally diverse. And you wouldn't necessarily expect the same microbial species to be in all of these cities.
KELLY: Some of these common core microbes come from human skin, but the researchers suspect the rest are just microorganisms that have adapted remarkably well to the metal and stone of urban environments.
CHANG: In total, over 11,000 new organisms were found, including bacteria and viruses. In light of the pandemic, that might sound alarming, but the researchers say it's quite the opposite.
MASON: These data really play into what's been described as the hygiene hypothesis. It's that, you know, these are the organisms that the immune system needs to learn about as they develop normally.
CHANG: Christopher Mason and his fellow scientists hope that the study will help change how we view many of our urban spaces.
MASON: Most people think of a rainforest as a source of drugs or new biology or new medicines. The same thing is true of every subway bench that you sit down on.
KELLY: So the subway as the new rainforest - try to remember that the next time the train's delayed.
(SOUNDBITE OF BRIAN REITZELL AND ROGER J. MANNING JR.'S "ON THE SUBWAY") Transcript provided by NPR, Copyright NPR.