Cities are home to uncounted millions of microorganisms, many of which are only there because we bring them. But the built environment is a uniquely challenging place for them to live
In the summer of 2013, several science students at Cornell University in New York found themselves tasked with an unusual project. They were asked to go into subway stations and trains, and swab surfaces for no less than three minutes.
Christopher Mason, the Cornell geneticist who devised the study, points out that three minutes is a long time when you are in the middle of a public place, swabbing away at a subway pole.
Some bemused looks and questions from commuters later, though, the team amassed a comprehensive collection of samples from 468 stations.
The point of the exercise?
To investigate the microbes that share our environments, riding the same subway trains that millions of New Yorkers use daily.
These microscopic urban commuters vastly outnumber humans but we still know little about them. The project is one of many now underway to explore the urban "microbiome" - the localised population of microorganisms - and its influence on human health.
The work is at an early stage. But the initial results suggest we might want to nurture the urban microbiome, rather than fight to remove the microbes from our cities.
Back in the lab, Mason and his colleagues analysed their subway samples for viral and bacterial DNA. A huge range of microbes showed up. For example, there was heaps of Pseudomonas stutzeri, which is quite common in soils, and lots of Acinetobacter radioresistens, a radiation-resistant bacterium found on human skin.
There were also many potentially pathogenic - illness-causing - microbes. But it was not clear that they were actually causing any illnesses.
For instance, they seemed to lack many of the specific chunks of DNA that trigger human disease. And, obviously, people who ride the subway are not as a rule in a constant state of poor health.
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