Teams of researchers and volunteers entered the mass transit systems of 60 cities, collecting thousands of samples from 2015 to 2017. They smoothed a variety of surfaces inside transit stations and metro cars, including turnstiles, railings, ticket kiosks and benches. (In a handful of cities that did not have metro systems, teams focused on bus or train systems.)
Underground sampling expeditions of scientists often attracted attention. The curiosity of some passengers increased so much that they joined the Volunteer Swabbing Corps, while others insisted that they did not want to know exactly what was staying on the pillars of the metro. Travelers sometimes misunderstood what researchers were doing with their small swabs. “A man thanked us for cleaning the metro,” Dr. Mason said.
The researchers also collected air samples for a companion paper on the “air microbiome” from the transit systems of six cities – New York, Denver, London, Oslo, Stockholm and Hong Kong. Published on Wednesday in the Microbiome Journal.
“It’s huge,” said Erica Hartmann, a microbiologist at Northwestern University, who was not involved in the study. “Number of samples and geographic diversity of samples – this is unprecedented.”
The team then extracted DNA from each sample and sequenced it to identify the species contained therein. In total, among all surface samples, they found 4,246 known species of microorganisms. Two-thirds of these were bacteria, while the rest were a mixture of fungi, viruses, and other types of microbes.
But that was just the beginning: they also found 10,928 viruses and 748 types of bacteria, which were never documented. “We could see that these were real – they are microorganisms – but they are nowhere in any database,” Said Daniela Bezdan, the former executive director of MetaSUB, who is now a research associate at University Hospital Tübingen in Germany.
Experts said that most of these organisms probably pose little risk to humans. Dr. Danko said that almost all new viruses he found were likely to be bacteriophages or viruses that infect bacteria. Furthermore, genetic sequencing cannot distinguish between organisms that have died and those that are alive, and none of the environment is sterile. In fact, our body relies on a rich and dynamic community of microbes to function properly.