Humans aren’t the only commuters making use of the metro. A new study that examined the Hong Kong subway system found distinct bacterial “fingerprints” in each line during the morning — distinctions that blurred over the course of the afternoon.
The findings, published this week in the journal Cell Reports, are part of a growing body of work that could have implications for containing the spread of infectious diseases and designing city infrastructure.
The microbes within and around us are critical to understanding human health. Those in our guts aid in digestion; those on our skin may help keep it healthy and balanced. We pick up microbes from our environment and leave many of our own behind, by touch or by breath.
As a result, the microbial communities that live in the spaces we build — homes, schools, trains — are a reflection of the people who pass through them.
They’re also places where humans can spread pathogens.
The deadly 2003 SARS epidemic had a lasting effect on Hong Kong and the way people move through public spaces, said Gianni Panagiotou, a systems biologist at the University of Hong Kong and the Hans Knoell Institute in Germany. People often wear masks when they have a cold, and the surfaces in subway cars are cleaned constantly.
But such tactics go only so far when it comes to keeping down the microbe load, he added.
“In the [subway] train compartments there is really little personal space, passengers are squashed there,” said Panagiotou, who designed the Cell Reports study. “We are talking about one of the busiest and most dense cities in the world.”
Panagiotou was interested in the mixing of microbe populations and the spread of pathogens through such a system. His colleague at the University of Hong Kong, architect Christopher Webster, was interested in how the design of the city might affect its microbial profile.
Hong Kong’s subway system made an ideal testing ground for both of them — it was used by about 5 million people each day and it even has a cross-border rail line that brings in commuters from mainland China.
Researchers have already been examining the microbial profiles, or microbiomes, of subway systems in Boston, New York and other cities.
“These are basically the first genetic maps of cities and high-density human environments,” said Christopher Mason, a geneticist at Weill Cornell Medicine who was not involved in the Hong Kong study.
With genetic maps from different cities, researchers can start to understand which markers of antibiotic resistance are common and largely harmless, and which ones are more rare and could potentially become a threat, said Mason, who previously studied the New York City subway’s microbiome.
Typically, researchers have tested the microbes on the surfaces of the train cars themselves. But that can’t tell them which microbes successfully hop from one person to another, said Regina Cordy, a microbiologist at Wake Forest University who has studied the Boston subway microbiome.
“Looking at how these microbes might really be transmitted to humans” is the next step, said Cordy, who wasn’t involved in the new work.
In Hong Kong, Panagiotou and his colleagues studied the microbes on the skin of passengers because they wanted to track which microbes were picked up from subway surfaces over the course of the day.
The scientists sent volunteers into the train cars of several subway lines for 30 minutes at a time, cleaning and sampling their palms before they boarded and testing them again after they stepped off.
The researchers found that the microbial communities were dominated by commensal bacteria — harmless microbes that live on or in the body.
Each subway line had its own specific microbiome signature during the morning hours, as people traveled from home to work. For example, the MOS line, which runs along Shing Mun channel, was full of aquatic bacteria — which wasn’t the case in more inland routes. The WR line, which passes through a mountainous region in the New Territories, had a relatively high abundance of species that prefer to live around 1,000 meters (about 3,280 feet) in altitude.
“Each line has its own topological characteristics: One is passing close to the sea, others close to the mountain; one is underground; others are above the ground,” Panagiotou said. “All these differences have an impact on the microbiome found in each line.”
The microbiota of the particular people from particular neighborhoods also contributed to each line’s individuality, he added.
But throughout the day, as people moved around, those distinctions began to fall away. Microbes that might have been largely seen in one region could be found all across the network by the day’s end.
“The morning signature is really reflecting the topology of the line,” Panagiotou said. “But in the evening after all the people have been moving around in the city we can see that the microbiome is becoming more similar.”
This was especially clear in the a.m.-to-p.m. spread of antibiotic resistance genes, he said.
That finding should not alarm people, he added. It merely illustrates the extent to which humans are exposed to a diverse array of microbes each day — and to show that the way we design our cities “can have a significant impact on the type of bacteria that we will encounter,” he said.
As far as the researchers could tell, the metro lines with higher traffic did not seem to carry higher health risks, either in pathogens or in antibiotic resistance genes, Panagiotou said.
In fact, the overall amount of microbes was surprisingly low for the number of travelers using the trains each day, the scientists said. That may be thanks to the antimicrobial nano-silver-titanium dioxide coating applied to surfaces in the subway. Without that coating, they theorized, the transmission of antibiotic-resistant microbes could potentially have been higher.
“I thought that was very interesting,” Cordy said, adding that such anti-microbial materials don’t seem to be a common feature of public surfaces in the United States. “There are public health implications for that.”
It will take more research to see whether these materials are reducing the density of microbes on surfaces, she said. But if they are, then planners should explore their potential use in a wide range of transit options, from subways to airplanes, she said.