Humans aren’t the only commuters making use of the metro.
A new study that examined the microbiome of 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 in the journal Cell Reports, are part of a growing body of work that could have implications for a host of efforts, from managing the spread of disease to designing city infrastructure.
The microbiomes within us and around us are critical to understanding human health. The microbes 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 we leave many of our own behind, by touch or by breath.
Because of this, 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 or pick up pathogens, some more dangerous or resistant to treatment than others.
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 at 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.
“Despite all these measures, in the (subway) train compartments there is really little personal space, passengers are squashed there,” Panagiotou, who designed the Cell Reports study, said in an email.
“We are talking about one of the busiest and most dense cities in the world.”
Panagiotou was interested in the mixing of microbe populations as well as the spread of pathogens through such a system.
But 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.
In either case, Hong Kong’s subway system made an ideal testing ground – it is used by about five 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 microbiomes of different subway systems, including Boston and New York in the United States, as well as Hong Kong.
“These are basically the first genetic maps of cities and high-density human environments,” said Christopher Mason, a geneticist at Weill Cornell Medicine, US, who was not involved in the study.
With genetic maps from different cities, researchers can start to understand which antibiotic-resistant markers are common and largely harmless, and which ones are rarer and could potentially become a threat, said Mason, who previously studied the New York subway microbiome.
Typically, however, previous studies have usually tested the surfaces of the train cars themselves, which isn’t quite the same thing as knowing which microbes successfully hop from person to another, said Regina Cordy, a microbiologist at Wake Forest University, US, who was not involved in the study.
“What really hasn’t quite happened, to as broad of an extent, is looking at how these microbes might really be transmitted to humans,” said Cordy, who previously studied the Boston subway microbiome.
For this paper, Panagiotou and his colleagues directly studied the microbes on the skin of passengers, because they wanted to track which microbes were actually picked up from subway surfaces over the course of the day.
The scientists sent volunteers into the train cars of different subway lines for 30-minute intervals, 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 seemed to have its own specific microbiome signature during the morning hours, as people left home for work.
For example, the MOS line, which runs along Shing Mun channel, was full of aquatic bacteria – an abundance that wasn’t found in the more inland routes.
The WR line, which passes through a mountainous region in the New Territories, had a relatively high abundance of those species that prefer to live around 1,000 metres (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(s); 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 neighbourhoods also contributed to each line’s individuality, he added.
But throughout the day, as people moved around, those distinctions in populations began to fall away, he said. 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,” he 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, due to the tidal effect.”
This was especially clear in the am-to-pm spread of antibiotic resistance genes, he said.
That finding alone should not alarm people, he said.
The idea behind this work was not to scare people, but to reveal 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 they could tell, the metro lines with higher traffic rates did not seem to carry higher health risks, whether in pathogens or in antibiotic resistance genes, he said.
In fact, the overall amount of microbes was surprisingly low for the number of travelers using it 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 theorised, 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 US. “There are public health implications for that.”
It will take further study to know if these materials are reducing the density of microbes on surfaces, she said.
But if they are, then “those types of materials should be further explored” for use in a wide range of transit options, from subways to airplanes, she said.
The results help to fill in our view of the ebb and flow of microbial populations on subway systems, Cordy added, though she said it will take more work to make the connection between the microbes on subway surfaces and the microbes on riders’ hands.
“It would have been nice to see paired data from those exact surfaces and the human hand so that we can, at the same time, directly bridge that gap,” she said. – Los Angeles Times/Tribune News Service