Changing populations of Hong Kong’s subway

Have you ever wondered about the bacteria you encounter on a train, particularly when holding hand rails used by countless others throughout the day?

Well, if the Hong Kong subway system is anything to go by, your train line could have its own unique bacterial profile in the morning rush hour. But by evening, it will have merged with microbes from other train lines to create a uniform microbiome for the whole system.

The finding, reported in the journal Cell Reports, may contribute to the design of public transport systems in future, including decisions about the use of antibacterial surfaces and the provision of hand sanitiser, to prevent antibiotic-resistant disease outbreaks.

Aside from finding many common and relatively harmless bacteria such as Propionibacterium acnes, which causes the skin condition acne, the researchers identified 136 antibiotic-resistant gene families, including 24 clinically important ones worrying infectious disease doctors around the world.

To map the movement of bacteria through the system, researchers led by Gianni Panagiotou from the University of Hong Kong, China, sent volunteers through different train lines for half an hour during both the morning and evening peak hours. They sampled the skin on their hands after each ride.

While previous studies of the Boston and New York City subways examined train compartment surfaces, these scientists wanted to know what was being transferred to commuters’ hands, because skin is so critical for immune function and disease prevention.

During the morning, each line had unique microbial features reflecting the regions it passes through. For example, the MOS line, which is entirely aboveground and runs alongside a polluted, brackish river, had the most aquatic and sewage species. In contrast, the WR line, which passes through a mountain region in the New Territories, had more species with a preference for altitude around 1000 metres.

But as more people used the system throughout the day, the microbial communities of all the lines became increasingly similar, dominated by human skin bacteria moving from train line to train line.

“The Metro is constantly cleaning every surface that we touch, but the train compartments have little personal space … we are talking about one of the busiest and densest cities in the world,” explains Panagiotou.

“With five million people riding the subway every day, the fingerprint of the whole city had to be there.”

The work also led to a new hypothesis that the ER train line – the only cross-border line linking Hong Kong to mainland China – could be transporting an important gene that is resistant to tetracycline, a commonly used antibiotic in China’s swine feedlots.

Panagiotou says that while the antibiotic resistant genes were the best illustration of the mixing pattern across the system throughout the day, people should not be scared to use the subway because the train lines carrying the most passengers did not carry higher health risks, neither in terms of pathogens nor antibiotic-resistant genes.

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