Bacteria spread across the subway network throughout the day Alan Graf/Getty

A study of the Hong Kong subway system has found that each line has its own microbial community, but that commuters mix bacteria across the whole network every day.

Gianni Panagiotou at the University of Hong Kong and his team asked volunteers to enter the Hong Kong subway with clean hands, ride around for 30 minutes while holding the handrails, and then swabbed their palms.

Analysing these swabs, they found that the majority of microbes they picked up were common skin bacteria, and the most abundant non-bacterial organisms were yeasts. In the morning rush hour, 140 species were detected, but by evening, many of those were no longer detectable and the populations of just 48 species had expanded to cover the entire system.


The particular microbial community seen in each route at the start of the day seems to be determined by where each train line begins. The highest abundance of soil species and antibiotic resistant bacteria were found on the uptown Ma On Shan line, and the East Rail line – the only route linked to mainland China.

Antimicrobial resistance

The resistance genes carried by these bacteria were mostly against medical antibiotic drugs, but the team did also detect resistance to tetracycline, which is commonly added to pig feed. These genes were mostly detectable on the northern train lines at the start of the day, but had dispersed throughout the city by evening.

The team also swabbed train surfaces, but they didn’t find much microbial DNA – perhaps because of the antibacterial coating that is applied to the surfaces of the Hong Kong subway. A similar study previously did the same to analyse the Boston subway system.

“These studies are the first maps of their kind, and what is striking is the ubiquity of these antimicrobial-resistant genes we see around the world, but also that each city is unique,” says Chris Mason at Cornell University, who has studied the spread of bacteria through the New York subway system. “We don’t yet know which ones are the most significant for human health,” he says, but adds that studies like this can help tease that out.

Journal reference: Cell Reports, DOI: 10.1016/j.celrep.2018.06.109

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