Every day, five million commuters ride the Hong Kong metro. Human commuters that is. Because there are countless more microbes riding the trains, too.

"Yes it seems they are also riding the metro to move around, without paying a ticket." Gianni Panagiotou is a systems biologist at the University of Hong Kong and the Hans Knoll Institute in Germany.

He and his team tracked the ebb and flow of microbes in the Hong Kong metro, by swabbing six volunteer commuters' palms as they commuted both day and night through the city's eight urban lines. 

DNA sequencing revealed a lot of harmless skin and soil microbes—but also other germs that harbored antibiotic resistance genes. And while some trains had unique microbial fingerprints in the morning—an above-ground line running near a polluted river had more aquatic and sewage-related species, for example—by the evening commute, the microbial footprint of all lines was nearly the same. In other words, microbes wind up commuting too. 

The study is in the journal Cell Reports. [Kang Kang et al., The Environmental Exposures and Inner- and Intercity Traffic Flows of the Metro System May Contribute to the Skin Microbiome and Resistome]

The authors also hypothesize that cross contamination can occur between regions with different antibiotics use or guidelines. For example, tetracycline is common on mainland China's pig farms. And it was the line crossing into Hong Kong from mainland China that shuttled the most tetracycline resistance genes into the city each morning…where they dispersed throughout the entire system. 

Panagiotou says people shouldn't worry too much about it—it's not a serious health risk. "Having said that, it's a good idea to wash your hands after you're back at home.”

—Christopher Intagliata

[The above text is a transcript of this podcast.]