Each line of Hong Kong’s transit system starts the day with a distinctive microbial profile — but this doesn’t last long.

In the morning, microbes found on various lines of Hong Kong’s rail network are characteristic of the neighbourhood each line serves. But during the day, the flora from each line mix, and by the evening, the network’s microbiome reflects the entire city.

A team led by Gianni Panagiotou at the University of Hong Kong sampled the bacteria and yeast on the hands of six riders for eight lines of Hong Kong’s Mass Transit Railway System. On three test runs per line, riders held handrails and touched surfaces for 30 minutes during the morning rush hour and again during the evening rush hour.

The team found that both microbes and antibiotic-resistance genes mingled freely in the system. The sampling ﻿also showed that the sole line linking Hong Kong with mainland China hosts high levels of genes associated with antibiotic resistance, suggesting that this rail segment may act as a potential source of such genes to other lines over the course of the day.