An international consortium has reported the largest-ever global metagenomic study of urban microbiomes, spanning both the air and the surfaces of multiple cities. The international project, which sequenced and analyzed samples collected from public transit systems and hospitals in 60 cities around the world, features comprehensive analysis and annotation for all the microbial species identified--including thousands of viruses and bacteria and two archaea not found in reference databases. The study was published online on May 26, 2021 in Cell. The open-access article is titled “A Global Metagenomic Map of Urban Microbiomes and Antimicrobial Resistance.” "Every city has its own 'molecular echo' of the microbes that define it," says senior author Christopher Mason (@mason_lab), Associate Professor at Weill Cornell Medicine (WCM) and Director of the WorldQuant Initiative for Quantitative Prediction. "If you gave me your shoe, I could tell you with about 90% accuracy the city in the world from which you came." The findings are based on 4,728 samples from cities on six continents taken over the course of three years, characterize regional antimicrobial resistance markers, and represent the first systematic worldwide catalogue of the urban microbial ecosystem. In addition to distinct microbial signatures in various cities, the analysis revealed a core set of 31 species that were found in 97% of samples across the sampled urban areas. The researchers identified 4,246 known species of urban microorganisms, but they also found that any subsequent sampling will still likely continue to find species that have never been seen before, which highlights the raw potential for discoveries related to microbial diversity and biological functions awaiting in urban environments.