An international team of researchers collected ambient total particulate matter samples from 19 cities in 13 countries. The researchers screened these samples using a high-throughput real-time qPCR platform for known antibiotic resistance genes and also performed 16S rRNA sequencing. Though all the cities had some antibiotic resistance genes, San Francisco had the highest overall abundance of resistance genes, while Bandung, Indonesia, had the least. Beijing harbored the widest range of resistance genes with 18 different subtypes, though Bandung only had five. The most common resistance genes were to β-lactams and quinolones, followed by macrolide, tetracycline, sulfonamide, aminoglycoside, and vancomycin, the researchers report.