Urban indoor airborne antibiotic resistance genes: Role of antibiotic use and outdoor air pollution

Antibiotic resistance genes (ARGs) in airborne dust represent an emerging concern for public health, particularly in indoor environments where human exposure is prolonged. While external environmental pressures are known to shape the abundance and diversity of microorganisms in indoor dust, their role in ARG dynamics remains underexplored. This study assessed the temporal and spatial patterns of airborne ARGs in indoor dust across four Nordic cities (Aarhus, Bergen, Reykjavik, and Uppsala) using electrostatic dust collectors (EDCs) in the same households at two time points: 2012 and 2022. Shotgun metagenomic sequencing was performed to profile ARGs. National antibiotic consumption data were obtained from the European Surveillance of Antimicrobial Consumption (ESAC-Net), outdoor air pollution data (PM2.5 and PM10) from the Copernicus Atmosphere Monitoring Service (CAMS), and meteorological parameters from the NASA POWER database. Beta diversity analysis revealed city-specific differences in ARG composition (PERMANOVA, R 2 = 0.18, P = 0.03), but no consistent temporal shift over the 10-year period. Macrolide, tetracycline, and aminoglycoside resistance genes were among the most abundant and persistent classes. A previously undetected macrolide resistance sequence was identified across all cities in 2022. Although national antibiotic consumption declined, cross-sectional correlations between national antibiotic consumption and ARG abundance strengthened from 2012 (Spearman's ρ = 0.25) to 2022 (ρ = 0.37), suggesting sustained ARG presence despite reduced antibiotic consumption. Several ARG classes showed associations with outdoor particulate matter, and these relationships were influenced by local meteorological conditions. For example, higher absolute humidity was associated with a weaker relationship between PM and polymyxin resistance genes, whereas stronger wind speeds were associated with stronger relationships between PM and Sulfonamide resistance genes. These findings highlight the complex environmental interplay between antibiotic consumption, air pollution, meteorological factors, and ARG dynamics in indoor air, emphasizing the need for integrated environmental and AMR surveillance, especially in the context of climate change.