Uncover landfilled antimicrobial resistance: a critical review of antibiotics flux, resistome dynamics and risk assessment

Municipal solid waste (MSW) landfill is one of the most important reservoirs of antimicrobial resistance (AMR) in urban environments. By reviewing ~120 published cases worldwide, we found that leachate-borne antibiotics were at the μg/L level, and meanwhile, around 8 tons of antibiotics (including the clinically relevant ones) annually leached from the MSW landfills in China. During a decade-long landfilling process, the leachate-borne bacteria mainly originating from human-associated waste (>40%) formed a community network being versatile to the drastic environmental changes. Among them, the keystone species (Proteobacteria subtaxa) functioned for metabolizing the most available substrate in leachates and were also the hosts of mobile antibiotic resistance genes (ARGs), which suggested the enduring and close associations between bacterial community and resistome. These leachate-borne ARGs were highly mobile via plasmid-mediated horizontal gene transfer, especially in less aged leachates (<10 yr). MetaCompare showed that the AMR-hazard index of landfill-specific airborne particles (index=20.5) was significantly higher than that of drinking water (index=17.81, P<0.01). Human daily exposure of ARGs amounted to an inhalation of (5.83±0.16)×10⁵ copies of ARGs, being tenfold higher than that ingestion of drinking water, which implies landfills as a non-ignorable AMR source.