Antibiotic resistance in wastewater: origins, fate, and risks

Antimicrobial resistance is increasing worldwide and threatens our capacity to treat infectious diseases. Recently, special attention has been given to the environment in the cycling of antibiotic resistance from and to humans. In urban areas, wastewater treatment plants represent a critical point of resistance propagation, since the discharge of domestic, hospital, and industrial effluents into municipal collectors mixes together human commensal, pathogenic, and environmental bacteria and high loads of antibiotic residues. Today we observe that wastewater bacteria have a high prevalence of resistance to antibiotics used in the last few decades. In parallel, resistance determinants recently detected in clinical settings are regularly observed in wastewater, suggesting a rapid propagation to the environment. The residues of antimicrobial compounds and some heavy metals play an important role in the propagation of resistance, not only during wastewater treatment but also after the discharge into the environment. Currently, it is not possible to estimate the probability of resistant bacteria or genes reaching humans, but direct transmission seems highly unlikely. However, the contamination of surface water and soil may represent a source of transmission to humans, via the food chain, drinking water, or leisure activities.