Antibiotic resistance has emerged as a serious global public health threat. The role of the environment in dissemination and emergence of antibiotic resistance has been highlighted, and numerous studies identified specific genes or resistant bacteria in environmental media (e.g.1,2). Still, little is known about the transmission dynamics of antibiotic resistance from water, air, and soil and their risks for humans in direct contact with these matrices. The key to determining human health impact lies in the application of epidemiological investigations, in which carriage of resistant bacteria in people exposed to a specific transmission route is tested in comparison to unexposed controls. Such studies have been carried out in travellers3 and farmers4, but environmental exposures have rarely been studied5.
Water from agriculture, hospitals, and the general population is collected at Wastewater Treatment Plants (WWTP), making them unintentional collection points for antimicrobials, antibiotic-resistant bacteria (ARB), as well as antibiotic resistance genes (ARGs, the collective of which is the resistome). Wastewater treatment processes are principally not developed to remove either of these, and studies indicate that even though reduction occurs through treatment processes, high amounts of antimicrobials and ARGs are still shed into environmental reservoirs, including recreational water as well as irrigated farmlands (e.g.6). Also, extended-spectrum beta-lactamase (ESBL) and carbapenemase-producing Enterobacteriaceae (CPE) have been detected in the influent and effluent of WWTPs and the receiving surface waters, with many of the resistance gene types linked to those found in clinical settings7. While the bacterial treatment efficiency of conventional treatment technologies greatly differs between plants, the role of specific treatment technologies for removal of bacteria remains poorly described8.
Workers at WWTPs are potentially exposed to polluted wastewater and aerosolized ARB/ARGs through inhalation and dermal routes but also via ingestion. Airborne bacteria have indeed been detected in WWTP, including fecal coliforms9, and an increased prevalence of gastrointestinal and respiratory diseases has been reported in WWTP workers, suspected to be linked with microbial exposures10. Although few studies so far addressed specific pathogens in WWTP workers and found an elevated carriage of Tropheryma whipplei11, high levels of antibodies against Helicobacter pylori12, HAV and HEV and positive stool PCR tests for Leptospira spirochete13, carriage of resistant bacteria in WWTP workers is yet unknown.
Furthermore, WWTPs are often located in urban settings in close proximity to the residents. As bacteria can be traced back up to 150 meters away from animal farms14, neighboring residents also face an exposure risk. Generally, city air represents a previously neglected potential transmission route for antibiotic resistance, as it can harbor a higher diversity of resistance genes than any other environment, including the human gut15. WWTPs, their workers, and nearby residents, therefore, represent an ideal - but yet unstudied - test case to investigate whether transmission via (waste)water actually increases ARB and ARG carriage.
Wastewater typically harbors a mix of residual antibiotics and other agents that are known to co-select for antibiotic resistance16,17. This provides the potential for selection and hence relative enrichment of resistant bacteria. Selection pressures, together with a high density and diversity of pathogens and environmental bacteria carrying various resistance factors provide a milieu where new forms of resistance may emerge18,19. From mining metagenomics data, the emergence of new resistance genes can be studied20. Also, changes in relative resistance gene abundance along the WWTP, coupled to data on bacterial community composition, can provide indications for selection of resistance.
Within this project, data on ESBL, CPE, and resistance gene prevalence gathered in air and water samples as well as in workers and residents of 80-100 different WWTPs in three countries enable us to assess the health impact of exposure in and around WWTPs. Through quantifying the contribution of different wastewater treatment processes to the ARB/ARG removal efficiency, we will provide evidence-based support for possible mitigations.
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