Sources, behaviour and health risks of antimicrobial resistance genes in wastewaters: A hotspot reservoir

Limited reviews have investigated sources, behaviour and health risks of antimicrobial resistance genes (ARGs) in the wastewater-human pathway. The current review seeks to: (1) determine hotspot sources of ARGs, and their behaviour and fate in wastewaters; (2) discuss ARG health risks, and risk factors; and (3) highlight future research directions. Diverse ARGs conferring single and multi-drug resistance to common and last-resort antimicrobials are widely reported. Hotspot sources of ARGs include medical facilities, pharmaceutical industries, aquaculture and animal systems. Non-engineered landfills, on-site sanitation, funeral parlours and gravesites constitute under-studied potential hotspot sources. Hydrological processes, wind and vectors mobilize ARGs from sources into aquatic systems. Wastewater treatment processes, the nature of ARGs and environmental conditions control the behaviour and fate of ARGs. Consumption of contaminated food and water, vectors, inhalation and dermal contact transfer ARGs from wastewaters to humans. Health risks include development of antimicrobial resistance, although evidence directly linking ARGs in wastewater to resistance remains weak. Ecological risks include shifts in physiological behaviour, community structure and ecosystem functions, including expression of shock-responses and virulence, biofilm formation and mutations. These shifts could increase colonization ability and survival of pathogenic microbes, thereby posing significant ecological, human and animal health risks. Overall, the behaviour and risks of ARGs are controlled by complex human-environmental interactions, and have far-reaching policy implications, hence ARGs can be considered as a 'super-wicked' problem. To minimize the risks, a conceptual framework entailing a ten-point strategy is highlighted. Under-studied hotspot sources, behaviour and fate, human ecotoxicology and risk assessments warrant further research.