Free-Living Protozoa as Important Drivers of Antimicrobial Resistance in Engineered Water Systems and Their Ecological Niches

Bacteria and free-living protozoa (FLP) have been coevolving in a predator–prey relationship for well over a billion years, facilitating an array of “arms race” mechanisms, including antimicrobial resistance (AMR). This perspective explores the role of FLP, including free-living amoebae (FLA), in promoting AMR with a focus on wastewater treatment plants (WWTPs), recognized hotspots for the release of AMR. Technological advances in WWTPs have changed ecological niches, impacting their microbial communities. Each process alters the diversity, abundance, and activity of FLP/FLA and bacteria, generally increasing the potential for horizontal gene transfer of antibiotic resistance genes (ARGs). Further, disinfection treatments such as chlorination, UV irradiation, and ozonation may inadvertently select for antibiotic-resistant bacteria (ARB) and multidrug resistance through natural stress responses, which are also enhanced and protected within FLP. Overall, there is a critical need to better understand the ecological impacts of biological wastewater treatment technologies and their associated interactions between FLP/FLA and ARB, and their pathways of AMR dissemination through engineered and natural water systems. This perspective underscores the importance of going beyond fecal indicator-ARG monitoring to control AMR in wastewater treatments and water reuse to mitigate risks associated with the dissemination of AMR via the environment.