Antibiotic resistant genes prevalent in groundwater

With climate change comes increasing water shortages, and potentially longer periods of drought. As policymakers look urgently to wastewater recycling to stem the gap in water resources, the question is -- how best to reuse water and ensure public safety. New and emerging contaminants like antibiotic resistant genes (ARGs) pose a potential hazard to public safety and water security. One concern is the spread of ARGs through the water system and an increase in development of antibiotic-resistant super bugs.

Researchers from the University of Southern California studied and compared samples from an advanced groundwater treatment facility in Southern California and groundwater aquifers to detect differences in ARG concentrations. While they found that the advanced groundwater treatment facility reduced nearly all targeted ARGs to below detection limits, groundwater samples had a ubiquitous presence of ARGs in both control locations and locations recharged with water from the advanced water treatment facility.

Historically, indirect reuse treatment methods in which an environmental barrier is an intermediary step in the water cleaning process have been more popular than the direct "toilet to tap" process. While indirect methods of water reuse treatment were, from a public perception and appetite, considered more reliable, it is actually direct reuse "toilet to tap" approaches which do not introduce an environmental buffer that produce safer, more pure water for potability. The reason for this lies in the way ARGs in the environment can contaminate potable reuse water.

While some ARGs are naturally occurring in microbial communities, antibiotics, ARGs and antibiotic resistant pathogens are on the rise in water sources as a result of the overuse of antibiotics in general. In a typical water treatment cycle, wastewater is treated first at a wastewater treatment facility. The study found that this water remains high in ARGs, as they persist throughout the treatment process. From here, water intended for potable reuse is further purified using advanced physical and chemical techniques including reverse osmosis -- a process that uses a partially permeable membrane to purify drinking water.

Since wastewater treatment plants are not generally designed for removal of micropollutants like antibiotics, they tend to persist in treatment systems, leading to high densities of ARG resistant bacteria at different stages of treatment. When this water is introduced into an aquifer, where ARGs are already naturally occurring, it can become contaminated with ARGs and antibiotic resistant bacteria. To further complicate the issue, ARGs are easily transferred through horizontal gene transfer, increasing the risk for antibiotic resistant pathogens.

See:

Moustapha Harb, Phillip Wang, Ali Zarei-Baygi, Megan H. Plumlee, Adam L. Smith. Background Antibiotic Resistance and Microbial Communities Dominate Effects of Advanced Purified Water Recharge to an Urban Aquifer. Environmental Science & Technology Letters, 2019; DOI: 10.1021/acs.estlett.9b00521

Posted by Dr. Tim Sandle, Pharmaceutical Microbiology