Engineers at Rice University found that composite wastewater samples taken over 24 hours give a much more accurate representation of the level of antibiotic-resistant genes (ARGs), according to a press release by Rice University’s Mike Williams.
The lab of Lauren Stadler at Rice’s George R. Brown School of Engineering reported seeing levels of antibiotic-resistant RNA concentrations 10 times higher in composite samples than what they see in grabs: snapshots collected when flow through a wastewater plant is at a minimum.
Stadler and lead authors Esther Lou and Priyanka Ali, both graduate students in her lab, reported their results in the American Chemical Society journal Environmental Science & Technology: Water.
The results could lead to better protocols for treating wastewater to lower the prevalence of antibiotic-resistant genes in bacteria that propagate at plants and can transfer those genes to other organisms in the environment. The issue is critical because antibiotic resistance is a global health threat responsible for millions of deaths worldwide.
The takeaway for testers is that snapshots can lead to unintended biases in their results, Stadler said.
“I think it’s intuitive that grabbing a single sample of wastewater is not representative of what flows across the entire day,” says Stadler, who is also a faculty member of the Rice-based, National Science Foundation-supported Nanotechnology Enabled Water Treatment (NEWT) Center. “Wastewater flows and loads vary across the day, due to patterns of water use. While we know this to be true, no one had shown the degree to which antibiotic-resistant genes vary throughout the day.”
The Study
For the study, the Rice team took both grab and composite samples in two 24-hour campaigns, one during the summer and another during winter, at a Houston-area plant that routinely disinfects wastewater.
They took samples every two hours from various stages of the wastewater treatment process and ran PCR tests in the lab to quantify several clinically relevant genes that confer resistance to key antibiotics, as well as a gene known as a mobile genetic element for its ability to move within a genome or transfer from one species to another.
Because the results from snapshots can vary significantly during any given day, they had to be collected at a steady pace over 24 hours. That required Lou and Ali to spend several long shifts at the City of West University Place wastewater treatment plant.
“They camped out,” Stadler says. “They set up their cots and ordered takeout.”
The Findings
The samples they collected allowed them to determine the concentration of ARGs and loads across a typical weekday, the variability in removal rates at plants based on the grab samples, and the impact of secondary treatment and chlorine disinfection on the removal of ARGs, as well as the ability to compare grabs and composites.
In the process the researchers discovered that, while secondary wastewater treatment significantly reduces the amount of target ARG, chlorine disinfectants often used in later stages of treatment can, in some situations, have a negative impact on water released back into the environment.
The team found that the vast majority of ARG removal occurred due to biological processes as opposed to chemical disinfection. In fact, they observed that chlorination, used as the final disinfectant before the treated wastewater is discharged into the environment, may have selected for antibiotic-resistant organisms.
