“Wrap, trap and zap” kills superbugs in wastewater

A shield of graphene helps particles destroy antibiotic-resistant bacteria and free-floating antibiotic resistance genes in wastewater treatment plants, researchers report.

The researchers have worked toward quenching antibiotic-resistant “superbugs” since first finding them in wastewater treatment plants in 2013. Their new work introduces microspheres wrapped in graphene oxide as a potential solution.

“Superbugs are known to breed in wastewater treatment plants and release extracellular antibiotic resistance genes (ARGs) when they are killed as the effluent is disinfected,” says Pedro Alvarez, a professor of civil and environmental engineering, of chemistry, of materials science and nanoengineering, of chemical and biomolecular engineering, and director of the Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment at Rice University.

“These ARGs are then discharged and may transform indigenous bacteria in the receiving environment, which become resistome reservoirs.

“Our innovation would minimise the discharge of extracellular ARGs, and thus mitigate dissemination of antibiotic resistance from wastewater treatment plants.”

The researchers showed their spheres—cores of bismuth, oxygen, and carbon wrapped with nitrogen-doped graphene oxide—inactivated multi-drug-resistant Escherichia coli bacteria and degraded plasmid-encoded antibiotic-resistant genes in secondary wastewater effluent.

The graphene-wrapped spheres kill nasties in effluent by producing three times the amount of reactive oxygen species (ROS) as compared to the spheres alone.

The spheres themselves are photocatalysts that produce ROS when exposed to light. Lab tests showed that wrapping the spheres minimised the ability of ROS scavengers to curtail their ability to disinfect the solution.

The researchers say nitrogen-doping the shells increases their ability to capture bacteria, giving the catalytic spheres more time to kill them. The enhanced particles then immediately capture and degrade the resistant genes released by the dead bacteria before they contaminate the effluent.

“Wrapping improved bacterial affinity for the microspheres through enhanced hydrophobic interaction between the bacterial surface and the shell,” says co-lead author Pingfeng Yu, a postdoctoral research associate at Rice’s Brown School of Engineering.

“This mitigated ROS dilution and scavenging by background constituents and facilitated immediate capture and degradation of the released ARGs.”

Because the wrapped spheres are large enough to be filtered out of the disinfected effluent, it’s possible to reuse them, Yu says. Tests showed the photocatalytic activity of the spheres remained relatively stable, with no significant decrease in activity after 10 cycles—significantly better than the cycle lifetime of the same spheres minus the wrap.

The research appears in the journal Water Research.

Additional coauthors are from Tongji University in Shanghai and Yale University.

The National Science Foundation, the National Natural Science Foundation of China, and the National Key R&D Program of China supported the work.