Antibiotic resistance genes are not completely eliminated by wastewater treatment. While some resistance genes are present in the influent, many others are found in activated sludge bacteria. A recent Eawag study shows that, rather than merely passing through, resistance genes are active and evolve within treatment plants.
Although wastewater treatment plants (WWTPs) remove over 95 per cent of human fecal bacteria, many resistant bacteria can still be detected in the final effluent. How is this to be explained? To find out, a group led by microbiologist Helmut Bürgmann investigated the fate and expression of antibacterial resistance genes in the course of treatment at twelve WWTPs. The scientists were also interested in whether the occurrence of resistance genes is influenced by stressors – such as antibiotics, biocides or heavy metals – in wastewater.
Core group of persistent resistance genes
Biomass samples were collected from the influent, the biological treatment steps and the effluent of the twelve WWTPs. DNA extracted from these samples was then analysed to identify sequences encoding antibiotic resistance. According to Bürgmann, while the levels of resistant bacteria found in treated wastewater were generally much lower than in the influent, “The relative abundance of resistance genes increases in the WWTP.”
The researchers found a wide variety of resistance genes, with the composition of the “resistome” varying widely across different WWTP compartments. However, a small group of resistance genes was found in all treatment steps. This core group traversing the WWTP is relatively abundant. Bürgmann notes that, while around 70 per cent of the resistance genes found in the influent are eliminated in the course of treatment, others arise within the WWTP: “About 40 per cent of the resistance genes in the effluent presumably originate in the activated sludge.”