While wastewater treatment essentially represents a barrier to the spread of antibiotic resistance genes, these genes are enriched in relative terms at WWTPs. (Photo: Werdhölzli WWTP)
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.
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