Large quantities of antibiotic-resistant bacteria enter the environment through municipal and hospital wastewater streams, which is demonstrated in a study carried out in Lake Geneva by a team of researchers from the Swiss Federal Institute of Aquatic Science and Technology (EAWAG).
Although wastewater treatment plants reduce the total number of bacteria, multi-resistant strains appear to withstand or even to be promoted by treatment processes, the research has found.
Every day around 90,000m3 of treated wastewater is released into Vidy Bay (Lake Geneva) from the city of Lausanne. The discharge point is 700m offshore, at a depth of 30m. The Lausanne region has neither a pharmaceutical industry nor intensive animal production, but the Lausanne treatment plant receives wastewater from the region’s 214,000 inhabitants, a number of smaller healthcare centres, and also from the University Hospital of Canton Vaud (CHUV).
Studies from the hospital and veterinary medicine sectors have indicated an increasing prevalence of antibiotic resistance, and researchers have now investigated whether resistance genes also enter Lake Geneva via wastewater treatment plants.
The research team performed resistance testing using both traditional culture methods and genetic analysis.
In the first sampling programme ever conducted on this scale in Switzerland, data were obtained for municipal wastewater, lake water and lake sediments.
As expected, particularly high levels of highly multi-resistant bacteria were found in wastewater at the CHUV. But the study also found that while, overall, more than 75% of bacteria are eliminated at the treatment plant, the proportion of especially resistant strains is increased in treated wastewater.
According to study author microbiologist Helmut Bürgmann, the treatment plant is likely to serve as a ‘hot spot’ for the exchange of antibiotic resis-tance genes. Here, bacteria which otherwise in-habit the human body encounter others already adapted to the environment, and mobile genetic elements can be readily transferred as a result of the high bacterial cell densities.
‘The uptake of resistance genes by bacteria is not unusual and doesn’t in itself pose a hazard; what wasn’t previously known was that levels of multi-resistant genes are elevated in the lake, and particularly also in the sediment, close to the waste-water discharge outlet,’ says Bürgmann.
He believes this increases the risk that resistance genes will also be transferred to pathogens, in Lake Geneva, or in the human body, if mobile genetic elements for antibiotic resistance find their way into drinking water.
Nadine Czekalski, who carried out most of the investigations, sees ‘no grounds for panic’. A drinking water pump is situated 3km from the point where wastewater is discharged into the lake and the researchers detected multi-resistance genes in sediment samples collected close to the pump, but not in lake water at this site. In addition, water taken from the lake is treated before being fed into Lausanne’s water supply system.
But Czekalski and Bürgmann agree that there is a need for caution and they welcome the Swiss federal government’s plans to introduce an additional process for the removal of micropollutants at selected treatment plants. This advanced treatment process largely inactivates not only micropollutants, but also resistant micro-organisms. However, as hospital wastewater contains the highest level of multi-resistant bacteria, they also recommend that it should undergo separate treatment.
Their research is published in the journal, Frontiers.