Final report: distribution and elimination of antibiotic resistance genes and antibiotic resistant bacteria in the drinking water supply of lausanne (St. Sulpice), Switzerland
Rationale: Pathogens with resistance against antibiotics are on the rise globally, creating a serious threat to public health. Antibiotic resistance is not a phenomenon only of clinical and medical environments. Resistance genes can be acquired from environmental bacteria and human activities release antibiotic resistance factors into the environment. This environmental dissemination of resistance factors has consequently been termed an emerging environmental contaminant. Recent studies conducted by Eawag have shown that antibiotic resistant bacteria are released with the treated wastewater of the city of Lausanne and affect the water and the sediment in the bay of Vidy. This water is one of the main sources of drinking water for Lausanne, raising questions whether antibiotic resistant bacteria or antibiotic resistance genes could be reaching consumers through the drinking water distribution network. While Eauservice Lausanne operates a modern water treatment facility that uses various filtration and disinfection procedures to produce hygienically safe drinking water, it was unclear how these procedures affect environmental bacteria carrying resistance factors. Similarly it was unclear to what extent bacterial regrowth takes place in the distribution system, and how this affects the presence and abundance of resistance factors. Purpose: In this project the water treatment facilities and the drinking water distribution network of Lausanne were studied with regards to the abundance of bacteria, of antibiotic resistant bacteria, and to assess the regrowth potential within the distribution network. The aim was to determine if there were antibiotic resistance factors present in the distribution system, how various treatment procedures change their absolute and relative abundance, and to check for seasonal changes in their abundance. Approach: Monthly sampling campaigns were conducted for almost a year. A wide variety of water samples was taken from different points of the drinking water network and treatment facilities over time. A focus was on the St. Sulpice plant and its distribution network, which was sampled repeatedly to asses temporal variability. Other samples included potential hotpots (dead ends and warm spots in the distribution system) and other treatment facilities (Lutry and Bret). A total of 63 samples were processed. A combination of cultivation-based (bacterial growth on agar media in the presence or absence of antibiotics) and cultivation independent (flow cytometric cell counts, quantification of resistance genes with quantitative real-time PCR) analyses were applied to obtain a multifaceted insight into the drinking water resistome. Assimilable Organic Carbon (AOC) and total cell numbers in the distribution system were determined to assess the regrowth potential within the treatment and distribution system. In addition to analyses done by Eawag, standard chemical and biological drinking water characteristics were measured by Eauservice. Results: Results from this study show that resistant bacteria, as well as known antibiotic resistance genes do occur in the drinking water network but at low abundance. The drinking water treatment as performed in Lausanne leads to a considerable reduction of the resistance load compared to the raw water used for production. However, chlorination was not an absolute barrier for resistance factors. Temporal and site-to-site dynamics in cell numbers, in the abundance of resistant bacteria and resistance genes, together with AOC data suggest bacterial regrowth occurs within the drinking water system. A selection for resistance (increase in the proportion of resistant bacteria relative to the raw water) was not observed. Evidence for problematic concentrations of resistant bacteria at certain spots within the treatment system were not discovered. Interpretation: While resistance factors do reach consumers with drinking water, including both viable bacteria and resistance genes (thought to at least partly be present in non-viable cells or in the form of free DNA), we have found no evidence of elevated risks. Future changes to the drinking water treatment and distribution system should remain mindful of maintaining or improving the barriers for resistance factors, and should aim to keep conditions that could be selective for resistant bacteria to a minimum.