Samples of water (250 ml) were collected in sterile conditions and stored at 4°C before analysis, which occurred within 8 h of collection. The total concentrations of E. coli, K. pneumoniae and
P. aeruginosa were measured as previously described (Slekovec et al. 2012; Bréchet et al. 2014). Aliquots of water (100 ml) were filtered through 0.45-μm membranes, which were then placed onto a Drigalski agar plate to measure the total E. coli and K. pneumoniae concentrations (Oxoid), a Cetrimide agar plate to measure the P. aeruginosa concentration (Bio-Rad) and a chromID ESBL agar plate to measure the concentration of ESBL-producing E. coli, and ESBL-producing K. pneumoniae, which are resistant to β-lactam antibiotics. When we suspected high bacterial concentrations, we plated 500 μl an additional of sample on the selective media. The plates were incubated for 24 h at 37°C. Colonies were counted and classified according to their phenotype. Each type of colony was identified using a MALDITOF MS spectrometer (Microflex, Bruker Daltonics, Bremen, Germany) according to the manufacturer’s recommendations. For each morphotype of E. coli and K. pneumoniae growing on chromID ESBL, the production of ESBL was detected using a synergy test following the recommendations of the Antibiogram Committee of the French Society for Microbiology (CA-SFM, http://www.sfm-microbiologie.org/
, last consultation: 10 November 2018).