Water samples were collected at the sampling site with steam-sterilized polypropylene carboys or bottles (Fisher scientific), and transferred to the laboratory immediately, then concentrated by 0.22 µm filter retention in a biological safety hood prepared by standard sterilization with UV and ethanol wiping. The filters used in this study were prepackaged sterilized polyethersulfone filter units (Millipore). Because of the need for other tests, the filtration volume for pre-stagnation was 40 liter per sample, and the volume of post-stagnation water samples were 80 mL for S1, 880 mL for S2, and 980 mL for S3 and S4 samples. Based on the flow cytometry data described in Results, this is estimated to retain 1.8 × 107–5.1 × 108 cells/ mL. Filters were removed from the filter unit with sterile scalpels and preserved in sterile falcon tubes −80 °C prior to DNA extraction. DNA was extracted using the Schmidt’s protocol [46 (link)] and purified with Promega Wizard DNA cleanup system (Promega) and was stored at −80 °C. The protocol was selected based on a previous publication that evaluated the different protocols for DNA extraction of drinking water samples from distribution systems [47 (link)].