To ensure sterility in the samples taken from the sink volume, samples were flash -frozen in liquid nitrogen and subsequently thawed in a hot water bath (60 ℃) to lyse any bacterial cells. This was performed three times. Cell fragments were removed by centrifugation (14800 rpm, 3 min), and the supernatant transferred to a 96-well plate for fluorescence measurement within an hour to prevent interference by further bacterial growth (BMG Labtech, CLARIOstar Plus).