The biofilms were grown inside microfluidic flow cells and were studied using a protocol adapted from a previous work. Using computer-aided design software (DraftSightTM, Dassault Systèmes, France), a flow cell containing two independent but closely spaced microchannels was designed to enable visualization of the sample and reference biofilms at the same time. A mould was fabricated using a dry (negative) photoresist film (Photopolymer Film 55, Mungolox, Germany) that was adhered to a glass slide with a heated laminator (304 Model, Fortex, UK). After lamination, the photoresist film was measured to have a thickness of 45 μm. This distance defined the channel height. The lateral dimensions of the microchannels were defined using a photomask, which was printed in-house using a high-resolution photoplotter (FPS 250000, Fortex, UK). A UV exposure system (UV-AY315, Fortex, UK) selectively crosslinked the photoresist through the photomask. After the unexposed photoresist was removed with a developer solution, the mould retained the features according to the design pattern. The flow cell was then made by casting a mixture of polydimethylsiloxane (PDMS) and a crosslinker against the mould. The PDMS device was subsequently sealed with a microscope glass coverslip by air plasma activation (PCD-001, Harrick Plasma, US). The final device featured two parallel microchannels with a height and width of 45 and 600 µm, respectively (Figure ). A small inter-channel separation distance (500 µm) enabled microscopy measurements of both channels in the same field of view while background light could be obtained from the space between them. Fluidic connections to the devices were made with perfluoro-alkoxy tubing (1514L, IDEX Corporation, Canada) at the entry and exit of each channel from the device PDMS side. Syringe pumps (SP300, Next Advance, USA) supplied a constant volumetric flow rate (0.3 mL•h -1 ) to the system. For each channel, a four-way diagonal flow valve (V-100D, IDEX Corporation, Canada) enabled a constant flow of liquid in the flow cell and air removal in the feed lines between syringe replacements. An upstream channel position was selected for observation to ensure contact with OD t