Device loading
Each PDMS device contained three sample chambers. When bonded to a 6-well glass-bottom plate, this allowed 18 conditions to be tested in parallel. Devices were first placed into a vacuum chamber for 20 mins prior to loading. To load the inner chambers, bacterial cocultures were then loaded into each main channel using a micropipette, and the devices allowed to equlibrate. We then checked that the bacteria had been successfully drawn into the inner chembers. If air bubbles still remained, the plate was placed under vacuum for further 5 minutes. During loading steps, care was taken to avoid mixing of different samples loaded into parallel chambers on the same PDMS device. After loading, the channels were washed thoroughly the media (IMDM with 20% FBS).