C, Widefield fluorescence microscopy images of E. coli JW2203 (+OmpF) and JW0912 (+OmpC) cells labelled with ColE9* K469CAF647 (1.5 µM) and Hoechst stain (20 µM) for 30 min at 37 °C with or without trypsin treatment. Each panel shows the same cell in DIC (grey), Hoechst DNA stain (blue) and fluorescence of ColE9* K469CAF647 (red). Overlays of Hoechst and ColE9* K469CAF647 fluorescence are also shown. Data show that following trypsin-treatment, significant ColE9* K469CAF647 fluorescence remains associated with ompF-expressing E. coli whereas little or no fluorescence remains associated with ompC-expressing E. coli. Scale bar, 1 µm. D, Box and whisker plots of E. coli JW2203 and E. coli JW0912 cells labelled with ColE9* K469CAF647 with and without trypsin treatment: whiskers represent minimum and maximum mean pixel intensity, box shows 1st and 3rd quartile with the median shown as a line. Microscopy data were collected as in c. The mean pixel intensity of ColE9* K469CAF647 per cell was measured for each cell condition. n, number of cells, typically from 3 or 4 biological replicates. Data show that ColE9* K469CAF647 translocates across the OM through OmpF and that this is impeded when E. coli has OmpC in the outer membrane. 