ColE9* denotes ColE9 H551A, an active site mutation that inactivates the toxin. A, Widefield fluorescence microscopy images of E. coli JM83 cells labelled with ColE9* K469CAF647 (1.5 µM) and Hoechst DNA stain (20 µM) for 30 min at 37 °C with or without trypsin treatment with or without prior treatment with CCCP. Each panel shows the same cell in DIC (grey), Hoechst DNA stain (blue) and ColE9* K469CAF647 fluorescence (red). Overlays of Hoechst and ColE9* K469CAF647 fluorescence are also shown. ColE9* K469CAF647 remains bound to the OM in the presence of CCCP but this signal is lost on treatment with trypsin. Trypsin treatment in the absence of CCCP yields some cell-associated ColE9* K469CAF647 fluorescence that likely represents internalised molecules (see text). Scale bar, 1 µm. 