B, Overnight plate assays comparing the cytotoxic activity of wild-type ColE9 with ColE9 K469C labelled with AF488, AF568 or AF647 against a lawn of E. coli with either OmpF (JW2203) or OmpC (JW0912) in the outer membrane (shown as +OmpF and +OmpC, respectively). Each plate was spotted with a serial dilution of the colicin (10 µM - 2.4 pM, 4-fold dilution per spot). Wild-type ColE9 was more active against E. coli cells with OmpF in the outer membrane than OmpC. Labelling ColE9 K469C with fluorophores reduced colicin activity in cells with OmpF in the outer membrane but largely abolished colicin activity in cells with OmpC. 