FIGURE 1.

Single toxin detection using fluorescence. A, domains of B. thuringiensis Cry1Aa toxin according to crystal structure (23). B, setup for photobleaching experiments with supported bilayers. A 532-nm laser excites fluorophores present in supported bilayers formed on a glass coverslip. A high numerical aperture objective collects the emitted light and directs it to an EMCCD camera. C, dispersion of fluorescent spots at incubation concentrations of 0.17 μg/ml (left panel), 1.21 μg/ml (center panel), and 2.33 μg/ml (right panel). D, for each spot, the fluorescence intensity time trace was determined. The time traces showed discrete photobleaching steps (arrows), and the number of steps was counted to determine the minimal number of subunits/oligomer. AU, arbitrary units.