Fluorescence measurements

Fluorescence spectra were recorded on a Shimadzu RF-5301 PC spectrofluorophotometer. In all the experiments, samples were measured in a 1 cm path-length quartz cell at 23 ± 1 °C. The concentration of all species was 0.05 mM in a phosphate buffer (10 mM) of pH 7.4. The reported values are the average of three measurements. Bacterial membrane vesicles were prepared from cultures of Bacillus cereus and E. coli as described previously³⁰. The fluorescence experiments were performed under the same concentration of Salivaricin B. This was achieved by dividing a buffer solution that contains Salivaricin B into three portions. One portion was used as a reference, or a control, without any membrane. Membranes of B. cereus and E. coli were then each added to one of the other portions. In order to compensate for the amount added from each membrane, similar volume form the buffer was added to the control solution. We checked the buffer alone and there was no fluorescence in the spectral region (295–550 nm) which rules out any contribution from the buffer to the observed signal. Since B. cereus and E. coli membranes show fluorescence signal in the same spectral region, in two other sets of experiments, the control in each one was either membranes of B. cereus or E. coli.