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Escherichia coli solutions with differing ratios of live:dead cells were stained with fluorescent dyes SYTO 9 and propidium iodide (PI), which label live and dead cells, respectively. Samples were measured using a LSR II Flow Cytometer (BD Biosciences); using 488 nm excitation with 20 mW power. Both SYTO 9 and PI fluorescence were collected and threshold was set to side scatter. Traditional culture-based plate count was done in parallel to the FCM analysis. The concentration of live bacteria from FCM was compared to that obtained by plate counts. Preliminary results show that the concentration of live bacteria obtained by FCM and plate counts correlate well with each other and indicates this may be extended to a wider concentration range or for studying other cell characteristics.
At high illumination power, saturation of the two-photon excitation probability of the fluorophore occurs and induces a strong nonlinear response. By temporally modulating the excitation laser-intensity and demodulating high-order harmonics from the saturated fluorescence signal, images with higher spatial resolution than with standard MPM can be obtained. We show, as expected from a two-photon excitation process, that resolution improvement arises when demodulating at least at the third harmonic and that linear combinations of harmonics provide further improvement of the resolution. Images of 200 nm fluorescent microspheres confirm the improvement of the spatial resolution.
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