Dear flowers, We have tagged E.coli with GFP. When we incubate the cells in the wells of a microplate and measure fluorescence with a plate reader fluorometer we observe a certain increase in fluorescence over incubation time (4-fold). We estimate that this increase reflects the growth of the cell population. We see just the same increase in OD at 620 nm read with a plate reader photometer. CFUs show the same increase, too. As well we see the same increase with flow cytometer when we measure the number of fluorescent cells from samples incubated identically. The problem is that the mean fluorescence intensity of the fluorescent cells increases over the same time period according to flow cytometer (2-fold). We anticipate that number of fluorescent cells should be multiplied with mean fluorescence intensity to get the total fluorescence of the cell population and the increase in the product (2 x 4 = 8-fold) should be the same as increase in fluorescence intensity in fluorometer. This is, however, not the case. Have we understood something wrong with the results obtained by flow cytometer? Esa-Matti Lilius
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