Figure 3

Figure 3: An Equilibrium Density Gradient of NaBr with an E. coli Band. In this 40 ml demonstration gradient, the bacterial band of about 1x10⁵ cells is shown. The density runs from 1.1 g/cc at the top to 1.5 g/cc at the bottom. On of the several methods for ascertaining the physical density of the cells is shown here: two droplets of immiscible organic solvents of precisely known different densities are seen. One floats above the bacterial band and the other below. The upper droplet has a density of 1.39 g/cc and the lower one is 1.45 g/cc. Interpolation indicates that these cells are close to having a density of about 1.41 g/cc, which is the maximum for E.coli. Note the thinness of the band, which indicates the homogeneity among the cells. Two other methods for ascertaining the cells' bouyant density are direct measurement of the liquid in which the cells float (a droplet is removed from the band and read refractometrically), and by spinning the cells with small bits of predetermined density markers. We have found that small bits of variously colored silk and wool work well once they have been assayed refractometrically. The density within a lot has little variance.