Membrane and DNA dyes were used concomitantly to visualise the cell periphery and the nucleoid (Figure 1B and

1C). Cells were classified into populations defined according to their number of foci, and the positioning of foci along the length of cells was evaluated for each population (Figures 1C and 2). The distances of the foci to the closest cell pole were scored on a five points scale along the long axis of the cell from the pole to mid-cell (Additional file 1, Figure S1). The ori, right and NS-right loci displayed 2 to 4 foci that mostly found at or near the quarter positions, whereas the ter locus displayed 1 or 2 foci, which were mostly located at mid-cell (Additional file 1, Figure S1). The proportion of mid-cell-located ter foci was lower for cells harbouring a single focus than for cells with two foci, consistent with a progressive migration of the ter region from the new cell MEK inhibitor pole to the GF120918 cell line mid-cell during the cell cycle [7, 8, 21]. These findings are

consistent with previous observations using similar [9, 20] or different detection systems and growth conditions [6, 10]. Positioning of chromosome loci along the cell diameter The position of a fluorescent focus along the width of the cell cannot be directly determined using 2-D wide-field microscopy. Indeed, a focus located near the cell periphery may appear at the centre of the cell diameter or at the edge according to the orientation of the cell cylinder with respect to the focal plan. Nevertheless, since the orientations of the cell cylinder are expected to be random for a population of rod-shaped bacteria deposited on a plane surface, the mean position of particular foci can be calculated from the apparent distributions of foci along the cell diameter. We therefore measured the apparent distance along the cell diameter between foci and the membrane (Figure 1C). The datasets obtained were then compared with distributions calculated for different models of positioning across

the width of the cell (Methods). We defined five slices of equivalent surface in a quarter of Methocarbamol the cell section and calculated the expected distributions of foci according to the various models of positioning (the 2-D apparent foci distributions for various 3-D localisation patterns are shown in Figures 2, 3 and 4). Figure 2 Distributions of foci along the cell diameter. (A) Drawing showing the measurement of the apparent positions of foci along the cell diameter. Distances along the cell diameter between the centres of foci and the nearest membrane were measured. (B) Distributions of foci along the cell diameter for the ori, right and NS-rigth loci in the various cell classes. Distributions are plotted as the percentage of total foci in each cell class (Y-axis). The sample size of the cell classes is given on each graph.