Figure 2.
Cells exhibit a broad distribution of motility bias within and across community locations. (A) Number of cells that traversed more than 2 μm from their original position over a 10 min imaging interval (see Fig. 1A for location definitions). Relatively few cells were motile in the back region. The total number of cells tracked in each region, including nonmotile cells, is indicated. Data are mean ± SE of n = 4 independent inoculations. Asterisks represent statistically significant differences, with ∗p < 10−2 and ∗∗p < 10−4. The back region had a significantly lower number of motile cells than the center region Student's t-test with six degrees of freedom (t(6) = 4.4, p < 10−2). (B) Fraction of segmented cells in each region that was motile. The midfinger region had a significantly higher fraction of motile cells than the front region (t(6) = 12.5, p < 10−4). (C) The distribution of step sizes over 1 s intervals was similar in all regions, with a change in slope at ∼0.2 μm that is likely attributable to progressive pilus retraction (Fig. S2 A). (D) Mean bias values were significantly higher in the fingertip and midfinger regions. The bias of cells within the midfinger region was significantly higher than cells within the front and center regions (t(6) = 9.0, p < 10−3), whereas the bias within the fingertip region was significantly higher than in the midfinger (t(6) = 6.3, p < 10−3). Mean bias perpendicular to the light was approximately zero in all regions, where positive directionality is defined as 90° clockwise from the incident light direction. (E) Fingertip and midfinger regions had a higher fraction of cells that moved processively toward the light (parallel bias close to 1), whereas all regions had approximately zero bias in the perpendicular direction. (F) The cell speed parallel to the axis of light incidence was approximately constant across the regions, whereas the velocity reflected the increase in bias toward the front of the drop shown in (D). (G) Rose plots of the distribution of step angles in directions relative to the light source. Regions are colored as in (A)–(F). 0° and 180° denote steps taken directly toward and away from the light, respectively.