Fig. 7.

Serratia marcescens colonies generate more force on stiffer substrates. (a) A representative movie z-stack showing the trajectories of tracer particles embedded in the underlying substrate and in the vicinity of the expanding edge of the bacteria colony. Overlaid on the image are PIV displacement maps at 3 time points—1 hour (green), 2 hours (blue), and 3 hours (magenta)—as the colony grows over time. Inset shows increasing length of the displacement arrows, as the beads move further toward the center of the colony with each increasing hour. Also overlaid on the image are the colony boundary positions at hours 1, 2, and 3. Scale bar, 200 μm. (b) Box-and-whisker plots of the stress map points for hours 1, 2, and 3 for colonies growing on soft (0.5 kPa; 3.5% PAA, and 0.15% bis) and stiff (5 kPa; 8% PAA, and 0.15% bis) PAA gels. The stress grows over time and is significantly higher for colonies on stiffer substrates than soft ones. Data curated from 3 separate colonies per condition.