Extended Data Fig. 10: Stiffer agar suppresses rosettes, increasing the fraction of ∆pilH cells at the front of mixed WT/∆pilH colonies as they expand.
From: Bacteria solve the problem of crowding by moving slowly
a, Subsurface colonies prepared with 0.8% (red), 1.2% (purple) and 1.6% (blue) agar were inoculated with equal fractions of WT-CFP and ∆pilH-YFP cells. Cells in colonies prepared with 1.6% agar are predicted to experience an approximately four-fold larger stabilizing torque compared to those with 0.8% agar, which acts to resist the formation of vertically oriented rosettes (Supplementary Notes). Here, we plot the packing fraction, ρ, at the front of colonies as they expand across the surface, aligning each by the radius at which the front becomes fully packed with cells, or confluent (rC). This collapses data from all colonies onto a single curve and allows us to control for variation across replicates (Supplementary Notes). b, Measurements of the ∆pilH to total cell fraction, fH, at the colony front shows that stiffer agar allows ∆pilH cells to migrate greater distances. The inset shows fH at rSUB − rC = 1200 µm, the final position for which data is available for all replicates. We find that fH is positively correlated with agar concentration at this position (p < 0.005, n = 9, Spearman’s rank correlation test), demonstrating that stiffer substrates allow ∆pilH cells to migrate further when competed against WT cells. c, The increased persistence of ∆pilH cells can readily be observed in representative images of the front of 0.8% (top), 1.2% (middle) and 1.6% agar (bottom) colonies. Dashed lines indicate the corresponding positions in a and b. d, e, Images of the region behind the front at the confluency time tC and tC + 100 min in colonies prepared with 0.8% (d) and 1.6% (e) agar. With 0.8% agar, ∆pilH cells form numerous discrete, tightly-packed rosettes. With 1.6% agar, ∆pilH cells still form aggregations but remain horizontally oriented, allowing them to escape and form plume-like patterns. To improve clarity, only YFP-expressing ∆pilH cells are shown in d and e. Thick lines in a, b show the mean of three separate colonies.
