Reciprocal invasion dynamics of resident biofilms. Resident biofilms of PA14 were grown for 12 h and invaded by PAO1 for 4 h (A, widefield fluorescence image; B, confocal optical section and z-projection). Resident biofilms of PAO1 were grown for 12 h (A, C, and E) or 24 h (B, D, and F) and invaded by PA14 for 4 h (C, widefield fluorescence image; D, confocal optical section and z-projection). (E) The invasion efficiency of PA14 (purple) and PAO1 (green) was measured over time by normalizing the change in fluorescence intensity from the start of the assay through 4 h of invasion. n = 3, P < 0.05 after 3 h by 2-way ANOVA with Sidak posttest. (F) The total invading strain biovolume of PAO1 and PA14 at the end of the invasion assay (n = 6 nonoverlapping image stacks from 4 separate microfluidic chambers). Error bars denote the standard deviations. P < 0.05 by Wilcoxon signed-rank test.