Fig. 4: Experimental trajectories in the presence of HC.

a and b show snapshots of experiments with ST-WT (a) and non-chemotactic ST-M935 (b) (black dots) in the presence T84 epithelial cells as HC (bright areas). Some representative bacterial trajectories—of a population that includes motile and non-motile bacteria—are indicated by solid lines: yellow for ∣Ω∣ < 0.3 s⁻¹ and black for ∣Ω∣ > 0.3 s⁻¹. The classification of trajectories in two categories, those with ∣Ω∣ smaller and larger than 0.3 s⁻¹, is simply to highlight the importance of ∣Ω∣. c Box plots of the (average) motility parameters in experiments: (i) with ST-WT in absence of T84 cells (ST-WT, n = 89), (ii) with ST-WT in the presence of T84 cells (ST-WT+T84, n = 60), and (iii) with the non-chemotactic mutant ST-M935 in the presence of T84 cells (ST-M935+T84, n = 75). d illustrates the temporal evolution of the distance l_min to the closest T84 cell and its derivative, m(t) = dl_min/dt for a representative bacterial trajectory; the average of m(t), which is our "bias" order parameter (\(\bar{m}\)) is shown as a dashed horizontal line. e The distribution of \(\bar{m}\) in experiments, with ST-WT (left) and ST-M935 (right), in the presence of T84 cells. The vertical red lines indicate the population average. f Same as e, the method is illustrated using simulated data with (SB-B) and without (SB-NB) a chemotactic bias. The black dashed line indicates \(\bar{m}=0\) as a reference. Box plots: the lower and upper limit of the boxes correspond to the 25th and 75th percentiles. The line inside indicates the median. The upper and lower whiskers extend values within 1.5 IQRs. The red asterisk indicates p value < 0.01. See Supplementary Table 1 for details about the statistical tests.