Fig. 4. Repolarization events in stationary cells exposed to an alternating succinate gradient preferentially re-localize PilT–YFP to the cell pole experiencing larger succinate concentrations, indicating that they are capable of spatial sensing.
a, We used a dual-flow microfluidic device to expose cells to a spatial gradient of succinate that alternates direction33. The dashed black box indicates the region downstream of the two inlets, where we imaged cells. b, In response to this alternating spatial gradient, stationary cells (blue) expressing PilT–YFP (white circles) can undergo either correct or incorrect repolarization events. c,d, The relative proportion of correct and incorrect repolarization events in this experiment can be used to determine whether cells use temporal (c) or spatial (d) sensing. c, Stationary cells using only temporal sensing could garner no information about a gradient’s spatial orientation and would therefore be equally likely to generate correct and incorrect repolarization events (prediction A). d, By contrast, stationary cells capable of spatial sensing could directly sense the gradient’s spatial orientation, allowing them to deploy correct repolarization events at a greater frequency than incorrect repolarization events (prediction B). e, Quantifying the behaviour of n = 171 stationary cells undergoing repolarization events within our alternating gradient experiments (see Supplementary Videos 1–16 and Supplementary Table 1) revealed that correct repolarization events occurred approximately 6 times more frequently than incorrect repolarization events, regardless of whether PilT–YFP localization was initially nonpolar, unipolar or bipolar (Fig. 3g–i). An exact two-tailed binomial test rejected the null hypothesis that correct and incorrect repolarization events were equally abundant with P = 2.37 × 10−7, 1.51 × 10−9 and 1.28 × 10−8 for nonpolar, unipolar and bipolar repolarization events, respectively. This is consistent with prediction B, indicating that cells are capable of directly sensing differences in concentration over the length of their bodies. Error bars show 95% confidence intervals about the proportion estimates. Source data provided as a Source data file.