The chemotaxis performance of E. coli in a linear concentration gradient of potassium. A. Design diagram of the microfluidic device. The source channel and sink were flowed with 100 mM KCl and motility buffer, respectively. The inlets of KCl, agarose, motility buffer and cells are denoted by (a), (b), (c) and (p), respectively. The outlets are labeled by the corresponding letters with the prime symbol. B. The average CMC of four datasets as a function of time for the wild-type strain (HCB1) under a linear concentration gradient of KCl. The shaded area denotes SEM. C. The cell density profile in the observing channel along the y-axis at the beginning (t = 1 s, green squares) and a steady state (t = 300 s, blue dots). The red solid line is an exponential fit to the data. Error bars denote SEM.

The response of motor rotational signal to potassium. A. Schematic diagram of the bead assay for the flagellar motor. B. Typical trace of rotational speed (blue line) and CW bias (purple line) of individual motors for the wild-type strain (JY26-pKAF131). The positive and negative values of speed denote CCW and CW rotation, respectively. 30 mM KCl was added at t =120 s and removed at t = 480 s. C. The average response of 83 motors from 5 samples for the wild-type strain to 30 mM KCl. The vertical purple (green) dashed lines indicate the moment of adding (removing) stimulus. The shaded areas denote SEM. D. The average response of 22 motors from 4 samples for the chemotaxis-defective strain (HCB901-pBES38) to 30 mM KCl. The vertical purple (green) dashed lines indicate the moment of adding (removing) stimulus. The shaded areas denote SEM.

The chemotactic response of the wild-type strain (HCB1288-pVS88) to potassium. A. Chemotactic response of the wild-type strain (HCB1288-pVS88) to stepwise addition and removal of KCl. The blue solid line denotes the orignal signal, and the red dots represent the pH-corrected signal, which was recalculated from the pH-corrected CFP and YFP channels using the response of the no-receptor strain. B. Comparison of the chemotactic response to 30 mM KCl and 60 mM sucrose. The vertical purple (green) dashed lines indicate the moment of adding (removing) stimulus. C. Quantitative comparison among the responses to 100 μM MeAsp, 30 mM KCl and 60 mM sucrose. The hollow and solid red bars represent the value calculated from the original signal and the pH-corrected signal, respectively. The errors denote SEM.

Quantitative results of the chemotactic response of the wild-type strain (HCB1288-pVS88) to potassium. A. A typical example of the dose-response measurement. The blue solid line denotes the original signal, and the red dots represent the pH-corrected signal, recalculated from the pH-corrected CFP and YFP signals using the response of the no-receptor strain. The vertical purple (green) dashed lines indicate the moment of adding (removing) stimulus. B. The dose-response curve of relative kinase activity to KCl. The blue dots and gray circles represent the pH-corrected and original experimental data, respectively. The red solid and gray dashed lines are the fit curves for the blue dots and gray circles, respectively, using a Hill function. The fitted Hill coefficient for original and pH-corrected response were 0.53±0.04 and 0.88±0.14, respectively, and the concentration for half-maximal response (K0.5) were 0.64±0.12 mM and 0.33±0.06 mM, accordingly. C. Definition of adaptation time in the step response. The gray and blue lines represent the original and pH-corrected signals, respectively. D. The adaptation level P = (R* − RL)/(1 − RL) and adaptation time (T) as a function of the concentration of KCl, calculated with pH-corrected data. The errors denote SEM.

A. The response of intracellular pH to 30 mM KCl for the wild-type strain (HCB33-pTrc99a_pHluorin2). The response to 20 mM sodium benzoate solution with pH=4.55 was used as a control. B. The chemotactic response of the Tsr-only strain (HCB1414-pPA114-pVS88) to four typical concentrations of potassium. The blue line denotes the orignal signal, and the red dots represent the pH-correcting signal. C. The chemotactic response of the Tar-only strain (HCB1414-pLC113-pVS88) to four typical concentrations of potassium. The blue line denotes the orignal signal, and the red dots represent the pH-correcting signal. D. The chemotactic response of the no-receptor strain (HCB1414-pVS88) to 30 mM KCl. The vertical purple (green) arrows denote the moment of adding (removing) stimulus.

A. The oscillating spatial gradient of potassium. B. The dose-response curve of receptor-kinase activity to potassium. Blue dots are pH-corrected experimental data (blue dots in Fig. 4B). Red solid line is the fitting curve with Eq. 2. C. Typical traces of simulated mean positions with L0=1.0 mM and T = 2 hrs. The blue and green line denote the mean x-position (1500 − ⟨x⟩) and mean y-position (⟨y⟩), respectively. The purple dashed line indicates the oscillating potassium source at x=0. The phase delay Δϕ is defined as the phase shift between the trough of the mean x-position and the trough of potassium source except for t/T=0. D. The comparison of the mean x-position under different periods of the driving source: T = 1, 2, 3 and 4 hrs. The purple dashed line indicates the oscillating potassium source at x=0. E. The relation between the phase delay Δϕ and the driving period T. Each data was calculated by the average of 10 simulations. The error denotes standard deviation. F. The relation between lag time (Δϕ ∙ T) and the driving period T. The error denotes standard deviation.

Strains and plasmids used in this study