Research ArticleMICROBIOLOGY

Response thresholds in bacterial chemotaxis

See allHide authors and affiliations

Science Advances  16 Oct 2015:
Vol. 1, no. 9, e1500299
DOI: 10.1126/sciadv.1500299
  • Fig. 1 Nonsigmoidal bias model.

    (A) Variation in FliM numbers as a function of CWbias. (B) Resultant motor bias versus CheY-P levels. The dashed lines indicate CheY-P concentration at CWbias = 0.5.

  • Fig. 2 Measurements of CWbias and FliM numbers.

    (A to C) Measured distributions of motor bias in cheR cheB cells (n = 17 to 35). The concentration of attractant used for each experiment is indicated. (D) Experimentally determined number of FliM molecules in individual tethered motors versus CWbias. Data were binned by CWbias (n = 81 motors, bin size ~0.15), and the average NS/NSCW is reported for each bin. Black curve represents predictions from Eq. 1, where NSCCW/NSCW ~1.32.

  • Fig. 3 Comparisons between nonsigmoidal and sigmoidal models.

    (A) Distributions of motor bias in a population of cells for decreasing mean values of [CheY-P] (left to right), computed from the data of Fig. 2 (A to C) (square symbols) or predicted (Appendix A) for a sigmoidal relationship between CWbias and [CheY-P] (shaded). The mean value for each distribution is reported in the figure; σ = 0.4 to 0.5 μCheY-P. (B) Distributions of motor bias in a population of cells for decreasing mean values of CheY-P (left to right), predicted for the nonsigmoidal model shown in Fig. 1B. The mean value for each distribution is reported in the figure, σ = 0.45 μCheY-P.

  • Fig. 4 Adaptation in wild-type cells subjected to exponential ramps of attractant.

    (A) Top panel: Response of a single motor to an up-ramp (0.005 s−1). Middle panel: Average response of four motors each subjected to two ramp cycles. Bottom panel: Ramp profile. (B) Top panel: Response of a single motor to a down-ramp (−0.005 s−1). Middle panel: Average response of five motors each subjected to a single ramp cycle. Bottom panel: Ramp profile. Dashed lines indicate prestimulus bias. Arrows indicate beginning of ramps. Faint gray lines indicate SD.

  • Fig. 5 Stochastic simulations of motor responses to ramps.

    (A) Two-state model for motor switching. In the absence of a ligand, the free-energy difference in the two states of the motor is E, and the CCW conformation is favored. (B) Average of simulated traces for motor adaptation in wild-type cells to exponential ramps (n = 11 motors). Reduction in CheY-P levels due to an exponential ramp in attractant level before t = 0 s causes an increase in N(t), the FliM/FliN content. (C) This results in a corresponding increase in CWbias. The solid black curve represents simulations with FliM/FliN remodeling. The solid gray curve represents simulations without FliM/FliN remodeling. The open circles indicate experimental measurements from Fig. 4A (middle panel).

  • Table 1 Parameters used in the study.
    ParameterValues
    koff = kon (for CW and CCW)~0.02 s−1 [Lele et al. (19)]
    M56 [Lele et al. (19)]
    NSCW34 (Thomas et al., 2006)
    NNECW, NNECCW12, 34 [Lele et al. (19)]
    E0.22 kBT (this work)
    Ψ0.35 kBT (this work)
    KCW2.61 (this work)

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/1/9/e1500299/DC1

    Fig. S1. CWbias versus CheY-P for fixed values of N (gray dotted and solid curves).

    Fig. S2. Motor switching trace simulated by alternately sampling exponential distributions of CW and CCW wait-time intervals.

    Fig. S3. Variations in mean intervals with CWbias.

    Fig. S4. Comparisons between experimental and simulation results for a down-ramp (−0.005 s−1).

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. CWbias versus CheY-P for fixed values of N (gray dotted and solid curves).
    • Fig. S2. Motor switching trace simulated by alternately sampling exponential distributions of CW and CCW wait-time intervals.
    • Fig. S3. Variations in mean intervals with CWbias.
    • Fig. S4. Comparisons between experimental and simulation results for a down-ramp (−0.005 s−1).

    Download PDF

    Files in this Data Supplement:

Navigate This Article