Figure 1
Experimental design. (a) Each flagellar motor of E. coli cells in steady state environment shows bi-directional motions stochastically (CW or CCW direction). When an attractant is added to the environment (chemical stimulus), cells detect differences in the amount of the attractant and bias flagellar rotation in the CCW direction. After a sufficient time (~ 400 s in case of c, d), under constant concentrations of attractants, each flagellar motor recovers bi-directional motion (adaptation). (b) Schematic drawing of the experimental setup (left) and tethered cell assay (right). Each experimental chamber consisted of a PDMS microchannel on a coverslip. E. coli cells applied to a chamber were adhered to a glass surface via cell bodies and/or flagella. The rotational motion of cell bodies, each tethered via flagellum by chance, was analysed. (c) Rotational directions of individual cells. Cells were stimulated by (left column) l-Glu (30 mM) and (right) l-Asn (3 mM). Vertical axes show the rotational directions (CW or CCW). Horizontal axes are common to (d) and represent time. Arrows indicate the times of chemical stimulations. We measured rotational motions of cells at every 10 ms (1 frame of our movies), and counted those more than 7.5 degrees/frame, which was more than two times larger than that of noise (< 3 degrees/frame). Most of cells rotated CCW immediately after chemical stimulation (attractant response) and recovered bi-directional rotations as in the initial phase over time (adaptation). Individual cells had varied response time courses. (d) Ensemble averages of rotational directions of cells. Vertical axes represent the CW bias, which was calculated as the fraction of CW motion (number of CW rotations) to all rotational motions (number of CW + CCW motions) at 1-s intervals. The motions of more than 100 of approximately 500 observed cells were averaged.
