From the Cover: Bacterial flagellum as a propeller and as a rudder for efficient chemotaxis

Abstract

We investigate swimming and chemotactic behaviors of the polarly flagellated marine bacteria Vibrio alginolyticus in an aqueous medium. Our observations show that V. alginolyticus execute a cyclic, three-step (forward, reverse, and flick) swimming pattern that is distinctively different from the run-tumble pattern adopted by Escherichia coli. Specifically, the bacterium backtracks its forward swimming path when the motor reverses. However, upon resuming forward swimming, the flagellum flicks and a new swimming direction is selected at random. In a chemically homogeneous medium (no attractant or repellent), the consecutive forward t(f) and backward t(b) swimming times are uncorrelated. Interestingly, although t(f) and t(b) are not distributed in a Poissonian fashion, their difference Δt = |t(f) - t(b)| is. Near a point source of attractant, on the other hand, t(f) and t(b) are found to be strongly correlated, and Δt obeys a bimodal distribution. These observations indicate that V. alginolyticus exploit the time-reversal symmetry of forward and backward swimming by using the time difference to regulate their chemotactic behavior. By adopting the three-step cycle, cells of V. alginolyticus are able to quickly respond to a chemical gradient as well as to localize near a point source of attractant.

Fig. 1.

Bacterial trajectories in a motility medium TMN (A) and in a steep chemical gradient created by a micropipette filled with 1 mM serine (B). The big solid circles are the starting points of the bacterial tracks, and the small solid circles represent the positions at an equal time interval of 0.067 s. The green and the red segments correspond to the forward and the backward trajectories, respectively. The large open circles marked the flicking events; for clarity, not all flicking events are marked in A.

Fig. 2.

Flicking angle θ distribution in a homogeneous medium (A) and in a sharp chemical gradient created by a point source of attractant (1 mM serine) (B). The distribution in both cases contains two peaks, a broad one at θ ≈ π/2 and a narrow one at π. The solid lines in the figure are fittings using a Gaussian function.

Fig. 3.

Statistical distribution of t_f and t_b in a homogeneous motility buffer (A–C) and near a sharp serine gradient (D–F). In A and D, pairs of consecutive forward t_f and backward t_b swimming intervals between two flicks are plotted with random shifts of less than 0.03 s to avoid data points overlapping. In the homogenous medium, there is no correlation between t_f and t_b, whereas in the attractant gradient there is a strong correlation between t_f and t_b. The biological cause of such a correlation is discussed in SI Results and Discussion. The solid line in D is a linear fit to the data points with large t_f and has a slope of 0.86. The PDFs of t_f and t_b in the two cases are plotted in B and E and in C and F, respectively. The Insets in B and C are semilog plots of the same datasets as in the main figures. The solid lines are exponential fits to the tail parts of the data. The good agreement between the data and the fits shows that for large t_f and t_b the PDFs are exponential. The solid line in F is a Gaussian fit. The same fitting curve was rescaled and replotted in E as the solid line. The broken line in E is a Gaussian fit to the short-time peak of P(t_f).

Fig. 4.

Fluorescent images of a swimming cell of V. alginolyticus (A–E). The bacterium is labeled with Nano Orange dye. The flagellum is visible when it is in the focal plane. A bend at the base of the flagellum is discernible and becomes amplified over time, as indicated by the red arrows. However, when the flagellum flicks, it moves out of the focal plane and becomes blurred, such as the one indicated by double arrows in B. The time interval between the images is 1/30 s, and the cell body length is ∼3 μm. The 3D drawing in F depicts the relative positions of the flagellum to the cell body at the last stage of flicking. Specifically, to align with the cell-body axis, the tip of the flagellum starts from position 1, traces out a hyperbolic spiral, and ends at position 3. The forward swimming ensues.

Fig. 5.

Normalized fluorescence intensity profiles I(r,t) for V. alginolyticus (A) and E. coli (B). The intensity profiles are proportional to the bacterial concentration profiles B(r,t). The individual runs are color coded according to the color bar (in seconds). Note the significantly different spatial and temporal scales used in these plots, indicating that the swarm size and the aggregation time are quite different between the two bacteria. As a comparison, the normalized steady-state bacterial and the serine profiles are displayed by the thick green and orange curves, respectively. The Insets represent the time-dependent half-height radii r_1/2(t) of the corresponding bacterial profiles B(r,t). The red lines are exponential fits, which are described in the main text.