Opposite and Coordinated Rotation of Amphitrichous Flagella Governs Oriented Swimming and Reversals in a Magnetotactic Spirillum

Abstract

Current knowledge regarding the mechanism that governs flagellar motor rotation in response to environmental stimuli stems mainly from the study of monotrichous and peritrichous bacteria. Little is known about how two polar flagella, one at each cell pole of the so-called amphitrichous bacterium, are coordinated to steer the swimming. Here we fluorescently labeled the flagella of Magnetospirillum magneticum AMB-1 cells and took advantage of the magnetically controllable swimming of this bacterium to investigate flagellar rotation in moving cells. We identified three motility behaviors (runs, tumbles, and reversals) and two characteristic fluorescence patterns likely corresponding to flagella rotating in opposite directions. Each AMB-1 locomotion mode was systematically associated with particular flagellar patterns at the poles which led us to conclude that, while cell runs are allowed by the asymmetrical rotation of flagellar motors, their symmetrical rotation triggers cell tumbling. Our observations point toward a precise coordination of the two flagellar motors which can be temporarily unsynchronized during tumbling.

Importance: Motility is essential for bacteria to search for optimal niches and survive. Many bacteria use one or several flagella to explore their environment. The mechanism by which bipolarly flagellated cells coordinate flagellar rotation is poorly understood. We took advantage of the genetic amenability and magnetically controlled swimming of the spirillum-shaped magnetotactic bacterium Magnetospirillum magneticum AMB-1 to correlate cell motion with flagellar rotation. We found that asymmetric rotation of the flagella (counterclockwise at the lagging pole and clockwise at the leading pole) enables cell runs whereas symmetric rotation triggers cell tumbling. Taking into consideration similar observations in spirochetes, bacteria possessing bipolar ribbons of periplasmic flagella, we propose a conserved motility paradigm for spirillum-shaped bipolarly flagellated bacteria.

FIG 1

Rotation direction of AMB-1 cell body during runs. (A) Electron micrograph of a spirillum-shaped AMB-1 cell showing its two polar flagella (black arrows) and its magnetosome chain (white arrow). Bar, 500 nm. (B) Swimming trajectory reflected by single polar fluorescence labeling (inset) of MamU-GFP recorded during cell translation with a 100-ms exposure. The arrow indicates the direction of cell movement. (C) Schematic representation of an AMB-1 cell showing cell body and flagellar rotation directions. • and × show views of the flagella out-off and into the image, respectively.

FIG 2

Rotation of AMB-1 fluorescently labeled flagella. (A) Superimposed images of phase-contrast (gray-scale) and fluorescence (green) microscopy of AMB-1 flagella labeled with Alexa 488 C₅ maleimide. Bar, 5 μm. (B) Frames extracted from Movie S2 in the supplemental material. Exposure time, 100 ms. Bar, 2 μm. (C) Time-lapse images extracted from Movie S4 showing the fluorescently labeled flagella of an AMB-1 cell swimming downward. In frame 106, the gray shape indicates the position of the unlabeled cell body. The arrow indicates the direction of swimming. P, parachute; T, tuft. The asterisk indicates the position of the flagellum anchoring point at the pole. The pictogram on the right is a model representing flagellar rotation at each cell pole. Exposure time, 80 ms. Bar, 2 μm. (D) AMB-1 lagging flagellum rotates in the CCW direction. Frames 19 to 26 (from right to left) were extracted from Movie S5. The arrows indicate the translation and rotation direction. In all panels, the number at the bottom left corner corresponds to the frame number in the movie. Bar, 3 μm.

FIG 3

Three distinct motility behaviors of AMB-1 cells. The traces describe the position of a cell (y axis, distance in micrometers) as a function of time (seconds). Each color-coded dot corresponds to the instantaneous speed of the cell. Shown are seven examples of motility groups 1, 2, and 3, consisting of uninterrupted runs (A), runs interrupted by pauses (B), and runs interrupted by reversals (C), which represent 586 tracks obtained from six independent experiments (see the text and Materials and Methods for details).

FIG 4

Cell reversals are triggered by simultaneous changes of the rotation direction of both flagella. Time-lapse images were extracted from Movie S7 in the supplemental material. Exposure time, 80 ms. Bar, 4 μm. The letter B and the vector above it indicate the orientation of the local magnetic field.

FIG 5

AMB-1 runs are interrupted by short pauses and tumbles. (A) Time-lapse images extracted from Movie S5 in the supplemental material showing an AMB-1 cell pausing in the midst of a run (frames 6 to 20). Exposure time, 100 ms. Bar, 4 μm. The letter B and the vector above it indicate the orientation of the local magnetic field. (B) Tumbles are caused by two flagella showing the same fluorescent pattern. Time-lapse images were extracted from Movie S8. The pictogram under each frame represents an interpretation of the flagellar motion depicted in the image above it. Exposure time, 80 ms. Bar, 2 μm.