Coexistence of Two Chiral Helices Produces Kink Translation in Spiroplasma Swimming

Abstract

The mechanism underlying Spiroplasma swimming is an enigma. This small bacterium possesses two helical shapes with opposite-handedness at a time, and the boundary between them, called a kink, travels down, possibly accompanying the dual rotations of these physically connected helical structures, without any rotary motors such as flagella. Although the outline of dynamics and structural basis has been proposed, the underlying cause to explain the kink translation is missing. We here demonstrated that the cell morphology of Spiroplasma eriocheiris was fixed at the right-handed helix after motility was stopped by the addition of carbonyl cyanide 3-chlorophenylhydrazone (CCCP), and the preferential state was transformed to the other-handedness by the trigger of light irradiation. This process coupled with the generation and propagation of the artificial kink, presumably without any energy input through biological motors. These findings indicate that the coexistence of two chiral helices is sufficient to propagate the kink and thus to propel the cell body.IMPORTANCE Many swimming bacteria generate a propulsion force by rotating helical filaments like a propeller. However, the nonflagellated bacteria Spiroplasma spp. swim without the use of the appendages. The tiny wall-less bacteria possess two chiral helices at a time, and the boundary called a kink travels down, possibly accompanying the dual rotations of the helices. To solve this enigma, we developed an assay to determine the handedness of the body helices at the single-wind level, and demonstrated that the coexistence of body helices triggers the translation of the kink and that the cell body moves by the resultant cell bend propagation. This finding provides us a totally new aspect of bacterial motility, where the body functions as a transformable screw to propel itself forward.

Keywords: cell polarity; cytoskeleton; helical shape; motility; video microscopy.

FIG 1

Kink propagation and reversal of the handedness of the body helix during migration. (a) Schematic. (b) Sequential micrographs of a single migrating cell in the vicinity of the glass. The cell shapes, bending to the left and right upward along the body axis, define RH and LH in our experimental setup (see more detail in Fig. S2). Purple arrowheads indicate the positions of the kink. (c) Kink diagram of panel b. The kink position from the lagging pole of the cell was plotted. (d) A kink diagram for 6 s. Kink_R→L and kink_L→R are defined as the kink generated at the cell pole of RH and at that of LH, respectively. (e) Histograms of the time that the cell stays only LH (blue) and only RH (red), respectively. The solid line shows the fit of double exponential decay with two parameters, n = const₁ · exp(−t/τ₁) − const₂ exp(−t/τ₂). The dashed line in the RH-only histogram shows the fit of single exponential decay, n = const · exp(−t/τ), where τ is estimated to be 0.27 s. Dwell times between the attainment of the kink to the rear end and the emergence of the next kink at the front end, distributed in a stochastic manner with single or double exponential functions. (f) Histograms of the time interval from kink_R→L to kink_L→R (LH only in panels d and e) and from kink_L→R to kink_R→L (RH only in panels d and e) generated at the leading pole of the cell. The line shows the Gaussian distribution. The average and its standard deviation (SD) were plotted in each panel.

FIG 2

Handedness of body helix stabilized in cells that have stopped moving. (a) Average swimming speed after motility inhibition treatment. (b) Helical morphology under phase-contrast microscopy. Arrows present the directions of slanted portions of the cell relative to the helical axis. Scale bar, 1 μm. (c) Experimental setup of the motility inhibition by light irradiation to the cell. BP, band-pass filter; DM, dichroic mirror. (d) Apparent pitch angle of cell shapes. (d, top) Schematic of the definition of the angle. (Bottom) Histogram of the pitch angle.

FIG 3

Photoinduced switch of the handedness of the body helix. (a) Single-cell images under phase-contrast microscopy. Cell was pretreated with CCCP and FM4-64 and was then submitted to strong-light irradiation. The irradiation was turned on at 2.69 s. Scale bar, 1 μm. (b) Kink diagram of the cell in panel a and Movie S4. (c) Speed of kink propagation. (d) Schematic illustration of the processive change in body helicity induced by light irradiation. RH and LH morphologies are represented by red and blue lines, respectively.

FIG 4

Kink propagation induced by partial-light irradiation. (a) Diagram of partial-light irradiation at cell edge. (b) Single-cell images under phase-contrast microscopy. The partial light was applied to the lower part represented by the green boxed areas in panels c and d. Scale bars, 1 μm. (c) Kink diagram of the cell in panel a and Movie S5. (d) Schematic illustration of the effect of partial-light irradiation. (e) Speed and frequency of kink propagation.

FIG 5

Kink propagation induced by spotlight irradiation. (a) Diagram of spotlight irradiation. Single-cell images under phase-contrast microscopy are shown. The middle region of the cell was irradiated by spotlight illumination in the presence of FM4-64 and CCCP. The data are from Movie S6. (b) Angle at cell bend. (c) “Cell bend” diagram. The light-illuminated region is shown in green. (d) Speed and frequency of cell bend position. (e, left) Schematic illustration of the effect of spotlight irradiation. (Right) Sharp cell bend explained by helical compositions.