Transformations in Flagellar Structure ofRhodobacter sphaeroides and Possible Relationship to Changes in Swimming Speed

Abstract

Rhodobacter sphaeroides is a photosynthetic bacterium which swims by rotating a single flagellum in one direction, periodically stopping, and reorienting during these stops. Free-swimming R. sphaeroides was examined by both differential interference contrast (DIC) microscopy, which allows the flagella of swimming cells to be seen in vivo, and tracking microscopy, which tracks swimming patterns in three dimensions. DIC microscopy showed that when rotation stopped, the helical flagellum relaxed into a high-amplitude, short-wavelength coiled form, confirming previous observations. However, DIC microscopy also revealed that the coiled filament could rotate slowly, reorienting the cell before a transition back to the functional helix. The time taken to reform a functional helix depended on the rate of rotation of the helix and the length of the filament. In addition to these coiled and helical forms, a third conformation was observed: a rapidly rotating, apparently straight form. This form took shape from the cell body out and was seen to form directly from flagella that were initially in either the coiled or the helical conformation. This form was always significantly longer than the coiled or helical form from which it was derived. The resolution of DIC microscopy made it impossible to identify whether this form was genuinely in a straight conformation or was a low-amplitude, long-wavelength helix. Examination of the three-dimensional swimming pattern showed that R. sphaeroides changed speed while swimming, sometimes doubling the swimming speed between stops. The rate of acceleration out of stops was also variable. The transformations in waveform are assumed to be torsionally driven and may be related to the changes in speed measured in free-swimming cells. The roles of and mechanisms that may be involved in the transformations of filament conformations and changes in swimming speed are discussed.

Bacterial motility: links to the environment and a driving force for microbial physics

“…A single cell may use all combinations. Flagellar conformation changes over the same timescale as speed (Armitage et al, 1999). This correlative link is supported by the mechanistic consistency of flagellar changes that are a tight, high-amplitude coil that does not propel the cell, a helix that propels the cell and a straight form that propels the cell.…”

“…Orientation control by speed modulation of a single conformationally dynamic flagellum was described in Rhodobacter sphaeroides (Armitage et al, 1999). Initially, R. sphaeroides was thought to turn by Brownian motion (Armitage & Macnab, 1987), but this is now known to be incorrect for this species.…”

“…The full implications of flagella-generated turbulence are unclear; but the consequences of speed variation are clear in some species. Rhodobacter sphaeroides shows a variety of swimming behaviours within and between individual cells (Armitage et al, 1999). Speeds were constant, oscillating or intermittent.…”

Bacterial Tracking of Motile Algae Assisted by Algal Cell’s Vorticity Field

“…Experiments indicate that another (unnamed) species of bacteria can sense an oxygen gradient over its body length and steer relative to the gradient in a continuous fashion [33], though it is quite different in shape from P. haloplanktis and S. putrefaciens. In another example, Rhodobacter sphaeroides has a similar shape to P. haloplanktis and S. putrefaciens, with a single flagellum, and is able to changes direction by altering the conformation of this helix [1], though there is no evidence of directed steering in this case.…”

Modelling and analysis of bacterial tracks suggest an active reorientation mechanism inRhodobacter sphaeroides

“…A related mechanism exists in the uniflagellate bacterium Rhodobacter sphaeroides, in which reorientations are, instead, caused by stopping the flagellar motor [6]. Upon ceasing to rotate, the flagellum undergoes a conformational change, transforming from a functional semi-rigid helix to a short wave length, high amplitude coil against the cell body [7]. This leads to reorientation by a mechanism that is not yet well understood.…”