In our laboratory, we’re interested in how the bacterial flagella have evolved to enable diverse species to swim through their equally diverse environments. Far from being a “one size fits all” organelle, there is striking structural diversity and embellishment from one species to the next. Historically, our tool of choice for studying flagellar form and function has been cryogenic electron tomography (cryoET), allowing us to compare and contrast the flagellar motors from different species. In our recent PLOS Pathogens publication, however, we’ve made a detour into the world of high-speed-video fluorescence microscopy to figure out how Campylobacter jejuni coordinates rotation of its two opposing flagella.
Campylobacter jejuni is a polar flagellate that causes several million cases of gastroenteritis annually and requires flagella-mediated motility for gut colonization. Nevertheless, its swimming was enigmatic. It was unclear how C. jejuni coordinates its two opposed flagella, with one at each pole, and was also unclear how it speeds up in more viscous fluids. Our study killed two birds with one stone: setting out to understand C. jejuni motility, we resolved these apparent paradoxes, and also shed light on a surprising significance of helical cell shape in swimming.
By combining bacterial genetics with high-speed-video fluorescence microscopy we resolved a surprisingly simple mechanism underlying C. jejuni’s characteristic swimming. By developing strains with fluorescently labelled flagella, we discovered that C. jejuni coordinates its apparently opposed flagella to achieve its distinctive rapid back-and-forth “darting motility” by wrapping its leading flagellar filament around its cell body, producing unified thrust from both flagella; back-and-forth motions result from polarity switching between wrapped and unwrapped filaments. These findings also contribute to understanding the confusing fact that C. jejuni speeds up in viscous media: viscosity drives wrapping of the leading filament; but in low-viscosity, neither filament is wrapped, resulting in opposed thrusts.
Our results also contribute insights to the enigmatic benefits of having a helical cell body. Although proposed to contribute to traction in solutions of long unbranched polymers by Howard Berg in the 1970s, more recent apparently paradoxical results suggested that helical cell shape contributes relatively little to propulsion. Here we show that a right-handed cell body is important for the left-handed flagellar filament of similar pitch to unwrap effectively, resolving this apparent contradiction. Using a novel, dual-labelling competition assay, we show that the inability to unwrap impairs a straight-cell mutant’s taxis, crucial for host colonization.
Finally, our results highlight the co-evolution and mutual interdependence of C. jejuni’s motility machinery. We previously demonstrated that larger novel structures in the C. jejuni flagellar motor have become co-dependent. Here we see that cell shape, the high-torque flagellar motor, and elaborate filament architecture have also become co-dependent, and disruption of any are detrimental to motility and therefore virulence. In other words, the various facets of C. jejuni motility have become “entrenched”.
The pivot from cryoET to high-speed-video microscopy was borne of necessity: our original model for coordination of opposing flagella, which was exquisitely suited to a cryoET approach, didn’t pan out. But, as is often the case with experimental science, our new results have opened up numerous new avenues for research on the flagellar evolution and function. As we embark down these new lines of inquiry, we’re back in the cryoET saddle.