Blog post: The nose of this wormy-shaped bacterium has a surprising symmetry (2024)

A blog by Alise Muok

‘Imagine having to survive in nature without the use of any of your senses. Of course, finding food and safety is a nearly impossible task without your sight and hearing. Yet, this is what bacteria are proficiently doing at all times. These organisms can find nutrients and ideal environmental conditions without eyes, ears, noses, or even brains! So how are they able to accomplish this complicated behaviour without such systems? Well, research over the last five decades has found out that bacteria possess an intricate protein apparatus that allows them to sense beneficial and harmful chemicals. This system is so highly evolved that the cells can even keep a temporal record of where the chemicals are located. That’s right: they effectively have a memory without a brain! This protein system—called the bacterial chemotaxis system—is also in pathogenic bacteria and allows the pathogen to survive in their host. For example, spirochetes are bacterial pathogens that utilise chemotaxis to bring about a range of horrible diseases, includingLyme’s disease, syphilis, yaws, and periodontal diseases.

But fortunately for all humans, microscopy methods have advanced very quickly, which has allowed researchers to actually see (with our eyes) what the chemotaxis apparatus looks like. Many of these experiments have been done by professor Ariane Briegel, both as a postdoc and a Principal Investigator, using cryo-electron tomography (cryo-ET). Ariane has examined the chemotaxis system in many many bacteria and found that they essentially have the same highly-conserved structure—the apparatus consists of three main proteins that assemble into an extended array with a six-fold symmetry in the cell membrane. A lot of biochemical and structural research has shown that this specific six-fold arrangement allows the cells to fine-tune their sensing mechanisms to the remarkable sensitivity and precision we observe.

However…for the first time in all of the Universe, Ariane and myself have found that spirochetes do not have a six-fold symmetry. Using cryo-ET, we see that they have a system with a two-fold symmetry. I know you’re probably thinking this is absurd and there must be some physical laws that can’t be broken. Well, don’t panic because there is a very logical method to this madness. In short, the reason for the divergent symmetry is physiological. While most bacteria are spheroidal or rod-shaped, spirochetes are extremely long and thin—they look like tiny worms. Because of their wormy shape, the spirochete cell membrane is highly curved in one direction (perpendicular to the cell axis) but nearly flat in the other (parallel to the cell axis). In accordance with this feature, we see that the two-fold axis of the chemotaxis array follows the cell axis. To put it plainly, the two-fold symmetry allows specific protein-protein interactions to occur with the least amount of curvature and strain as possible by following the path of least membrane curvature. In other organisms, cell curvature is fairly consistent over the entire surface, so distinctions in orientation are unnecessary.

This is not the only unusual feature of the spirochete chemotaxis system. Through bioinformatics (Davi Ortega, California Institute of Technology), genetics (Chris Li and Kurni Kurniyati, Philips Research Institute for Oral Health), biochemistry (Brian Crane and Zach Maschmann, Cornell University) and crystallography, our team was able to see that the spirochete chemotaxis proteins possess atypical features that likely help assemble an extended array in a highly curved membrane. Collectively, these results illustrate a transmembrane system that has specifically evolved to suit a physiological feature.

Since all previous kinetic and biochemical experiments with chemotaxis proteins assume a six-fold symmetry, it’s important to question how a two-fold symmetry apparatus functions. This finding also leaves open the possibility for additional array symmetries, or mixed symmetryarrangements.’

Science Signalling

Simultaneously with the Nature Communications’ paper, the Briegel Lab had another publication in a renowned journal about the bacterial chemotaxis system. In Science Signalling, the team answers questions on the detailed structural changes of the chemotaxis system using chemotaxis proteins from E.coli and Thermotoga Maritima.