Bacteria come in a surprising variety of shapes: the more commonly known are coccus, bacillus. But curved cell shapes are also widespread among bacteria and important for cellular motility, virulence, and fitness in their respective ecological niches. However, despite intensive research, the factors that determine the shape of bacterial cells remain, in many cases, unknown.

Curvature is crucial to the ability of bacteria to colonize surfaces and move in viscous environments—and also to cause disease, as is the case for Vibrio cholerae or Helicobacter pylori. A team of researchers has now discovered the mechanism that determines the spiral shape photosynthetic bacterium Rhodospirillum rubrum—species is widespread in the environment and has biotechnological potential because it can utilize carbon monoxide, fix nitrogen, and produce both hydrogen and building blocks for bioplasticsshedding new light on the link between cell shape and fitness.

The study has uncovered a novel mechanism of shape determination in bacteria that is based on the direct influence of outer membrane proteins on the spatial control of cell growth.

This work is published in Nature Communications in the paper, “An outer membrane porin-lipoprotein complex modulates elongasome movement to establish cell curvature in Rhodospirillum rubrum.

The researchers found that in Rhodospirillum, two porins—channel-like proteins that are known to be responsible for the exchange of nutrients across the outer membrane of bacteria—are arranged helically in the outer curvature of the cell. These structures are closely connected to the cell wall by another protein, the lipoprotein PapS. When PapS was missing, or when the researchers prevented it from binding to the porins, the cells became completely straight.

More specifically, the team showed that “porins Por39 and Por41 form a helical ribbon-like structure at the outer curve of the cell that recruits the peptidoglycan-binding lipoprotein PapS, with PapS inactivation, porin delocalization or disruption of the porin-PapS interface resulting in cell straightening.”

“The porins seem to have evolved to perform a second function apart from exchanging nutrients,” explains Martin Thanbichler, PhD, professor of microbiology, University of Marburg, Germany. “Together with PapS, they control the movement of a molecular machine that travels in circles around the cell body. This machine incorporates new material into the existing cell wall and thus leads to cell elongation. In rod-shaped bacteria such as E. coli, this machine moves uniformly in all areas of the cell, resulting in a straight shape. In R. rubrum, by contrast, the helical Porin-PapS structure forms a kind of molecular cage.”

The paper also notes that “porin-PapS assemblies act as molecular cages that entrap the cell elongation machinery, thus biasing cell growth towards the outer curve.”

Thanbichler continues: “Due to its dense packing, it surrounds the machinery that is normally responsible for the longitudinal cell growth and partially fixates it in the outer curve of the cell. This results in a local increase in cell elongation around the Porin-PapS structure, which ultimately bends the cell body into a spiral shape.”

The findings are likely to apply to all curved relatives of Rhodospirillum, and it will be exciting to see whether this mechanism is also used by other bacterial groups with more complex cell shapes.

“We now have the opportunity to modify the cell shape of R. rubrum,” says Sebastian Pöhl, PhD, a postdoc in the Thanbichler lab. “This will give us the opportunity to study the selective advantage of the helical cell shape for bacteria in their habitat.” This could provide important insights into how cell shape affects the colonization of ecological niches, the establishment of symbiotic interactions with plants, or the cause of disease.

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