Architecture and assembly of the Gram‐positive cell wall

Abstract

The bacterial cell wall is a mesh polymer of peptidoglycan – linear glycan strands crosslinked by flexible peptides – that determines cell shape and provides physical protection. While the glycan strands in thin “Gram-negative” peptidoglycan are known to run circumferentially around the cell, the architecture of the thicker “Gram-positive” form remains unclear. Using electron cryotomography, here we show that Bacillus subtilis peptidoglycan is a uniformly dense layer with a textured surface. We further show it rips circumferentially, curls and thickens at free edges, and extends longitudinally when denatured. Molecular dynamics simulations show that only atomic models based on the circumferential topology recapitulate the observed curling and thickening, in support of an “inside-to-outside” assembly process. We conclude that instead of being perpendicular to the cell surface or wrapped in coiled cables (two alternative models), the glycan strands in Gram-positive cell walls run circumferentially around the cell just as they do in Gram-negative cells. Together with providing insights into the architecture of the ultimate determinant of cell shape, this study is important because Gram-positive peptidoglycan is an important antibiotic target crucial to the viability of several important rod-shaped pathogens including Bacillus anthracis, Listeria monocytogenes, and Clostridium difficile.

Coarse-grained simulations of bacterial cell wall growth reveal that local coordination alone can be sufficient to maintain rod shape

“…The development of all-atom force fields for PG (6,36) allowed us to match the mechanical properties of the coarse-grained model to those exhibited by all-atom MD simulations. To calculate the stiffness of glycan, a fully solvated system of a 160-disaccharide strand without stem peptides was equilibrated for 6.6 ns using the software NAMD (37) (Fig.…”

Staying in Shape: the Impact of Cell Shape on Bacterial Survival in Diverse Environments

“…Gram-positive bacteria lack an outer membrane and have a thicker PG layer that is exposed to the external environment and includes additional structural constituents, such as wall teichoic acids and mycolic acids (15). Recent work suggests that even though the Gram-positive cell wall has multiple layers, it has a similar organization of circumferentially oriented glycan strands, with newly synthesized material incorporated on the inner face (16). PG is thought to be the structural determinant of cell shape because purified PG sacculi retain the shape of the cell from which they are isolated, be it coccoid or a straight, curved, or helical rod (17)(18)(19).…”

Getting into shape: How do rod-like bacteria control their geometry?

“…In Gram-positive bacteria the cell wall is much thicker than in Gram-negatives (e.g., in B. subtilis the cell wall approximately 30 nm thick (Beeby et al 2013;Misra et al 2013). Recent electron cryotomography and surface AFM experiments have revealed circumferential furrows in the cell-wall surface (Beeby et al 2013;Andre et al 2010;Hayhurst et al 2008) with a spacing of roughly 50 nm.…”

“…Recent electron cryotomography and surface AFM experiments have revealed circumferential furrows in the cell-wall surface (Beeby et al 2013;Andre et al 2010;Hayhurst et al 2008) with a spacing of roughly 50 nm. While this observation is in agreement with the model of circumferential glycan strands it also suggests a higherorder three-dimensional structure, which is not understood yet.…”