Dynamics Characterization of Fully Hydrated Bacterial Cell Walls by Solid-State NMR: Evidence for Cooperative Binding of Metal Ions

The bacterial cell wall maintains a cell's integrity while allowing growth and division. It is made up of peptidoglycan (PG), a biopolymer forming a multigigadalton bag-like structure, and, additionally in Gram-positive bacteria, of covalently linked anionic polymers collectively called teichoic acids. These anionic polymers are thought to play important roles in host-cell adhesion, inflammation, and immune activation. In this Article, we compare the flexibility and the organization of peptidoglycans from Gram-negative bacteria (E. coli) with its counterpart from different Gram-positive bacteria using solid-state nuclear magnetic resonance spectroscopy (NMR) under magic-angle sample spinning (MAS). The NMR fingerprints suggest an identical local conformation of the PG in all of these bacterial species. Dynamics in the peptidoglycan network decreases from E. coli to B. subtilis and from B. subtilis to S. aureus and correlates mainly with the degree of peptide cross-linkage. For intact bacterial cells and isolated cell walls, we show that P-31 solid-state NMR is particularly well adapted to characterize and differentiate wall teichoic acids of different species. We have further observed complexation with divalent ions, highlighting an important structural aspect of Gram-positive cell wall architecture. We propose a new model for the interaction of divalent cations with both wall teichoic acids and carbonyl groups of peptidoglycan.