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Looking at bacterial cell walls with NMR

​We have succeeded in observing by NMR the wall of living bacterial cells. They were able to significantly increase the sensitivity of the NMR signal by a so-called hyperpolarization technique. In the presence of cells, the polarizing agent binds to the polymers of the wall and, under microwave irradiation, selectively amplifies the response of neighboring atomic nuclei. Such analyzes could contribute in particular to the development of new antibiotics.

Published on 29 May 2013
The cell wall of bacteria is composed of several biopolymers building a shell around the cell, which allows recognition and adhesion to hosts, as well as regulation of other important cellular functions. A better understanding of the complex cell wall structure and interactions may help scientist to develop new antibiotics. Solid-state nuclear magnetic resonance (NMR) offers an interesting tool to look at this huge and complex molecular system in intact conditions, as it is principally not limited by the size of the object and do not require any crystallization. However, it suffers from its intrinsic low sensitivity, especially for the case of cell wall studies in living cells. Using high-field dynamic nuclear polarization (DNP), an emerging technique aimed at improving sensitivity of NMR, researchers of Inac demonstrated in collaboration with IBS that signal intensity from the bacterial cell wall can be 24-fold enhanced, opening the possibility of atomic-scale studies of cell-wall interactions.

To obtain this gain in sensitivity, the research teams of Inac and IBS mixed Bacillus subtilis bacterial cells with a biradical commonly used for DNP and called TOTAPOL, which has the ability to hyperpolarized nearby nuclei when properly irradiated with micowaves, boosting thus the NMR signal. This interdisciplinary team demonstrated that TOTAPOL molecules stick selectively to the cell-wall polymers. As the radical only polarizes nuclei in a certain proximity, this binding affinity allows the researches to enhance selectively signals from the cell wall in living cells, or in the opposite to suppress them by saturation, leaving only signals from inside the cell. In addition to this selectivity, they discover that the combination of spectra acquired at different radical concentrations increases the spectral resolution, one of the bottlenecks of DNP-enhanced solid-state NMR, by removing some broad signal components. This study opens new opportunity to investigate more generally cell surfaces or other compartments in living cells by adjusting the radical affinity to the polymers of interest.

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