Cryo-FIB-tomography reveals the assembly of protective cellular structures during bacterial spore development

Bacterial spores are dormant cells that can resist to multiple stresses, including antibiotics, disinfectants, irradiation and high temperatures. Such resilience proves advantageous when spores are used for human benefit, as in probiotics, but it poses a major problem for public health, food safety or biowarfare when it comes to spores of pathogenic bacteria.

The spore owes its toughness to durable intracellular and extracellular assemblies that protect the cell and its genetic material, while keeping it receptive to its environment, so that it can germinate and resume vegetative growth under appropriate conditions. Two major molecular mechanisms required for the acquisition of resistance properties are the transition of the chromosome from a compact structure to a crystalline arrangement, and the deposition of a thick, multi-layered protein shell called the coat. The assembly of these protective structures remains enigmatic because they involve molecular complexes of nanometric dimensions that do not form or do not function properly outside the cellular environment.

Two teams led by Cecile Morlot and Guy Schoehn at the IBS, in collaboration with the ESRF’s CM01 facility (Grenoble, FR) and the CEITEC (Brno, CZ), have performed cryo-electron tomography on cell lamellae produced by cryo-FIB/SEM (Focused Ion Beam micromachining monitored by Scanning Electron Microscopy) to study the ultrastructure of the bacterial spore. In particular, this work has revealed that at intermediate stages of spore development, the chromosome is organized like a torus made of fibrils, and the coat proteins form a stack of amorphous or structured layers of distinct dimensions and composition.

Only a few cryo-electron tomography studies of sporulating bacterial cells have been reported so far and this work is the first one that addresses the ultrastructure of the spore chromosome and coat layers, laying foundations for the dissection of mechanisms involved in the resistance of the bacterial spore.

Ultrastructure of macromolecular assemblies contributing to bacterial spore resistance revealed by in situ cryo-electron tomography. Bauda E, Gallet B, Moravcova J, Effantin G, Chan H, Novacek J, Jouneau PH, Rodrigues CDA, Schoehn G, Moriscot C, Morlot C (2024). Nat. Commun. 15(1):1376. doi : 10.1038/s41467-024-45770-6.

Contact : Cécile Morlot (IBS/Pneumococcus Group)