Ultrastructural details of resistance factors of the bacterial spore revealed by in situ cryo-electron tomography

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Abstract
The bacterial spore owes its incredible resistance capacities to various molecular structures that protect the cell content from external aggressions. Among the determinants of resistance are the quaternary structure of the chromosome and an extracellular shell made of proteinaceous layers (the coat), the assembly of which remains poorly understood. Here, in situ cryo-electron tomography (cryo-ET) on bacteria lamellae generated by cryo-focused ion beam micromachining (cryo-FIBM) provides insights into the ultrastructural organization of Bacillus subtilis sporangia, including that of the DNA and nascent coat layers. Analysis of the reconstructed tomograms reveal that rather early during sporulation, the chromosome in the developing spore (the forespore) adopts a toroidal structure harboring 5.5-nm thick fibers. At the same stage, coat proteins at the surface of the forespore form a complex stack of amorphous or structured layers with distinct electron density, dimensions and organization. We investigated the nature of the nascent coat layers in various mutant strains using cryo-FIBM/ET and transmission electron microscopy on resin sections of freeze-substituted bacteria. Combining these two cellular electron microscopy approaches, we distinguish seven nascent coat regions with different molecular properties, and propose a model for the contribution of the morphogenetic proteins SpoIVA, SpoVID, SafA and/or CotE. Significance statement: Bacterial spores are dormant cells that can resist to multiple stresses, including antibiotics, detergents, irradiation and high temperatures. Such resilience is an asset when spores are used for the benefit of humans, as in the case of probiotics, or a major problem for public health, food safety or biowarfare when it comes to spores of pathogenic bacteria. In this study, we combined state-of-the-art cryo-electron tomography and conventional cellular electron microscopy to provide insights into intermediate stages of spore development. Our data reveal the intracellular reorganization of the chromosome into a toroidal fibrillar structure and the complex assembly of the multi-protein, multilayered extracellular coat, shedding light on the mechanisms by which the spore acquires its incredible resistance capacities.

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