No abstract

Bacteria of the Bacillus cereus family form highly resistant spores, which in the case of the pathogen B. anthracis act as the agents of infection. The outermost layer, the exosporium, enveloping spores of the B. cereus family as well as a number of Clostridia, plays roles in spore adhesion, dissemination, targeting, and germination control. We have analyzed two naturally crystalline layers associated with the exosporium, one representing the "basal" layer to which the outermost spore layer ("hairy nap") is attached, and the other likely representing a subsurface ("parasporal") layer. We have used electron cryomicroscopy at a resolution of 0.8-0.6 nm and circular dichroism spectroscopic measurements to reveal a highly α-helical structure for both layers. The helices are assembled into 2D arrays of "cups" or "crowns." High-resolution atomic force microscopy of the outermost layer showed that the open ends of these cups face the external environment and the highly immunogenic collagen-like fibrils of the hairy nap (BclA) are attached to this surface. Based on our findings, we present a molecular model for the spore surface and propose how this surface can act as a semipermeable barrier and a matrix for binding of molecules involved in defense, germination control, and other interactions of the spore with the environment.electron crystallography | cryoelectron microscopy | two-dimensional crystal

The physical properties of semicrystalline polymers depend on the organisation of chains within the crystal and amorphous regions, on the interface between the two, and on the location and nature of defects. Here, torsional tapping atomic force microscopy has been used to image crystalline lamellae and the crystal-amorphous-region interface at the single-chain level with resolution down to 3.7 Å. Defects within the crystalline phase, such as buried folds and chain ends, are revealed. Imaging at the chain level also allows direct measurement of crystalline stem lengths, providing a potential route to test theories of crystal thickness selection.

Torsional-tapping AFM with supersharp carbonwhisker tips is used to explore the molecular conformations at the surface of a semicrystalline polymer. Images of the crystal− amorphous interface of oriented polyethylene have allowed us to measure hitherto inaccessible parameters that can be directly compared to polymer crystallization theories and molecular simulations, such as the length of stem-to-stem overhang. It has also been possible to identify both first-and second-neighbor folds and to determine the surface roughness of lamellae which we find approximately doubles the interfacial area. Finally, we calculate the interfacial density profile from the images and find it to be sigmoidal but narrower than values reported by SAXS measurements.

The nanoscale mechanical properties of live Staphylococcus aureus cells during different phases of growth were studied by atomic force microscopy. Indentation to different depths provided access to both local cell wall mechanical properties and whole-cell properties, including a component related to cell turgor pressure. Local cell wall properties were found to change in a characteristic manner throughout the division cycle. Splitting of the cell into two daughter cells followed a local softening of the cell wall along the division circumference, with the cell wall on either side of the division circumference becoming stiffer. Once exposed, the newly formed septum was found to be stiffer than the surrounding, older cell wall. Deeper indentations, which were affected by cell turgor pressure, did not show a change in stiffness throughout the division cycle, implying that enzymatic cell wall remodeling and local variations in wall properties are responsible for the evolution of cell shape through division.