Vortex beams of nonelementary particles The discovery of photon and electron vortex beams carrying orbital angular momentum (as a result of a twisting wave front) has led to appreciable advances in optical imaging, optical and electron microscopy, communications, quantum optics and micromanipulation, and more advances are expected. In an effort to extend this progress to other types of beams, Luski et al . demonstrate the production of vortex beams of helium atoms and dimers formed from supersonic beams with large coherence lengths diffracted off of specifically nanofabricated gratings with fork dislocations (see the Perspective by Kornilov). Vortex beams made of nonelementary particles with internal degrees of freedom represent a direct manifestation of quantum mechanics on macroscopic scale, and their production paves the way for many long-awaited applications. —YS

Bacterial pathogens inject virulence factors into host cells during bacterial infections using type III secretion systems. In enteropathogenic Escherichia coli, this system contains an external filament, formed by a self-oligomerizing protein called E. coli secreted protein A (EspA). The EspA filament penetrates the thick viscous mucus layer to facilitate the attachment of the bacteria to the gut-epithelium. To do that, the EspA filament requires noteworthy mechanical endurance considering the mechanical shear stresses found within the intestinal tract. To date, the mechanical properties of the EspA filament and the structural and biophysical knowledge of monomeric EspA are very limited, mostly due to the strong tendency of the protein to self-oligomerize. To overcome this limitation, we employed a single molecule force spectroscopy (SMFS) technique and studied the mechanical properties of EspA. Force extension dynamic of (I91)-EspA-(I91) chimera revealed two structural unfolding events occurring at low forces during EspA unfolding, thus indicating no unique mechanical stability of the monomeric protein. SMFS examination of purified monomeric EspA protein, treated by a gradually refolding protocol, exhibited similar mechanical properties as the EspA protein within the (I91)-EspA-(I91) chimera. Overall, our results suggest that the mechanical integrity of the EspA filament likely originates from the interactions between EspA monomers and not from the strength of an individual monomer.

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