Bacterial spreading with randomly oriented motors
How can randomly oriented molecular motors generate cellular movement? We have addressed this question in the human pathogen Neisseria gonorrhoeae which employs grappling hooks called type IV pili for pulling itself over surfaces. We found that up to twenty randomly oriented pili generate persistent cellular movement and we suggest a tug-of-war mechanism for movement.
Many bacterial pathogens live at surfaces. On the one hand they must attach firmly to avoid clearance but on the other hand they must be motile to spread. The type IV pilus is a polymeric cell appendage that has solved this problem; it elongates through polymerization, attaches to the surface and generates force on the cell body by retraction.
In this paper we showed that pilus-mediated movement is persistent even though the orientation of pilus motors is random. Persistence increased with the number of pili per cell. To resolve this discrepancy, we demonstrated that the force at which a single pilus detached from the surface was 10 times smaller than the motor force. We suggest a tug-of-war mechanism, in which the winning motors pull the opposing motors off the surface thereby generating directed movement.
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LM12636
First Experimental Demonstration of Selective and Efficient Quantum
Evolution Characterization
The characterization of the temporal evolution of quantum systems is
not only one of the most important tasks in physics, it is also one of
the most difficult ones. In fact, the implementation of 'Quantum
Process Tomography' (the technical name under which the above task is
known) requires resources that scale exponentially with the size of
the system, making it practically impossible for large systems.
Moreover even extracting partial useful information about the
evolution also turns out to be exponentially hard. Quantum Process
Tomography is essential to achieve quantum information processing and
quantum computation (as it is required to device appropriate quantum
error correcting strategies to protect quantum information from
natural degradation inducing decoherence). Only recently methods have
been developed which can extract useful partial information about
quantum processes efficiently for systems of any size. In this paper
the authors present the first experimental implementation of a quantum
algorithm of this kind. The experiment, performed at a quantum optics
laboratory recently set up in Buenos Aires (Argentina) involves the
manipulation and detection of single photons where quantum information
is encoded both in the polarization and momentum degree of freedom.
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LL11804
Bacteria stampede
Who gets trapped in the burning building: the stampeding crowd or the calm individual? The answer might be obvious if you're dealing with human beings, but we show that in the microscopic world of bacteria, it pays to stampede.
We create a microchannel no wider than a few human hairs, but more than one centimeter long, and fill it with about one hundred fish-trap like barriers that are difficult for incoming E. coli bacteria to cross. Although E. coli bacteria can migrate great distances, it is virtually impossible for a single cell to cross all 100 fish traps in sequence. If we add food on the other side of each barrier, however, something interesting happens: several hundred cells team up and collectively swim against all the barriers.
We show that the bacteria achieve collective escape because they are attracted by the food: if we allow the cells to deplete the nutrients, they no longer defeat the fish traps. This behavior also depends on the number of cells present: too few bacteria in the initial injection means they lack the critical density they need to escape the burning building and reach the banquet next door.
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