Structure of sticky-hard-sphere random aggregates: The viewpoint of contact coordination and tetrahedra

International audienceWe study more than 10 4 random aggregates of 10 6 monodisperse sticky hard spheres each, generated by various static algorithms. Their packing fraction varies from 0.370 up to 0.593. These aggregates are shown to be based on two types of disordered structures: random regular polytetrahedra and random aggregates, the former giving rise to δ peaks on pair distribution functions. Distortion of structural (Delaunay) tetrahedra is studied by two parameters, which show some similarities and some differences in terms of overall tendencies. Isotropy of aggregates is characterized by the nematic order parameter. The overall structure is then studied by distinguishing spheres in function of their contact coordination number (CCN). Distributions of average CCN around spheres of a given CCN value show trends that depend on packing fraction and building algorithms. The radial dependence of the average CCN turns out to be dependent upon the CCN of the central sphere and shows discontinuities that resemble those of the pair distribution function. Moreover, it is shown that structural details appear when the CCN is used as pseudochemical parameter, such as various angular distribution of bond angles, partial pair distribution functions, Ashcroft-Langreth and Bhatia-Thornton partial structure factors. These allow distinguishing aggregates with the same values of packing fraction or average tetrahedral distortion or even similar global pair distribution function, indicative of the great interest of paying attention to contact coordination numbers to study more precisely the structure of random aggregates

Contribution of the FeoB transporter to Streptococcus suis virulence

The contribution of iron transporter systems encoded by feo genes to the pathogenic traits of streptococci is largely unknown, despite the fact that those systems are required for the full virulence of several gram-negative bacterial species. In this work, we show that the swine pathogen and zoonotic agent Streptococcus suis has a feoAB operon similar to that encoding an iron transporter system in Escherichia coli. Electrophoretic mobility assays and transcriptional analyses confirmed that the expression of S. suis feo genes is under the negative control of the ferric uptake regulator (Fur) protein. In vivo trials in mice using a feoB defective mutant strain were carried out to investigate the contribution of this gene to the virulence of S. suis. The results showed that the median lethal dose (LD50) of the mutant was approximately 10-fold higher than that of the wild-type parent strain. These data suggest that the Feo metal transporter plays a significant role in streptococcal infectious disease. This is in contrast to previous results reported for this same gene in other gram-positive bacterial species. [Int Microbiol 2009; 12(2):137-141

Plant-associated microbiota as a source of antagonistic bacteria against the phytopathogen Erwinia amylovora

Control of bacterial plant diseases is a major concern, as they affect economically important species and spread easily, such as the case of fire blight of rosaceous caused by Erwinia amylovora. In the search for alternatives to the use of agrochemicals and antibiotics, this work presents a screening of natural bacterial antagonists of this relevant and devastating phytopathogen. We recovered bacterial isolates from different plant tissues and geographical origins and then selected those with the strongest ability to reduce fire blight symptoms ex vivo and remarkable in vitro antagonistic activity against E. amylovora. None of them elicited a hypersensitivity reaction in tobacco leaves, most produced several hydrolytic enzymes and presented other biocontrol and/or plant growth-promoting activities, such as siderophore production and phosphate solubilization. These isolates, considered as biocontrol candidates, were identified by 16S rRNA sequencing as Pseudomonas rhizosphaerae, Curtobacterium flaccumfaciens, Enterobacter cancerogenus, Pseudomonas azotoformans, Rosenbergiella epipactidis and Serratia plymuthica. This is the first time that the last five bacterial species are reported to have biocontrol potential against E. amylovora

Classical Limit of Demagnetization in a Field Gradient

We calculate the rate of decrease of the expectation value of the transverse component of spin for spin-1/2 particles in a magnetic field with a spatial gradient, to determine the conditions under which a previous classical description is valid. A density matrix treatment is required for two reasons. The first arises because the particles initially are not in a pure state due to thermal motion. The second reason is that each particle interacts with the magnetic field and the other particles, with the latter taken to be via a 2-body central force. The equations for the 1-body Wigner distribution functions are written in a general manner, and the places where quantum mechanical effects can play a role are identified. One that may not have been considered previously concerns the momentum associated with the magnetic field gradient, which is proportional to the time integral of the gradient. Its relative magnitude compared with the important momenta in the problem is a significant parameter, and if their ratio is not small some non-classical effects contribute to the solution. Assuming the field gradient is sufficiently small, and a number of other inequalities are satisfied involving the mean wavelength, range of the force, and the mean separation between particles, we solve the integro- partial differential equations for the Wigner functions to second order in the strength of the gradient. When the same reasoning is applied to a different problem with no field gradient, but having instead a gradient to the z-component of polarization, the connection with the diffusion coefficient is established, and we find agreement with the classical result for the rate of decrease of the transverse component of magnetization.Comment: 22 pages, no figure