43 research outputs found
Modeling the Role of the Cell Cycle in Regulating Proteus mirabilis Swarm-Colony Development
We present models and computational results which indicate that the spatial
and temporal regularity seen in Proteus mirabilis swarm-colony development is
largely an expression of a sharp age of dedifferentiation in the cell cycle
from motile swarmer cells to immotile dividing cells (also called swimmer or
vegetative cells.) This contrasts strongly with reaction-diffusion models of
Proteus behavior that ignore or average out the age structure of the cell
population and instead use only density-dependent mechanisms. We argue the
necessity of retaining the explicit age structure, and suggest experiments that
may help determine the underlying mechanisms empirically. Consequently, we
advocate Proteus as a model organism for a multiscale understanding of how and
to what extent the life cycle of individual cells affects the macroscopic
behavior of a biological system
The three- and four-nucleon systems from chiral effective field theory
Recently developed chiral nucleon-nucleon (NN) forces at next-to-leading
order (NLO) that describe NN phase shifts up to about 100 MeV fairly well have
been applied to 3N and 4N systems. Faddeev-Yakubovsky equations have been
solved rigorously. The chiral NLO forces depend on a momentum cut-off \Lambda
lying between 540-600 MeV/c. The resulting 3N and 4N binding energies are in
the same range as found using standard NN potentials. In additon, low-energy 3N
scattering observables are very well reproduced like for standard NN forces.
Surprisingly, the long standing A_y-puzzle is resolved at NLO. The cut-off
dependence of the scattering observables is rather mild.Comment: 4 pp, revtex, 3 figure
N-d scattering above the deuteron breakup threshold
The complex Kohn variational principle and the (correlated) Hyperspherical
Harmonics technique are applied to study the N--d scattering above the deuteron
breakup threshold. The configuration with three outgoing nucleons is explicitly
taken into account by solving a set of differential equations with outgoing
boundary conditions. A convenient procedure is used to obtain the correct
boundary conditions at values of the hyperradius fm. The
inclusion of the Coulomb potential is straightforward and does not give
additional difficulties. Numerical results have been obtained for a simple
s-wave central potential. They are in nice agreement with the benchmarks
produced by different groups using the Faddeev technique. Comparisons are also
done with experimental elastic N--d cross section at several energies.Comment: LaTeX, 13 pages, 3 figure
Three-Nucleon Forces from Chiral Effective Field Theory
We perform the first complete analysis of nd scattering at
next-to-next-to-leading order in chiral effective field theory including the
corresponding three-nucleon force and extending our previous work, where only
the two-nucleon interaction has been taken into account. The three-nucleon
force appears first at this order in the chiral expansion and depends on two
unknown parameters. These two parameters are determined from the triton binding
energy and the nd doublet scattering length. We find an improved description of
various scattering observables in relation to the next-to-leading order results
especially at moderate energies (E_lab = 65 MeV). It is demonstrated that the
long-standing A_y-problem in nd elastic scattering is still not solved by the
leading 3NF, although some visible improvement is observed. We discuss
possibilities of solving this puzzle. The predicted binding energy for the
alpha-particle agrees with the empirical value.Comment: 36 pp, 20 figure
The cross section minima in elastic Nd scattering: a ``smoking gun'' for three nucleon force effects
Neutron-deuteron elastic scattering cross sections are calculated at
different energies using modern nucleon-nucleon interactions and the
Tucson-Melbourne three-nucleon force adjusted to the triton binding energy.
Predictions based on NN forces only underestimate nucleon-deuteron data in the
minima at higher energies starting around 60 MeV. Adding the three-nucleon
forces fills up those minima and reduces the discrepancies significantly.Comment: 11 pages, 6 figure
Effect of exogenous surfactants on viability and DNA synthesis in A549, immortalized mouse type II and isolated rat alveolar type II cells
<p>Abstract</p> <p>Background</p> <p>In mechanically ventilated preterm infants with respiratory distress syndrome (RDS), exogenous surfactant application has been demonstrated both to decrease DNA-synthesis but also and paradoxically to increase epithelial cell proliferation. However, the effect of exogenous surfactant has not been studied directly on alveolar type II cells (ATII cells), a key cell type responsible for alveolar function and repair.</p> <p>Objective</p> <p>The aim of this study was to investigate the effects of two commercially available surfactant preparations on ATII cell viability and DNA synthesis.</p> <p>Methods</p> <p>Curosurf<sup>® </sup>and Alveofact<sup>® </sup>were applied to two ATII cell lines (human A549 and mouse iMATII cells) and to primary rat ATII cells for periods of up to 24 h. Cell viability was measured using the redox indicator resazurin and DNA synthesis was measured using BrdU incorporation.</p> <p>Results</p> <p>Curosurf<sup>® </sup>resulted in slightly decreased cell viability in all cell culture models. However, DNA synthesis was increased in A549 and rat ATII cells but decreased in iMATII cells. Alveofact<sup>® </sup>exhibited the opposite effects on A549 cells and had very mild effects on the other two cell models.</p> <p>Conclusion</p> <p>This study showed that commercially available exogenous surfactants used to treat preterm infants with RDS can have profound effects on cell viability and DNA synthesis.</p
Social Motility in African Trypanosomes
African trypanosomes are devastating human and animal pathogens that cause significant human mortality and limit economic development in sub-Saharan Africa. Studies of trypanosome biology generally consider these protozoan parasites as individual cells in suspension cultures or in animal models of infection. Here we report that the procyclic form of the African trypanosome Trypanosoma brucei engages in social behavior when cultivated on semisolid agarose surfaces. This behavior is characterized by trypanosomes assembling into multicellular communities that engage in polarized migrations across the agarose surface and cooperate to divert their movements in response to external signals. These cooperative movements are flagellum-mediated, since they do not occur in trypanin knockdown parasites that lack normal flagellum motility. We term this behavior social motility based on features shared with social motility and other types of surface-induced social behavior in bacteria. Social motility represents a novel and unexpected aspect of trypanosome biology and offers new paradigms for considering host-parasite interactions