11 research outputs found
High-speed motility originates from cooperatively pushing and pulling flagella bundles in bilophotrichous bacteria.
Funder: Max-Planck-Gesellschaft; FundRef: http://dx.doi.org/10.13039/501100004189Funder: IMPRS on Multiscale BiosystemsFunder: French National Research Agency; FundRef: http://dx.doi.org/10.13039/501100001665; Grant(s): ANR Tremplin-ERC: ANR-16-TERC-0025-01Bacteria propel and change direction by rotating long, helical filaments, called flagella. The number of flagella, their arrangement on the cell body and their sense of rotation hypothetically determine the locomotion characteristics of a species. The movement of the most rapid microorganisms has in particular remained unexplored because of additional experimental limitations. We show that magnetotactic cocci with two flagella bundles on one pole swim faster than 500 µm·s-1 along a double helical path, making them one of the fastest natural microswimmers. We additionally reveal that the cells reorient in less than 5 ms, an order of magnitude faster than reported so far for any other bacteria. Using hydrodynamic modeling, we demonstrate that a mode where a pushing and a pulling bundle cooperate is the only possibility to enable both helical tracks and fast reorientations. The advantage of sheathed flagella bundles is the high rigidity, making high swimming speeds possible
Chiral Structure of F-actin Bundle Formed by Multivalent Counterions?
The mechanism of multivalent counterion-induced bundle formation by
filamentous actin (F-actin) is studied using a coarse-grained model and
molecular dynamics simulation. Real diameter size, helically ordered charge
distribution and twist rigidity of F-actin are taken into account in our model.
The attraction between parallel F-actins induced by multivalent counterions is
studied in detail and it is found that the maximum attraction occurs between
their closest charged domains. The model F-actins aggregate due to the
like-charge attraction and form closely packed bundles. Counterions are mostly
distributed in the narrowest gaps between neighboring F-actins inside the
bundles and the channels between three adjacent F-actins correspond to low
density of the counterions. Density of the counterions varies periodically with
a wave length comparable to the separation between consecutive G-actin monomers
along the actin polymers. Long-lived defects in the hexagonal order of F-actins
in the bundles are observed that their number increases with increasing the
bundles size. Combination of electrostatic interactions and twist rigidity has
been found not to change the symmetry of F-actin helical conformation from the
native 13/6 symmetry. Calculation of zero-temperature energy of hexagonally
ordered model F-actins with the charge of the counterions distributed as
columns of charge domains representing counterion charge density waves has
shown that helical symmetries commensurate with the hexagonal lattice
correspond to local minima of the energy of the system. The global minimum of
energy corresponds to 24/11 symmetry with the columns of charge domains
arranged in the narrowest gaps between the neighboring F-actins.Comment: 9 pages, 10 figures, Published online in Soft Matter journal:
http://pubs.rsc.org/en/content/articlelanding/2012/sm/c2sm07104
Orientationally ordered aggregates of stiff polyelectrolytes in the presence of multivalent salt
Aggregation of stiff polyelectrolytes in solution and angle- and
distance-dependent potential of mean force between two like-charged rods are
studied in the presence of 3-valent salt using molecular dynamics simulations.
In the bulk solution, formation of long-lived metastable structures with
similarities to the raft-like structures of actin filaments is observed within
a range of salt concentration. The system finally goes to a state with lower
free energy in which finite-sized bundles of parallel polyelectrolytes form.
Preferred angle and interaction type between two like-charged rods at different
separations and salt concentrations are also studied, which shed some light on
the formation of orientationally ordered structures.Comment: 18 pages, 8 figures, accepted for publication in Soft Matte
Stokesian dynamics simulations of a magnetotactic bacterium
International audienceThe swimming of bacteria provides insight into propulsion and steering under the conditions of low-Reynolds number hydrodynamics. Here we address the magnetically steered swimming of magnetotactic bacteria. We use Stokesian dynamics simulations to study the swimming of single-flagellated magnetotactic bacteria (MTB) in an external magnetic field. Our model MTB consists of a spherical cell body equipped with a magnetic dipole moment and a helical flagellum rotated by a rotary motor. The elasticity of the flagellum as well as magnetic and hydrodynamic interactions is taken into account in this model. We characterized how the swimming velocity is dependent on parameters of the model. We then studied the U-turn motion after a field reversal and found two regimes for weak and strong fields and, correspondingly, two characteristic time scales. In the two regimes, the U-turn time is dominated by the turning of the cell body and its magnetic moment or the turning of the flagellum, respectively. In the regime for weak fields, where turning is dominated by the magnetic relaxation, the U-turn time is approximately in agreement with a theoretical model based on torque balance. In the strong-field regime, strong deformations of the flagellum are observed. We further simulated the swimming of a bacterium with a magnetic moment that is inclined relative to the flagellar axis. This scenario leads to intriguing double helical trajectories that we characterize as functions of the magnetic moment inclination and the magnetic field. For small inclination angles ( ≲ 20 ∘ ) and typical field strengths, the inclination of the magnetic moment has only a minor effect on the swimming of MTB in an external magnetic field. Large inclination angles result in a strong reduction in the velocity in direction of the magnetic field, consistent with recent observations that bacteria with large inclination angles use a different propulsion mechanism. Graphic abstrac
Impact of flexibility on the aggregation of polymeric macromolecules
Dependence of the dimerization probability and the aggregation behavior of polymeric macromolecules on their flexibility is studied using Langevin dynamics simulations. It is found that the dimerization probability is a non-monotonic function of the polymers persistence length. For a given value of inter-polymer attraction strength, semiflexible polymers have lower dimerization probability relative to flexible and rigid polymers of the same length. The threshold temperature of the formation of aggregates in a many-polymer system and its dependence on the polymers persistence length is also investigated. The simulation results of two- and many-polymer systems are in good agreement and show how the amount of flexibility affects the dimerization and the aggregation behaviors of polymeric macromolecules