181 research outputs found
Collective force generation by groups of migrating bacteria
From biofilm and colony formation in bacteria to wound healing and embryonic
development in multicellular organisms, groups of living cells must often move
collectively. While considerable study has probed the biophysical mechanisms of
how eukaryotic cells generate forces during migration, little such study has
been devoted to bacteria, in particular with regard to the question of how
bacteria generate and coordinate forces during collective motion. This question
is addressed here for the first time using traction force microscopy. We study
two distinct motility mechanisms of Myxococcus xanthus, namely twitching and
gliding. For twitching, powered by type-IV pilus retraction, we find that
individual cells exert local traction in small hotspots with forces on the
order of 50 pN. Twitching of bacterial groups also produces traction hotspots,
however with amplified forces around 100 pN. Although twitching groups migrate
slowly as a whole, traction fluctuates rapidly on timescales <1.5 min. Gliding,
the second motility mechanism, is driven by lateral transport of substrate
adhesions. When cells are isolated, gliding produces low average traction on
the order of 1 Pa. However, traction is amplified in groups by a factor of ~5.
Since advancing protrusions of gliding cells push on average in the direction
of motion, we infer a long-range compressive load sharing among sub-leading
cells. Together, these results show that the forces generated during twitching
and gliding have complementary characters and both forces are collectively
amplified in groups
Predictability and hierarchy in Drosophila behavior
Even the simplest of animals exhibit behavioral sequences with complex
temporal dynamics. Prominent amongst the proposed organizing principles for
these dynamics has been the idea of a hierarchy, wherein the movements an
animal makes can be understood as a set of nested sub-clusters. Although this
type of organization holds potential advantages in terms of motion control and
neural circuitry, measurements demonstrating this for an animal's entire
behavioral repertoire have been limited in scope and temporal complexity. Here,
we use a recently developed unsupervised technique to discover and track the
occurrence of all stereotyped behaviors performed by fruit flies moving in a
shallow arena. Calculating the optimally predictive representation of the fly's
future behaviors, we show that fly behavior exhibits multiple time scales and
is organized into a hierarchical structure that is indicative of its underlying
behavioral programs and its changing internal states
Long time scales, individual differences, and scale invariance in animal behavior
The explosion of data on animal behavior in more natural contexts highlights
the fact that these behaviors exhibit correlations across many time scales. But
there are major challenges in analyzing these data: records of behavior in
single animals have fewer independent samples than one might expect; in pooling
data from multiple animals, individual differences can mimic long-ranged
temporal correlations; conversely long-ranged correlations can lead to an
over-estimate of individual differences. We suggest an analysis scheme that
addresses these problems directly, apply this approach to data on the
spontaneous behavior of walking flies, and find evidence for scale invariant
correlations over nearly three decades in time, from seconds to one hour. Three
different measures of correlation are consistent with a single underlying
scaling field of dimension
Curvature and torsion in growing actin networks
Intracellular pathogens such as Listeria monocytogenes and Rickettsia
rickettsii move within a host cell by polymerizing a comet-tail of actin fibers
that ultimately pushes the cell forward. This dense network of cross-linked
actin polymers typically exhibits a striking curvature that causes bacteria to
move in gently looping paths. Theoretically, tail curvature has been linked to
details of motility by considering force and torque balances from a finite
number of polymerizing filaments. Here we track beads coated with a prokaryotic
activator of actin polymerization in three dimensions to directly quantify the
curvature and torsion of bead motility paths. We find that bead paths are more
likely to have low rather than high curvature at any given time. Furthermore,
path curvature changes very slowly in time, with an autocorrelation decay time
of 200 seconds. Paths with a small radius of curvature, therefore, remain so
for an extended period resulting in loops when confined to two dimensions. When
allowed to explore a 3D space, path loops are less evident. Finally, we
quantify the torsion in the bead paths and show that beads do not exhibit a
significant left- or right-handed bias to their motion in 3D. These results
suggest that paths of actin-propelled objects may be attributed to slow changes
in curvature rather than a fixed torque
Bayesian Estimation of the Axial Position in Astigmatism-Based Three-Dimensional Particle Tracking
Accurate estimation of the axial position of a molecule using a single lateral image remains a challenge in fluorescent single particle tracking. Here, a principled algorithm for the Bayesian estimation of the axial position of a molecule in three-dimensional astigmatism-based particle tracking is proposed. This technique uses the data from a calibration image set to derive the position without assuming a functional form for the abberated defocusing curve. Using a calibration image set from forty 57 nm beads, the axial position is calculated,
and the error associated with position estimation is discussed. This method is compared to previously published algorithms
Phase transitions during fruiting body formation in Myxococcus xanthus
The formation of a collectively moving group benefits individuals within a
population in a variety of ways such as ultra-sensitivity to perturbation,
collective modes of feeding, and protection from environmental stress. While
some collective groups use a single organizing principle, others can
dynamically shift the behavior of the group by modifying the interaction rules
at the individual level. The surface-dwelling bacterium Myxococcus xanthus
forms dynamic collective groups both to feed on prey and to aggregate during
times of starvation. The latter behavior, termed fruiting-body formation,
involves a complex, coordinated series of density changes that ultimately lead
to three-dimensional aggregates comprising hundreds of thousands of cells and
spores. This multi-step developmental process most likely involves several
different single-celled behaviors as the population condenses from a loose,
two-dimensional sheet to a three-dimensional mound. Here, we use
high-resolution microscopy and computer vision software to spatiotemporally
track the motion of thousands of individuals during the initial stages of
fruiting body formation. We find that a combination of cell-contact-mediated
alignment and internal timing mechanisms drive a phase transition from
exploratory flocking, in which cell groups move rapidly and coherently over
long distances, to a reversal-mediated localization into streams, which act as
slow-spreading, quasi-one-dimensional nematic fluids. These observations lead
us to an active liquid crystal description of the myxobacterial development
cycle.Comment: 16 pages, 5 figure
Bayesian Estimation of the Axial Position in Astigmatism-Based Three-Dimensional Particle Tracking
Accurate estimation of the axial position of a molecule using a single lateral image remains a challenge in fluorescent single particle tracking. Here, a principled algorithm for the Bayesian estimation of the axial position of a molecule in three-dimensional astigmatism-based particle tracking is proposed. This technique uses the data from a calibration image set to derive the position without assuming a functional form for the abberated defocusing curve. Using a calibration image set from forty 57 nm beads, the axial position is calculated, and the error associated with position estimation is discussed. This method is compared to previously published algorithms
- …