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Theory of Intelligent Design, the best explanation of Origins » Photosynthesis, Protozoans,Plants and Bacterias » Bacterial motility and behavior

Bacterial motility and behavior

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1 Bacterial motility and behavior on Sun Mar 02, 2014 7:31 pm


Gliding Cyanobacterial filaments

A thin slime sheath, external to the oscillin layer, encloses the cell or filament. Cyanobacteria may be
immotile, or they move by gliding motility. A good example of this is seen in the Oscillatoria filament. These
filaments glide along a solid surface, leaving the exuded slime sheet behind them as a collapsed tube as they go.
The slime sheath is continuously secreted during these movements. Filaments will also glide vertically within their
slime sheaths, an important adaptation in some forms for altering their height above the surface. One of the chief
reasons why some bacteria have evolved into filaments of cells is to increase their length, allowing them to reach
above the stagnant boundary layer. Gliding also helps cyanobacteria to reach optimal lighting levels for photosynthesis.

junctional pores and slime jets                            
Gliding Cyanobacterial filaments appear to
be driven by jet propulsion! In oscillatoria,
for example, there are rows of pores on
either side of the annular groove which
demarcates one cell from a neighbouring
cell. Slime jets from these pores in a
controlled manner, propelling the filaments
in one direction or the other. The oscillin
channels the slime jets in a helix around the
cell, causing the filament to rotate on its
axis as it glides. The slime eventually forms
a collapsed tube trailing behind the
filament, as secretion of new slime
continues. The slime is not too expensive to
produce, as it consists of polysaccharides
9chains of sugar molecules) that expand
massively upon mixing with water. earlier
authors claimed that slime production was a
consequence of and not the course of
motility, and controversy still surrounds the

In Oscillatoria the cross-walls, which divide one cell from the next in the filament, consist of peptidoglycan lined by
the inner membrane of each cell on each side. The outer membrane, S-layer and oscillin layer do not extend into
the cross-walls, but instead form continuous layers along the whole filament. This means that the periplasm (the
'space' between the inner and outer membranes is continuous along the filament and may act as a channel for
communication from one cell to the next. However, in at least some cases tiny pores, called
microplasmodesmata, have been seen to span the dividing cross-walls, connectying the cytoplasms of
neighbouring cells together. These junctions may function as electrical contacts, allowing electrochamical signals
(e.g. hydrogen or calcium ions) to diffuse from one cell to the next, enabling the cells to communicate via
electrical signals and so synchronsie their activity. This is important if the filament is to move in a given direction -
each cell needs to 'know' which direction to move in.

Motility mechanism: Direct observation of mucilage secretion (Hoiczyk and Baumeister 1998) during gliding motility of unbranched trichomes, including mucilage-secreting pores and mucilage trails, along with electron micrographs of the pores at trichome cell junctions, enabled a model in which helical proteins in the outer cyanobacterial wall control secretion of mucilage to propel the trichome reversibly, with a reversing rotation and translocation. How all the cells in the trichome are communicating to act synergically (to propel reversibly) is not explained. Also, trichome bending may require a different model. How external stimuli (light, temperature, solutes) are sensed by the motility system is another series of questions.

Bacterial motility and behavior

Escherichia coli. Swarming is a common yet specialized form of surface translocation exhibited by flagellated bacteria, distinct from swimming. When grown on a moist nutrient-rich surface, cells differentiate from a vegetative to a swarm state: they elongate, make more flagella, secrete wetting agents, and move across the surface in coordinated packs. In our lab, we focus on the mechanics of bacterial swarming, as exhibited by the model organism Escherichia coli. The chemotaxis system is thought not to be required. At the edge of the swarm E. coli forms a monolayer where cells move in packs. More centrally, closer to the point of inoculation, cells pile up in multilayers and move in active swirls.

How do cells in E. coli swarms move across an agar surface? Nick Darnton and Linda Turner sought to answer this question by performing a global analysis of videotaped data (of phase-contrast images) collected from 5 regions of 2 swarms, plotting body lengths, speeds, propulsion angles, local track curvatures, and temporal and spatial correlations, finding that cells reorient on the time scale of a few tenths of a second, primarily by colliding with one another.

What are their flagella doing? Using flourescent labeling of flagella Linda found that most of the time, swarm cells are driven forwards by a flagellar bundle in the usual way. Flagellar filaments from different cells can intertwine and form common bundles, but this is rare. However, cells in swarms do something not ordinarily seen with swimming cells: they back up. They do this without changing the orientation of the cell body by moving back through the middle of the flagellar bundle. This involves changes in filament shape (in polymorphic form), from normal, to curly, and back to normal. The changes observed with swarm cells are driven by application of torque, i.e. when motors switch from CCW to CW. When swimming cells tumble, polymorphic transformations also occur, in the order normal, semi-coiled, curly, and back to normal. But we rarely see the semi-coiled form with cells in swarms, and when it appears it is quite transient. We wonder whether polymorphic transformations evolved to enable cells to escape when trapped in confined environments, when the only way out is to back up, keeping the filaments close to the sides of the cell body.

Rongjing Zhang found that the upper surface of a swarm is stationary. If small particles, i.e. smoke particles, are put on the top of the swarm then visualized in dark-field, the particles diffuse locally but are not perturbed by the cells swarming. We think the surface of the swarm is covered by a surfactant monolayer pinned at its edges.

Yilin Wu studied fluid motions ahead of swarms and within swarms. Uptake of fluid from the underlying agar appears to be driven by cell growth, presumably through local perturbations of osmolarity, a hypothesis currently under test.

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2 Re: Bacterial motility and behavior on Tue Mar 04, 2014 7:59 pm


Bacterial Motility

Many but not all bacteria exhibit motility, i.e. self-propelled motion, under appropriate circumstances. Motion can be achieved by one of three mechanisms:

Most motile bacteria move by the use of flagella (singular, flagellum), rigid structures 20 nm in diameter and 15-20 µm long which protrude from the cell surface (e.g. Chromatium).
Spirochaetes are helical bacteria which have a specialized internal structure known as the axial filament which is responsible for rotation of the cell in a spiral fashion and consequent locomotion (e.g. Rhodospirillum).
Gliding bacteria all secrete copious slime, but the mechanism which propels the cells is not known (M.J. McBride Bacterial Gliding Motility: Multiple Mechanisms for Cell Movement over Surfaces. Annu. Rev. Microbiol. 2007. 55:49-75).

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3 Re: Bacterial motility and behavior on Tue Mar 04, 2014 8:17 pm


The surprisingly diverse ways that prokaryotes move

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