Myxococcus cells respond to elastic forces in their substrate

Abstract

Elasticotaxis describes the ability of Myxococcus xanthus cells to sense and to respond to elastic forces within an agar gel on which they rest. Within 5 min of the application of stress, each cell begins to reorient its long axis perpendicular to the stress force. The cells then glide in that direction, and the swarm becomes asymmetric. A quantifiable assay for the strength of elasticotaxis is based on the change in swarm shape from circular to elliptic. By using a collection of isogenic motility mutants, it has been found that the ability to respond to stress in agar depends totally on adventurous (A) motility, but not at all on social (S) motility or on the frz genes. In fact, S ؊ mutants (which are moving only by means of A motility) respond to the applied stress more strongly than does the wild type, despite the fact that their spreading rates are slower than that of the wt strain. Based on the swarming and elasticotactic phenotypes of isogenic frizzy strains that were also defective either in A or S motility, frz behaves as if part of the S motility system.

Chemotaxis as an Emergent Property of a Swarm

“…S1 in the supplemental material). Because the nutritive disk is covered by nonnutritive agar, the assay surface is smooth, thus eliminating any possible swarm response to surface irregularities, such as elasticotaxis (11,13). To initiate a tracking assay, a swarm is created by spotting M. xanthus liquid culture onto the nonnutritive-agar surface approximately 1 mm from the edge of the nutritive disk.…”

“…Several physical properties are known to contribute to a swarm's context: the mass of a swarm that exerts stress on the agar substrate, which individual cells can sense and respond to through elasticotaxis (11,13); the exopolysaccharide matrix generated by the swarm, which cells require for normal motility (21,31); and the quorum sensing and cell-cell contact signals that require groups of cells in close proximity, which are known to alter individual cell behavior (21). Perhaps there is some subset of these properties that enables a single M. xanthus cell to direct its reversal frequency so that it moves up a gradient of nutrients.…”

“…This defined two equal areas of the swarm that were geometrically opposite, one being closest to the nutritive disk (leading edge) and the other being furthest from the nutritive disk (lagging edge). The swarm expands during the assay through a series of group translocations called flares (13), and symmetry breaking is detected by measuring the ratio of the furthest distance traveled by a flare on the leading edge to the furthest distance traveled by a flare on the lagging edge from T 0 to T n (flare length line), where n is the number of hours required for the first flare of the leading edge to reach the nutritive disk, with a maximum T n of 24 h (Fig. 1C).…”

Aggregation during Fruiting Body Formation in Myxococcus xanthus Is Driven by Reducing Cell Movement

“…Several speculative mechanisms have been proposed to describe cell movements leading to fruiting body formation. For example, elasticotaxis, wherein cells follow stress line cues in the substratum, was suggested as a possible mechanism that can direct cells to aggregate into fruiting bodies (3). Alternatively, it has been suggested that streaming can lead to fruiting body formation, since M. xanthus cells frequently form networks of aligned cells that gather into streams, the reversal frequency of cells in streams is reduced, and aggregation centers form where the streams intersect (8,19).…”

Dissipative shocks behind bacteria gliding

“…The theory developed here identifies with W 0 the rate at which such an active energy is produced to sustain the bacterium locomotion. In the bacterium rest frame, where the kinetic energy of the slime filament extruded between pore and foot is constant in time, balance of energy requires thatK 22) where K is the extra kinetic energy associated with the sliding motion of the filament on the substrate. In (3.22), W 0 is the power supply at the bacterium pore, given by (2.6), W s is the concentrated power supply involved in the dissipative internal shock at the filament's foot, given by (2.7), W g is the power expended by the gravitational forces and τ + v is the power expended by the free end of the adhered filament (seen gliding with velocity v in the bacterium rest frame).…”