Myxobacteria are one of the prime model systems for studying cell-cell
interaction and cell organization preceding differentiation.
Myxobacteria are social bacteria which swarm, feed and develop
cooperatively.
When starved, myxobacteria self-organize into a three
dimensional fruiting body structure. Fruiting body formation is a
complex multi-step process of alignment,
rippling, streaming and aggregation that culminates in the
differentiation of highly elongated, motile cells into round,
non-motile spores. A successful model exists for the fruiting body
formation of the eukaryotic slime mold Dictyostelium
discoideum . Understanding the formation of fruiting
bodies in myxobacteria, however, would provide a new insight since
collective myxobacteria motion depends not on chemotaxis as in
Dictyostelium but on contact-mediated signaling.
We use a lattice-gas cellular automaton (LGCA) model to simulate rippling in myxobacteria. We developed an efficient way of representing cells of different cell size, shape and orientation that may be easily extended to model later stages of fruiting body formation.
This LGCA model is designed to investigate whether a refractory period, a minimum response time, a maximum oscillation period and non-linear dependence of reversals of cells on C-factor are necessary assumptions for rippling. We find the optimal model conditions that best reproduce rippling in the experiments of Myxoccoccus xanthus .
We further developed an LGCA that models alignment, streaming and aggregation. The simulations of this LGCA model resulted in two types of cell aggregates: stationary orbits of cells and motile streams of cells. We developed statistical techniques to differentiate between the two types of aggregates and we studied their relative lifespans, cell density and areas.
In order to evaluate the effect of microscopic (cell level) noises on aggregate pattern formation, we also converted our stochastic LGCA model into a deterministic Lattice Boltzmann model. Similar patterns developed with streams and orbits, indicating that microscopic cell level noises are not responsible for the formation of the aggregate patterns.
A three dimensional LGCA further enables us to investigate details of aggregation, including how cells form "traffic jams" and circular "skirts" around the jams, and how these jams become the mound-shaped aggregates, which eventually become the fruiting body.
Vegetative myxobacteria cells differentiate into round environmentally-resistent spores and build large multicellular fruiting bodies. The ignition of sporulation is governed by the level of C-signal protein accumulated in each cell. Internally, a nascent fruiting body consists of two concentric domains: an outer motile shell with high cell density surrounds a three times less dense hemispherical mound [3,4]. Differentiation of elongated motile cells into spherical spores begins in outer domain densely packed with circling cells, during which cells lose their gliding motility.
We further extend our 3D model to model cell differentiation and spores' passive transportation. Starting from mature 3D aggregate we trigger cell differentiation according to c-signal levels. Spores first appear in the "skirt" region and the number increase in time. Spores lose both the ability of C-signaling and their motility. They are pushed around by moving cells mechanically. Slime plays an important role in keeping spores toward the center of aggregates.
Swarming is many bodies moving collectively as a coherent 'structure'. Bird, fish, and many insects (ants, bees) are but a few familiar examples. Many bacteria swarm too. Myxobacteria are one of such species. In the wild, myxobacteria hunt in wolf packs secreting digestive enzymes to lyse other cells and then absorbing the nutrients released. In lab conditions, M. xanthus swarms outward with a dependence on the initial density of the colony. The major features of our cell-based model for swarming are that our cells are propelled by the two motility systems of M. xanthus which can be turned off independently, cells interacting via physical contact, growth, division and nutrient consumption.
Besides the intrisic similarities between bacterial swarming and bird flocking, Myxobacteria's periodic reversal also brings an additional, unique mechanism for swarming dynamics.