Viscoelasticity of <i>Myxococcus xanthus </i>Fruiting Bodies

When starved, the social bacteria Myxoccocus xanthus mounts a population-level, collective response through the formation of multicellular fruiting bodies. Beginning with the aggregation of motile cells into mounds and culminating in the constituent cells' sporulation, the process of fruiting body formation both provides the environment within which cells sporulate and creates a structure that then buffers the spore population against environmental stresses. We use atomic force microscope-based microrheology to characterize the mechanical state of these structures throughout their development. Doing so reveals that the process of fruiting body formation is mechanically akin to a gelation process wherein nascent mounds of cells show a rheodictic creep response while mature fruiting bodies exhibit more elastic, arrheodictic behavior. We further show that these changes are endogenously driven by energy expenditure of the structure's constituent cells. Finally, we demonstrate that changes in how these cells expend energy—whether that be towards motility at early times or sporulation at later times—governs the transition to a solid, mechanically resilient structure.