Comparative Analysis of Bacterial Self-Organization and Crystallization

When starved, a biofilm of the delta-proteobacterium Myxococcus xanthus executes a genetically-driven process of self-organization called development, during which cells coalesce to form multicellular, spherical structures called fruiting bodies. This is a survival mechanism that initiates with the aggregation of cells into small clusters. Over a period of hours, a subset of these clusters grow larger with the recruitment of more cells, but this appears to occur at the expense of smaller cell clusters, which destabilize and disperse back into the biofilm. In this way, development bears a striking resemblance to particle growth during Ostwald Ripening, when a subset of larger particles grow at the expense of smaller ones in a saturated solution during crystallization.

M. xanthus development is a genetic process that must operate within the constraints of physical laws. Therefore, understanding the similarities that exist between M. xanthus cell aggregation and crystallization will provide essential insight into the forces that guide self-organization in living systems. We therefore examined the following hypotheses: M. xanthus development is comparable to crystallization, and initiating development by removing nutrients (metabolic energy) is comparable to initiating crystallization by removing thermal energy; therefore, development and crystallization should behave in a similar and predictable manner resulting from minimization of free energy.

To test these hypotheses biofilms of M. xanthus were starved at three different rates, and relevant characteristics of fruiting bodies were analyzed. Data regarding the timing, size, and placement of fruiting bodies throughout this process support our hypotheses.