No abstract

Frustration due to differences in cell motility within clusters gives rise to novel collective motion of migrating cell clusters.

Swarming is a phenomenon where collective motion arises from simple local interactions between typically identical individuals. Here, we investigate the effects of variability in behavior among the agents in finite swarms with both alignment and cohesive interactions. We show that swarming is abolished above a critical fraction of non-aligners who do not participate in alignment. In certain regimes, however, swarms above the critical threshold can dynamically reorganize and sort out excess non-aligners to maintain the average fraction close to the critical value. This persists even in swarms with a distribution of alignment interactions, suggesting a simple, robust and efficient mechanism that allows heterogeneously mixed populations to naturally regulate their composition and remain in a collective swarming state or even differentiate among behavioral phenotypes. We show that, for evolving swarms, this self-organized sorting behavior can couple to the evolutionary dynamics leading to new evolutionarily stable equilibrium populations set by the physical swarm parameters.

Collective and directed motility or swarming is an emergent phenomenon displayed by many self-organized assemblies of active biological matter, such as clusters of embryonic cells during tissue development, cancerous cells during tumor formation and metastasis, colonies of bacteria in a biofilm, or even flocks of birds and schools of fish at the macro-scale. Such clusters typically encounter very heterogeneous environments. What happens when a cluster encounters an interface between two different environments has implications for its function and fate. Here, we study this problem by using a mathematical model of a cluster that treats it as a single cohesive unit that moves in two dimensions by exerting a force/torque per unit area whose magnitude depends on the nature of the local environment. We find that low speed (overdamped) clusters encountering an interface with a moderate difference in properties can lead to refraction or even total internal reflection of the cluster. For large speeds (underdamped), where inertia dominates, the clusters show more complex behaviors crossing the interface multiple times and deviating from the predictable refraction and reflection for the low velocity clusters. We then present an extreme limit of the model in the absence of rotational damping where clusters can become stuck spiraling along the interface or move in large circular trajectories after leaving the interface. Our results show a wide range of behaviors that occur when collectively moving active biological matter moves across interfaces and these insights can be used to control motion by patterning environments.

No abstract

Most bacteria surround themselves with a tough cell wall made of peptidoglycan that preserves cellular integrity and maintains cell shape. Peptidoglycan must be dynamic to accommodate cell growth and division. Enzymes that hydrolyze peptidoglycan are crucial for these processes, but their activities can be lethal if not tightly controlled. In Gram-positive coccus Staphylococcus aureus, cell division can be classified into three stages: septation, daughter cell separation and finally disassociation. Previous Cryo-EM data has indicated that prior to cell separation the two daughter cells are only connected through the peripheral peptidoglycan. This result has led to the hypothesis that there are two classes of cell wall hydrolases: one class that splits the majority of the septum and the other class that resolves the final connecting ring to trigger cell separation. The identities of the hydrolases involved in these two stages and how the cell coordinates and regulates them are still not clear. We have examined the major cell wall hydrolases Atl and Sle1 in S. aureus and found that a sle1 deletion mutant is delayed in cell separation while an atl mutant separated normally but was impaired in cell disassociation.