Active Smectic Hydrodynamic Models of Colony Growth.

Complex spatial patterns emerge in bacterial communities in response to internal and external pressures. The resulting structure is instrumental in everything from interspecies competition to acquisition of antibacterial resistance. While current massive sequencing efforts yield vast data about community composition, they fail to provide positional information, despite its integral role in bacterial communities. This project will study the spatiotemporal patterning of growing communities by resolving the essential physical principles that drive shape and dynamics on a microcolony-wide scale.

Understanding community-wide self-organization within host environments requires predictive mathematical theories that balance conceptual simplicity and polymicrobial complexity. While recent theories for the collective dynamics of cell motion have found some success by employing continuum active nematic liquid crystal models for colony dynamics, there are clear insurmountable shortcomings to reproducing the multiplex internal organization of real microbial colonies. In particular, these nematic models limit the internal ordering of baciliforms to orientational alignment. However, brightfield fluorescence and microscopy images of microcolonies of Pseudomonas aeruginosa for example show internal structuring, including rafts (small clusters of coherent motile baciliforms groupings), multilayers (distinct tiers of vertical strata), interfacial counter-sliding (single lane of baciliforms at the very edge of the colony moving in the opposite direction to the bulk), and in-plane smectic layering (stacked in well-defined planes parallel to the surface). Each of these observed phenomena involves layer ordering of baciliforms, which fundamentally cannot be accounted for by previous nematohydrodynamic theories.

Thus, this project proposes that a novel theory for active smectic fluids will allow us to understand active turbulence in baciliform colonies, including intrinsic length scales of in-plane smectic layering and associated collective dynamics. Jack Paget will extend 2D active smectic theory to account for both the in-plane smectic layering and also the multilayer colonies. By implementing a phase field hybrid model to simulate colony/fluid interfaces, he reproduce and understand counter-flowing interfacial monolayers and active stresses on colony interfaces. Our theoretical framework will tightly couple growth to orienational order. Future work will be to hybridize smectic code with reaction-diffusion equations, to produce chemotactic communication between competing colonies and within each colony.