Role of cellular filamentation in bacterial aggregation and cluster-cluster assembly

Bacterial aggregate formation and surface accumulation are increasingly viewed as alternative pathways for biofilm colonization. However, little is known about the dynamics of bacterial aggregate cluster-cluster assembly and their subsequent microstructural and mechanical properties. To this end, we studied experimentally and computationally an aggregating bacterial system that forms a space-spanning interconnected network via cluster-cluster assembly. By controllably inducing bacterial filamentation, we aimed to understand how cell length distribution and cell surface hydrophobicity control the dynamics of aggregation and sedimentation, as well as the microstructure and mechanics of the settled bacterial networks. We found that filamentation lowers the percolation threshold, leading to gelation at a lower number density with distinct assembly dynamics and lower network connectivity. Furthermore, we analyzed the mechanical properties of the bacterial networks. Static stress tests reveal three yielding modes: discrete cluster-cluster disassembly, collective delamination, and subregional network fracture. The yielding modes are consistent with the gel-like viscoelastic properties of the cluster-cluster assembled networks observed during macroscale rheometry. In particular, we observe a scaling relationship between the storage modulus and the volume fraction, characteristic of an attractive rod gel. Our experimental observations are supported by Langevin dynamic simulations, providing mechanistic insights into the factors determining network self-assembly and connectivity. Our findings elucidate the gel-like structure-function dynamics in cluster-cluster aggregated bacterial systems, and they underscore the fundamental importance of filamentation in their properties and mechanical behavior.