Across many different habitats bacteria are often found within surface-attached communities known as biofilms . Biofilms greatly enhance bacterial resistance to harsh environmental conditions and antimicrobial treatments, which makes their removal more difficult in industrial and clinical settings. The protection from mechanical and chemical harm is mainly due to the presence of a matrix of extracellular polymeric substances (EPS) secreted by the bacteria, which represents a cohesive polymer network  that interconnects biofilm cells. Although bacteria are ubiquitously exposed to liquid flow in natural environments, the human body, and artificial systems, the influence of hydrodynamics on the transport and attachment of bacteria to surfaces and the formation of biofilms remains poorly investigated and understood.
Here, we show that a laminar flow around a pillar can trigger the formation of suspended filamentous biofilm structures known as streamers . We have developed a microfluidic setup that allows the visualization, in real time and at the single-cell level, of the trajectories of swimming cells (such as the human pathogen Pseudomonas aeruginosa) around a precisely fabricated microfluidic pillar and the subsequent formation of biofilm streamers. Experiments with pillars of different diameters and with different flow rates allowed us to assess the effect of hydrodynamics on bacterial transport and biofilm formation and to discover with the latter dominated by the interplay between a secondary flow around the pillar and the viscoelastic nature of EPS. In addition, since in this geometry we can study freestanding, single biofilm filaments, we can probe the shear-induced deformation and detachment of bacterial streamers to investigate their intrinsic material properties and viscoelastic nature. We discovered that the formation time and mechanical properties of streamers are strongly related to the diameter of the pillar: streamers form more rapidly around smaller pillars and can withstand stronger hydrodynamic stress before detachment. Understanding the roles of geometry and liquid flow in biofilm initiation offers an opportunity to optimize the design of industrial and medical systems to minimize their formation.
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