Surface defects enable biofilm growth under high shear

Bacteria commonly form surface-attached biofilms by encasing themselves in a protective polymer matrix, enhancing their resistance to threats such as antibiotics. These communities are often found in harsh flow environments and can withstand substantial shear stresses once established. However, the initial stage of biofilm development – when cells attach to a surface – requires relatively calm flow conditions. Here, we show that biofilm formation under high shear is enabled by local hydrodynamic shelters that act as nucleation sites for system-wide colonisation. Using millifluidic experiments, we demonstrate that initial colonisation within surface grooves enables biofilm growth in downstream regions that would otherwise remain uncolonised. These sheltered regions act as local hotspots that promote growth inthe high-shear environment. Our results reveal biofilm formation as a non-local, transport-mediated process, highlighting the need for models that couple far-field hydrodynamics with microbial growth.