Stress-hardening behaviour of biofilm streamers

Bacteria’s ability to withstand mechanical challenges is enhanced in their biofilm lifestyle, where they are encased in a viscoelastic polymer matrix [1]. Under fluid flow, biofilms can form as streamers – slender filaments tethered to solid surfaces and suspended in the flowing fluid [2, 3]. Streamers thrive in environments subjected to intense hydrodynamic stresses, such as medical devices and water filters, often resulting in catastrophic clogging [4]. Their colonisation success may depend on a highly adaptable mechanical response to varying stress conditions, though the evidence and underlying mechanisms of this adaptation remain elusive. Here, we demonstrate that biofilm streamers exhibit a stress-hardening behaviour, with both differential elastic modulus and effective viscosity increasing linearly with external stress. This stress-hardening is consistent across biofilms with different matrix compositions, formed by various bacterial species, and under diverse growth conditions. We further demonstrated that this mechanical response originates from the properties of extracellular DNA (eDNA) molecules [5], which constitute the structural backbone of the streamers. Our findings reveal an instantaneous, purely physical mechanism enabling streamers to adapt to hydrodynamic stresses. Given the ubiquity of eDNA in biofilms [1, 6], this discovery prompts a re-evaluation of its functional role in biofilm mechanics. Moreover, the stress-hardening behaviour may have broad implications for biofilm development, survival, and ecology, paralleling its role in tissues where it regulates processes such as morphogenesis, cancer formation and wound healing [7, 8].