An article published in Nature communications (Savorana, G., Redaelli, T., Truzzolillo, D. et al. Stress-hardening behaviour of biofilm streamers. Nat Commun 16, 9497 (2025). reports that bacteria’s ability to withstand mechanical challenges is enhanced in their biofilm lifestyle, where they are encased in a viscoelastic polymer matrix. Under fluid flow, biofilms can form as streamers – slender filaments tethered to solid surfaces and suspended in the flowing fluid. Streamers thrive in environments subjected to intense hydrodynamic stresses, such as medical devices and water filters, often resulting in catastrophic clogging. 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. The article demonstrates that biofilm streamers exhibit stress hardening behavior, with both the differential elastic modulus and the 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. The article further demonstrates that this mechanical response arises from the properties of extracellular DNA (eDNA), which constitutes the structural backbone of the streamers. In addition, the results identify extracellular RNA (eRNA) as a modulator of the matrix network, contributing to both the structure and rheological properties of the eDNA backbone. The findings reveal an instantaneous, purely physical mechanism enabling streamers to adapt to hydrodynamic stresses. Given the ubiquity of extracellular nucleic acids (eNA) in biofilms, this discovery prompts a re-evaluation of their functional role in biofilm mechanics, with potential implications for biofilm structural integrity, ecological resilience, and colonisation dynamics. @ https://www.nature.com/articles/s41467-025-64557-x
