Bacterial biofilms are microbial communities consisting of cells embedded in a self-produced extracellular polymeric substance (EPS). Understanding biofilm mechanics is crucial to predicting and controlling biofilm response to the chemical and physical stressors that they face in nature. Significant challenges are encountered when attempting mechanical testing on biofilms due to their inherent fragility. The majority of previous work has focused on the low-strain response, e.g. shear modulus, and less work has explored how the failure behavior of biofilms depend on their EPS composition and structure. Here, using a custom-built mechanical tester, The Uniaxial Tensile Tester for UltraThin films (TUTTUT), we directly measure how the complete uniaxial stress strain relationship and fracture response of liquid-supported Bacillus subtilis pellicles depend on their EPS composition. From our measurements, we determine fracture energy, elastic modulus, and yield stress and find that the addition of metal ions to EPS alters the failure behavior of the biofilms. Overall, this work provides fundamental knowledge on how the failure of biofilms depends on the external environment.
