Strengthening biofilms with selective metal ions

Biofilms are structured microbial communities consisting of bacteria embedded in a self-produced extracellular polymeric substance (EPS) that enables survival in diverse environments. The EPS can integrate materials from the surrounding environment, such as metal ions, which can provide additional mechanical protection to the embedded bacteria from environmental stressors. While previous studies demonstrated that metal ions impact the erosion behavior of biofilms, key quantitative properties, such as failure strain, remain largely undocumented due to difficulties in handling these viscoelastic and soft biomaterials. In this work, we introduce a technique to characterize the impact of metal ions on the uniaxial stress-strain response of bulk bacterial biofilms. Through applying this method to Bacillus subtilis pellicles, we demonstrate that exposure to selective metal ions increases both the low strain elastic modulus and maximum stress, while decreasing failure strain. These effects are consistent with ion-mediated EPS crosslinking and are reversible through the introduction of a strong chelating agent, while variations in pH alone have a negligible impact on measured mechanical properties. We compare our results to previous biofilm erosion studies and provide insights into how metal ion interactions can alter the mechanical behavior of biofilms, which will aid in future biofilm mitigation strategies for biofouling or healthcare applications.