Bacteria hide in their sturdy house to survive the harsh environment

Biofilms are collections of surface-associated bacterial cells enclosed in an extracellular matrix (ECM). Biofilm cells cause numerous tenacious problems in the medical and healthcare industry because they are more resistant to antibiotics and they can thrive in various dynamic environmental conditions, by living together in a community. On the other hand, the unique material properties of biofilm also help biofilm-dwelling cells survive in various natural environments. Biofilms have also been proposed to have beneficial engineering applications such as those in wastewater treatment, as well as hold promise for other applications, such as biocatalysis, which includes the production of biofuels. To take advantage of the potential of biofilms, their formation must be controlled; therefore, studying biofilms from a mechanical perspective provides insights into how their mechanical properties contribute to cell viability and persistence in diverse environmental conditions. Additionally, understanding biofilm mechanics is crucial in designing effective biofilm removal strategies and different applications of biofilm materials.

Previous findings suggested that the ECM, consisting mainly of polysaccharides and accessory proteins, plays a vital role in determining the structural and functional integrity of biofilms. For instance, the ECM polymeric network facilitates the glueing together of bacterial cells to form hydrated, gel-like biomaterial. This indicates that the ECM components also influence the mechanical and viscoelastic robustness of biofilms. The biofilm rheology is also dependent on the stiff bacterial cells that produce the ECM in the first place.

While ECM plays a vital role in determining biofilm mechanics, how it functions to endow biofilm with such remarkable resilience in diverse and harsh environmental perturbations is still poorly understood. To this end, a team of Yale University researchers: Dr. Qiuting Zhang, Dr. Jung-Shen Tai, Mr. XJ Xu, Dr. Japinder Nijjer, Dr. Xin Huang and Professor Jing Yan in collaboration with Dr. Danh Nguyen and Dr. Ying Li from Institute of Materials Science University of Connecticut (now at the University of Wisconsin-Madison) investigated the mechanical resilience of biofilms toward environmental perturbations, mediated by ECM. Their research work is currently published in the journal Advanced Functional Materials.

The research team used a facultative human pathogen, Vibrio cholerae (abbreviated as Vc), as the biofilm-forming organism model. An ancient disease, cholera still affects millions of people worldwide annually, particularly in economically disadvantaged regions with improper water treatment system. Biofilm formation is involved in the survival of Vc cells both in nature and during an infection, inspiring the current study. Another reason for choosing Vc as the biofilm-former model is the well-defined biochemistry and genetics of its ECM components. The contribution of each ECM component to the mechanical properties of the biofilm, including nonlinear viscoelasticity and recovery after heating and large deformation, was systematically investigated in this study. To do so the authors have used a combination of rheological measurements, mutagenesis and molecular dynamic simulations.

The authors observed that the bacteria utilized the ECM polymeric network as a protective shield for the embedded cells by ensuring improved mechanical resilience and survival in various environmental perturbations. Many unique mechanical features were observed when focusing on the large deformation regions. This includes the self-reinforcing response that is essential for enhancing the biofilm mechanical robustness in response to shear stress. Such features were never observed before in the linear region. Each individual ECM matrix component contributed differently to specific nonlinear viscoelastic behavior, especially during shear stiffening to softening transition. Complementing the experimental measurements, the computer simulations gave cinformation at individual cell scale that cannot be accessed experimentally – and they match.

In summary, the Yale University study reported a systematic investigation of the mechanical behaviors of biofilm mutants in Vibrio cholerae subjected to diverse environmental challenges. The ECM network improved mechanical robustness and promoted cell viability under high temperatures. The findings provided more physical insights into the relationship between the property and structure of biofilms. “We like to know more about how biofilms work so that we can either remove them or use them, in the way we want” said Professor Jing Yan to Advances in Engineering.