Bacteria are able to colonize any surfaces and produce extracellular matrix in which they may embed and survive in many different environments. This extracellular matrix is mostly composed of water, polysaccharides, proteins and DNA and it confers to the colony a true cohesion and properties of solid and soft matter. Biofilms are ubiquitous and may play positive or negative role to their environment depending on the settings, context and biofilms nature. For that reasons, most research studies on biofilms have been conducted at the level of applied sciences since the last fifteen years and large part of biofilm biophysics remain to be discovered. In the laboratory, we are interested in different biofilm properties, in particular in their mechanical properties.

Biofilms are living systems with a heterogeneous and multicomponent structure. This makes challenging the understanding of their mechanical properties as compared to other inert and homogeneous materials. We are currently studying their mechanical behaviors, how they may be compared to the other systems or described by simple laws. Most of the work is done on biofilms named pellicles standing on the top of liquid, at the air-liquid interface, and on biofilms formed by Bacillus subtilis.

Wrinkled morphology of pellicles on top of liquid. Growth in a confined space induces in-plane compressive stress which generates vertical structures, wrinkles and folds.

Our first study, published in 2013, focussed on the mechanical forces determining the biofilm morphologies at a macroscopic scale. Wrinkled morphology is a distinctive phenotype observed in mature biofilms produced by a great number of bacteria. We have shown how these ripples might be explained by the buckling mechanical instability and hypothesized the presence of stress inside the biofilm. In a further paper (2015), we have explicitly shown the existence of a growth stress within the pellicles : bacteria multiply and secrete extracellular matrix in a confined environment which generate a compressive force.
As pellicles are pre-stressed due this growth, it was interesting to study how they would response to mechanical strain (2016). Based on the response to cyclic stimuli, we arrived to the conclusion that they are elastic in the compressive regime while being plastic in the tensile regime.

This video shows examples of biofilm reorganization after the puncturing of compressed (A) and released (B) pellicles. The internal compression helps maintain the biofilm integrity. Scale bars represent 2 mm.