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Internal stress in a floating biofilm of bacteria

In collaboration with a scientist at the Ecole Polytechnique, a team of the Laboratoire de Physique des Solides in Orsay has discovered the existence of a new mechanical force within biofilms. The authors measured it using an original setup and identified its origin without hypothesis or modelling.

Biofilms are the most prevalent mode of life of micro-organisms like bacteria existing well before plants, animals and other complex multicellular forms of life. Bacteria colonize any surfaces and produce a protective gel allowing them to survive in hostile conditions. Biofilms have negative (infectious source) as well as positive (green source of useable power and chemistry) effects on our society and on the environment.

The experiments demonstrate that biofilms are subject to internal mechanical forces and suggest that biofilms are engineered by bacteria like strengthened edifices combining prestrain and internal stress. The force is generated by the biofilm growth : bacteria are embedded in a developing network, secrete extracellular matrix, and in turn get stressed mechanically (compressed). By monitoring the response of biofilms to an injury, we show the benefit of a biofilm to be naturally compressed rather than released. The force maintains its integrity and contributes to self-repairing.

A key question in understanding why biofilms are the most prevalent mode of bacterial life is the origin of their degree of resistance and protection that bacteria gain in self-organizing the community in biofilms. Our experiments suggest that one of the reasons comes from their mechanical properties. When multiplying and secreting extracellular substances, experiments on pellicles or floating biofilms of Bacillus subtilis show that bacteria create themselves an internal force (associated to a - 80 ± 25 Pa. stress) within the biofilms, similar to the forces that self-equilibrate and strengthen plants, organs and some engineered buildings. This force or stress is here associated to a growth-induced pressure. Our observations indicate that biofilms spread after any cuts and ablations up to 15-20% of their initial sizes due to such forces. The force relaxes over very short timescales (tens of milliseconds). We conclude that the forces help bacteria to shape the biofilm, improve its mechanical resistance and facilitate its invasion and self-repairing.

Reference :

Carine Douarche, Jean-Marc Allain, and Eric Raspaud.
Bacillus subtilis Bacteria Generate an Internal Mechanical Force within a Biofilm
Biophysical Journal 109, 2195 (2015).

Contact :

Eric Raspaud