Biomechanics of collective motility in systems of microorganisms
Mercredi 11 janvier 2012
à 11h en salle C. Brot
The evolution of living organisms is shaped by the physical environment where they live. Microbial systems live in the realm of small Reynolds numbers, where viscous drag and diffusion dominate over inertia. I will focus on collective motility in systems of microorganisms that cooperate to elude the physical constraints on locomotion of individual cells. Bacterial bioﬁlms are organized communities of cells living in association with surfaces. The hallmark of bioﬁlm formation is the secretion of a polymeric matrix rich in sugars and proteins in the extracellular space. In the soil bacterium Bacillus subtilis, secretion of polymers is coupled to the inhibition of ﬂagella-mediated motility. The onset of this switch results in slow expansion of the bioﬁlm on a substrate. Multiple functions have been attributed to the polymers, but none of these provide a physical mechanism for generating spreading. We propose that the secretion of polymers drives surface motility by generating osmotic pressure gradients in the extracellular space. A mathematical model based on the physics of polymer solutions shows quantitative agreement with experimental measurements of bioﬁlm growth, thickening, and spreading. I will discuss the implications of this osmotically driven type of surface motility for nutrient uptake that may elucidate the reduced ﬁtness of the matrix-deﬁcient mutant strains.
Voir en ligne : School of Engineering and Applied Sciences Harvard University
Fluides & Matériaux Complexes
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