Speaker
Description
Bacteria are well-recognised as having a beneficial effect on the structure of soil in that they favour soil aggregation and increase soil pore connectivity1,2. Soil opacity renders its dynamic imaging at the microscale difficult, so our knowledge on bacterial activity in soil largely results from end-point measurements. Microfluidic chambers enable the dynamic observation of bacteria in model porous environments at fine temporal and spatial resolutions3. Microfluidic-based investigations have revealed some of the biophysical principles governing bacterial growth in porous media, including under fluid flow4, amongst grains of sand-mimicking shape5, and in packed soft particles6. However, the mechanical interactions between growing bacterial colonies and rigid moving grains, akin to sand grains, remain unexplored. Here, we incorporate grain mobility into the microfluidic toolkit. We form mobile divided media in microfluidic chambers by polymerising hydrogel grains (approx. 40 µm in diameter and height) in situ and let Green Fluorescent Protein (GFP)-expressing Bacillus subtilis colonise the interstitial space between the resulting hydrogel grains. We observe grain movement along the axes of bacterial density gradients. Grains move at velocities of up to a few µm/h for several hours. We make the novel observation of a "granular respiration" where pores occupied by dense bacterial colonies widen before partially shrinking back. We link the direction of movement of grains to bacterial growth kinetics and propose a simple theoretical model linking bacterial growth pressure to the elastic deformation of the grain network to interprete the observed displacements. The balance between the time of bacterial division and the time of growth-pressure relaxation into the adjacent pores determines whether the substrate is compressed or relaxes. That relaxation time scales with the effective viscosity of the colony inside the divided medium, and determining how that viscosity varies with colony growth is a key objective of our current work. This work provides a first insight into the effect of bacterial growth-induced pressure7 onto divided media and suggests a mechanism by which bacteria could mechanically modify soil structure.
| Country | France |
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