Collective movement by soil bacteria during the colonisation of the rhizosphere

Abstract

<p>Migrations are essential for soil microorganisms to exploit scarce and heterogeneously distributed resources in the rhizosphere, but mobility in soil remains poorly studied due to experimental limitations. In this study, time-lapse images collected using live microscopy techniques captured collective and coordinated groups of <em>B. subtilis</em> cells exhibiting “crowd movement” during rhizosphere colonisation. Groups of <em>B. subtilis</em> cells moved through transparent soil toward plant roots and re-arranged dynamically around root tips in the form of elongating and retracting “flocks” resembling collective behaviour usually associated with higher organisms (e.g., bird flocks or fish schools). These <em>B. subtilis</em> flocks are likely driven by the diffusion of extracellular signalling molecules (e.g., chemotaxis and quorum sensing) and may be impacted by the physical properties of the environment. Our findings advance the understanding of bacterial migration through soil matrices and expand known behaviours for coordinated bacterial movement.</p><p> </p>

Microcosm Fabrication Platform for Live Microscopy of Plant-Soil Systems

“…The physical model constructed from transparent and soil-like substrates is able to mimic the heterogeneous nature of soil, something which is the key to forming boundaries in physiochemical gradients, root exudates, and microbial communities 38 . Indeed, spatial and temporal associations in their rhizosphere were observed in our microcosms chambers, where root systems developed more lateral roots 14 , bacteria cells formed collective movements 20,21 , and rhizosphere acidification was from root exudate 37 . Spatial and temporal dynamics in transparent soil present a smaller but more dynamic rhizosphere when compared to a gel-based system, where homogenous diffusion might lead to overestimated rhizosphere ranges 39 and other non-physiologically representative behaviours.…”

“…stimulated emission depletion, fluorescence lifetime, Raman spectroscopy, or scattering [16][17][18][19] which increase the richness of information collected from a sample. Optical sectioning has also been used to obtain data from plant roots and soil microbes in situ using transparent soils 20,21 .…”

Scaling up and down: movement ecology for microorganisms

“…The production of quorum sensing molecules then can synchronize cell movements, and rapidly reorient individual movement pathways, transitioning from a gradient-seeking random walk to a directed walk up a signaling gradient. In soils, B. subtilis followed a gradient of quorum sensing molecules to localize on the tips of plant roots [51]. Thus, taxic behavior is an important link between individual-level movements and the collective movements of the population.…”