Studying the mechanisms of bacterial mobility in the rhizosphere using soil model systems

The mobility of soil bacteria plays a crucial role in rhizosphere colonisation, as exemplified by the complex chemotactic machinery required for effective biofilm formation on growing roots [1]. However, the nature of soil bacterial movements in the soil pore space is poorly characterised. It is difficult to observe the trajectories of individual cells, and the complexity of soil structure makes any observation difficult to replicate. To overcome these difficulties, we have developed a range of microcosm systems that allow live observation of bacterial movement in porous media with precisely controlled physical or chemical properties. Paper-based microfluidic systems have been developed for the in situ extraction of root exudates, which act as a potent chemoattractant for bacteria [2]. Microfluidic devices made of semi-permeable materials allowed the construction of pores through which nutrient release can be controlled, and transparent soil and custom-made microscopes were used to observe bacterial movements when co-cultured with plants [3]. Using Bacillus subtilis as a model organism, we observed that the bacterium occupies very specific regions of pore space in relation to distance from the root [4] or the presence of nutrients, and that movements can be coordinated at a population level [3]. Future work will focus on understanding the conditions under which collective movements of bacteria occur in soil.

References

[1] Allard-Massicotte, R, et al (2016) “Bacillus subtilis early colonization of Arabidopsis thaliana roots involves multiple chemotaxis receptors” MBio, 7(6), 10-1128. [2] Patko, D, et al (2024) "Spatial and temporal detection of root exudates with a paper-based microfluidic device" Soil Biology and Biochemistry 195 (2024): 109456. [3] Liu, Y, et al (2021) "Plant–environment microscopy tracks interactions of Bacillus subtilis with plant roots across the entire rhizosphere" Proceedings of the National Academy of Sciences 118.48: e2109176118. [3] Engelhardt, I C, et al (2022) "Novel form of collective movement by soil bacteria" The ISME Journal 16.10: 2337-2347. [4] Engelhardt, I C, et al (2024) “Mobility and growth in confined spaces are important mechanisms for the establishment of Bacillus subtilis in the rhizosphere” Microbiology, 170(8), 001477.