Soils-on-a-chip reveal unforeseen motility parameters of microconfined Bradyrhizobium diazoefficiens

Soil bacteria of the order of the Rhizobiales associate symbiotically with legume plants. Particulary, Bradyrhizobium diazoefficiens is a nitrogen-fixing symbiont of soybean, that helps to improve grain quality among other benefits. This bacterium possess two flagellar systems, which enable it to swim in water-saturated pores. However, the motility of B. diazoefficiens, which may be crucial for its competitiveness in root nodulation, has not been well understood. To address this knowledge gap, we designed and fabricated microfluidic soil-on-a-chip (SOC) devices that offer sustainable agriculture an original tool for directly visualizing bacterial behavior in confined-environments. Using these microdevices, we measured the population velocities and changes of direction along their paths for two strains of B. diazoefficiens, namely the wild-type and a mutant with only one flagellar system. Our detailed statistical analysis revealed that both strains exhibited reduced speeds and increased changes of direction of 180{degrees}, in channels of decreasing microscopic cross sectional area, down to a few microns. Interestingly, while the wild-type strain displayed faster swimming speeds in unconfined spaces, this advantage was negated in the SOCs that mimicked porous soils. Moreover, we employed the measured motility parameters to model and simulate B. diazoefficiens motion in SOC devices for extended periods and larger scales, enabling further predictions of diffusion in real soils. Thanks to miniaturization, microfabrication, and multidisciplinary knowledge, this study represents a significant breakthrough in soil bacteria field and methods, useful both for farmers and environment. Furthermore, the potential applications of this work extend to multiple beneficial bacteria widely used as biofertilizers.