Mechanisms and ecological implications of the movement of bacteria in soil

The extent of translocation of bacteria through soil strongly affects parameters of bacterially-mediated bioremediation and biocontrol. Here, we discuss the main strategies that bacteria use for their dispersal through the soil matrix. Cell dispersal mechanisms are scale-dependent and may be either growth- or cell organelle-driven (active, small scale) or water/wind/vector-organism-driven (passive, large scale). The active modes of dispersal (growth, swimming, swarming and twitching motility) are limited to the scale of (connected) soil pores or aggregates. In contrast, water-or wind-driven transport dominates large-distance bacterial dispersal. Remarkably, the association of bacteria with other (vector) organisms, in particular plant roots, fungal hyphae and moving organisms like earthworms, assists them in the crossing of soil matrix discontinuities. We posit that a dynamic "underground transport web" exists in soil, which is organism- (plant-, fungal- and/or soil animal-) driven. In particular, we examine the role of fungal hyphae as facilitators of short-or long-distance bacterial migration through soil. The transport web thus has strong implications for the occupancy of novel ecological niches and nutrient cycling dynamics. The effects of this underground transport web are examined.