Assessing the speed of individual bacteria dispersing on mycelial networks

The movement of bacteria on the hyphae of fungi and other mycelial-forming organisms is an important process that determines their ability to actively disperse in water-unsaturated habitats. However, direct observation and characterization of bacterial cell movement on mycelial networks have been difficult to achieve. In this study, we developed a new method that uses high-speed video recording to track the dispersal of individual fluorescently tagged cells of two closely related strains of Pseudomonas putida (UWC1 and KT2440) over the mycelial network of the oomycete Pythium ultimum. We found high intra-population heterogeneity and between-population differences in dispersal speeds for the two bacterial strains. The fitting of the speed distribution functions led to the separation of speeds into two ranges (fast/slow) at an intersection of the fitted curves. In the lower speed range, the UWC1 strain dispersed faster, while the KT2440 strain moved faster in the higher speed range. This finding helps explain conflicting competition outcomes revealed in previous studies and suggests that population mean speed alone does not capture key aspects of bacterial dispersal in mycelial networks. Our new method opens the possibility of studying bacterial dispersal, competition, and other social interactions in spatially heterogeneous environments, such as soils.