Abstract
Mechanistic understanding of bacterial spreading in soil is critical to control pathogenic contamination of groundwater and soil as well as design bioremediation projects. However, our understanding is currently limited by the lack of direct bacterial imaging in soil conditions. Here, we overcome this limitation by directly observing the spread of bacterial solution in a transparent chamber with varying corner angles designed to replicate soil-like conditions. We show that two common soil bacteria, Bacillus subtilis and Pseudomonas fluorescens, generate flows along sharp corners (< 60°) by producing surfactants that turn nonwetting solid surfaces into wetting surfaces. We further show that a surfactant-deficient mutant of B. subtilis cannot generate corner flows along sharp corners, confirming that the bacteria-generated corner flows require the production of bacterial surfactants. The speed of biosurfactant-induced corner flow at the sharp corner is about several millimeters per hour, similar to that of bacterial swarming, the fastest mode of known bacterial surface translocation. We further demonstrate that the bacteria-generated corner flow only occurs when the corner angle is less than a critical value, which can be predicted from the contact angle of the bacterial solution. Furthermore, we show that the corner flow has a maximum height due to the roundness or cutoff of corners. The mechanistic understanding and mathematical theories of bacterial spreading presented in this study will help improve predictions of bacterial spreading in soil, where corners are ubiquitous, and facilitate future designs of soil contamination mitigation and other bioremediation projects.
