A trace amount of surfactants enable diffusiophoretic swimming of bacteria*

From birth to health, from personal care products to pharmaceutical and petroleum industries, surfactants are all around us. Due to the importance, their environmental impacts are extensively studied and well understood. One of the aspects that have been studied in recent years is their impact on bacteria, particularly the swimming behavior of motile bacteria. Here, we uncover an alternate chemotactic strategy triggered by the presence of surfactants, namely diffusiophoresis. We show that even a trace amount of ionic surfactants, down to a single ppm level, can impact the bacteria diffusiophoresis by boosting the surface charge of the cells. Because diffusiophoresis is driven by the surface-solute interactions, the surfactant-enhanced diffusiophoresis is observed regardless of the types of bacteria. Whether gram-positive or -negative, motile or non-motile, the surfactants enable fast migration of bacteria even under non-swimming conditions, suggesting a ubiquitous mechanism that has been largely overlooked.