Depletion Zone Following the Spread of a Bacterial Colony

Many species of bacteria have developed strong abilities to spread on solid surfaces, via a mode of motility called swarming. This study focuses on the swarming, or colony expansion, of a human pathogen Pseudomonas aeruginosa, on an agar surface. We observed occurrence and spread of a central region where the thickness of the bacteria containing fluid film notably drops after the colony has spread to cover most of the agar plate. We call this region “depletion zone”, where bacterial density is lower than the outer region. We propose that occurrence of a depletion zone within an expanding bacterial colony is a de-wetting phenomenon. It is caused by a higher concentration of surface-active molecules, most likely surfactants, that have been secreted by bacteria and accumulated in the central region of the colony. These molecules infiltrate the agar underneath, reducing the surface tension of the agar surface in the central region, causing a radially outward flow to the extent that most of the bacteria laden fluids clear out. In essence, we offer a physical model to a phenomenon prone to misinterpretation by invoking chemotaxis or quorum sensing.