Mucin Promotes Bacterial Swarming by Making the Agar Surface More Slippery

When inoculated on the surface of soft agar containing nutrients, many species of motile bacteria can grow into a dense population and spread across the surface by a form of motility called swarming. We study the swarming behavior of Enterobacter sp. SM3, a species of bacteria that exhibits a swarm-dependent reduction in symptoms associated with inflammatory bowel disease (IBD). In this report, we focus on how incorporating mucin into agar gels affects the swarming motility of SM3. We found that mucin enhances SM3's swarm rate, defined as the rate at which bacteria cover an agar surface. We show that mucin promotes wetting of aqueous droplets by inhibiting the pinning of the contact line, which is caused by structural or chemical inhomogeneity. This effect results in a more slippery agar surface. As a macromolecular biosurfactant, mucin promotes an increase in the bacterial swarm rate on agar by masking surface inhomogeneities, thereby inhibiting contact line pinning and allowing for better spreading of an expanding bacterial swarm.