ABSTRACT Swarming bacteria move on agar surfaces in groups, using flagella as motive organelles. Motility depends critically
on surface wetness, which is enabled by osmotic agents and surfactants secreted by the bacteria. In a recent study, the
upper surface of an Escherichia coli swarm was found to be stationary, as determined from the motion of MgO particles deposited
on the swarm. This led to the remarkable conclusion that the bacteria move between two stationary surfaces’the agar gel
below and the liquid/air interface above. That study suggested that secreted surfactants may contribute to immobilizing the
upper surface of a swarm. Here, we test this proposition using two robust surfactant-producing bacteria. We find antithetically
that the upper surfaces of both these swarms are mobile, showing a superdiffusive behavior in swarms with stronger surfactant
activity. Superdiffusive behavior was not observed on the surface of a drop of bacterial culture, on bacteria-free culture supernatant,
or on nonswarming surfactant-producer colonies, which suggests that superdiffusion is an emergent property resulting
from the interaction of the collective motion of the bacteria within the swarm with the surfactant layer above. Swarming not only
allows bacteria to forage for food, but also confers protective advantages against antimicrobial agents. Our results are therefore
relevant to superdiffusive strategies in biological foraging and survival.