There has been much theoretical interest in understanding the dynamics of a passive object in a nonequilibrium bath. This is biologically relevant in the context of bacterial swarming, where a bath of self-propelled bacteria interacts with a passive water-air interface and deforms it into a complex space-filling pattern. Before addressing this problem, I will first discuss the dynamics of a passive tracer in the simplest kind of nonequilibrium bath, namely, a bath of particles that are coupled to a thermal reservoir at a higher temperature. I will show that such a tracer exhibits nonequilibrium dynamics reminiscent of active particles, but that an effective equilibrium picture emerges in an appropriate thermodynamic limit. Following these results, I will return to bacterial swarming and present a numerical model of active particles interacting with a passive phase-boundary, focusing on mappings to equilibrium and on novel nonequilibrium phenomena.