Bacterial swarming is a collective mode of motion in which dense populations of self-propelled bacteria migrate on surfaces. Under sub-lethal antibiotics concentrations, the statistics of swarming Bacillus subtilis transitions from normal to anomalous, with a heavy-tailed speed distribution and a two-step temporal correlation of velocities. The transition is due to the formation of a motility defected sub-population that self-segregates into regions. The phenomenon suggests a new strategy bacteria employ to fight antibiotic stress using the physical properties of driven particle systems.