A protein called CheW is essential for swarming, catalyzing the mass movement of bacteria across the surface of an organ. But when bacteria encounter antibiotics, the drugs initiate a DNA repair system in bacteria known as SOS response, which is supposed to stop the infection. But the Spanish researchers found that when SOS response kicks in, levels of another protein known as RecA begin to increase. RecA interferes with CheW, causing the colony to stop moving forward. As a result, only the front line of the colony is affected by the antibiotics; the rest of the colony stops marching until the concentration of antibiotics decreases, at which point RecA concentrations fall and the colony starts attacking again.