How bacteria break down organohalides Anaerobic bacteria can break down a range of organohalide pollutants. To do so, they use unusual reductive dehalogenase enzymes that remove the halogen ion from the molecule, making the pollutants less toxic. Bommer et al. describe x-ray crystal structures of one such enzyme from Sulfurospirillum multivorans (see the Perspective by Edwards). Vitamin B 12 , present near the substrate binding site, catalyzes the reduction of trichloroethylene in concert with two iron-sulfur clusters. The structures provide mechanistic clues for how to engineer enzymes to recognize other pollutants. Science , this issue p. 455 ; see also p. 424