Bacteria can organize into structured communities, called biofilms, that protect them from antibiotics and from immune attack by the host. The biofilms are embedded in a matrix containing a complex mixture of macromolecules including exopolysaccharides and proteins. In the February 22 Science, Whitchurch et al. report that extracellular DNA is a major component of the biofilms of Pseudomonas aeruginosa (Science 2002, 295:1487). They demonstrate that adding DNase I to P. aeruginosa cultures inhibited biofilm formation and bacterial colonization. The enzyme could also dissolve established biofilms. The extracellular DNA is thought to be derived from membrane vesicles. Whitchurch et al. propose that DNase I treatment may be beneficial to prevent biofilm formation in infection-linked diseases such as cystic fibrosis.