Biofilms are surface-attached communities of bacteria, encased in an extracellular matrix of secreted proteins, carbohydrates, and/or DNA, that assume phenotypes distinct from those of planktonic cells. These phenotypes include a slower growth rate, increased antibiotic resistance, and elevated frequency of lateral gene transfer (15, 20, 33, 38). The ability of certain bacterial strains to form biofilms has been associated with virulence in a number of pathogens, such as Pseudomonas aeruginosa, Staphylococcus aureus, and Streptococcus mutans (21). An association between biofilm formation and virulence has also been reported for Enterococcus faecalis, a gram-positive bacterium that has recently emerged as a leading cause of nosocomial infections (7, 26). E. faecalis biofilms on dental root canals (6), urethral catheters (36), ureteral stents (28), and heart valves (8) have been observed. While it is not clear that the ability of E. faecalis to form biofilms is essential for virulence, it appears that a majority of clinical isolates do possess the ability to form a biofilm in vitro (18, 35). Efforts to identify the molecular entities critical for development into this insidious and persistent mode of existence have been recently undertaken. In a previous issue of this journal, Hancock and Perego provided strong evidence that the activity of a single enzyme controlled by a single signal transduction pathway plays a key role in the formation of E. faecalis biofilms (11). This finding establishes a new focus for investigating the molecular mechanisms of biofilm development and raises the possibility for development of a targeted therapeutic agent to prevent the establishment of biofilms in vivo. The culprit enzyme is a secreted zinc metalloprotease, gelatinase, a thermolysin-like M4 protease similar to those found in other bacterial pathogens (2). Gelatinase cleaves a broad range of substrates in vitro, including Azocoll, casein, gelatin, hemoglobin, plasmid conjugation factors, collagen, fibrin, and an autolysin (17, 30, 37). The expression of the gene encoding gelatinase, gelE, is dependent on the fsr genes that encode a two-component signal transduction system (24, 25). Hancock and Perego determined a role for fsr and gelE in biofilm formation after carrying out a systematic inactivation of each of the 18 genes predicted by a homology search of the fully sequenced genome of E. faecalis strain V583 to encode re-