Virstatin is an inhibitor of the ToxT transcriptional regulator of V. cholerae, which regulates transcription of virulence factors that enable intestinal colonization. It inhibits ToxT dimerization and decreases expression of cholera toxin (CT) and toxin coregulated pilus (TCP) when used at a concentration of 50 μM but virstatin does not inhibit growth of V. cholerae at this concentration (MBCs = 600 and 1,200 μM for O395 and C6706 strains, respectively). Virstatin administration protects infant mice (5 to 6 days old) from intestinal colonization by ToxT-dependent V. cholerae but not from strains that colonize via non-ToxT-dependent mechanisms. Virstatin (100 μM) inhibits biofilm formation by A. baumannii by 38% under static conditions, which is at a lower concentration than that which inhibits growth (MIC = 1.6 mM). It decreases the motility of 60% of 30 mobile A. baumannii strains. Virstatin also binds to the accessory cholera enterotoxin (Ace) from V. cholerae (K = 9 x 10 M; K = 11 µM).