The Gram-negative bacterium Acinetobacter baumannii is a leading cause of nosocomial infections in humans. The emergence of A. baumannii strains resistant to most, and in some cases, all available antibiotics has made the treatment of these infections exceedingly difficult. This underlies the importance of finding new ways to combat these infections. Our work has demonstrated that A. baumannii can rapidly switch between virulent (VIR-O) and avirulent (AV-T) subpopulations. This switch is controlled, in part, by the stochastic activation of a family of TetR-type transcriptional regulators (TTTRs). Remarkably, these TTTRs can be activated alone or in different combinations, representing a new mechanism for the generation of phenotypic heterogeneity in bacteria. Expression of the TTTRs is primarily controlled at the level of Rho-dependent transcriptional termination within the mRNA leader regions of each TTTR. In VIR-O cells the level of termination is high and the TTTRs are OFF. In AV-T cells, termination is blocked by an unknown mechanism and the TTTRs are expressed. The goals of this proposal are to understand the role of a regulatory cascade beginning with the GacSA two-component system. We hypothesize that GacA exists in a bistable ON state in VIR-O cells and an OFF state in AV-T cells. In the ON state, a set of GacA regulated sRNAs are expressed which inhibit the CsrA protein. In AV-T cells, GacA is OFF and CsrA is active, and we hypothesize CsrA can then block Rho-dependent termination in the TTTR leader regions. This switch represents a possible Achilles-heel for A. baumannii pathogenesis and an increased understanding of the mechanisms that regulate the switch may lead to interventions that drive virulent cells to the avirulent state and cripple the ability to cause disease.