PROJECT SUMMARY Acinetobacter baumannii is among the most antibiotic-resistant pathogens known, and the emergence of isolates with enhanced virulence poses an urgent public health challenge. Understanding how the microorganism thwarts antibiotic and immune attack via its protective cell envelope is essential to developing new strategies for controlling this threat. Envelope synthesis and integrity in bacteria are typically maintained by a large number of response systems that control specific aspects of the envelope. A. baumannii, however, has diverged substantially from this paradigm. The pathogen lacks orthologs of many canonical envelope response proteins and instead relies on a single two-protein regulatory system to globally modulate every layer of the envelope and control both antibiotic resistance and ability to cause disease. This unique system, known as BfmRS, lowers susceptibility to a wide range of drugs, antagonizes innate immune killing, and facilitates development of lethal disease in mice. Intriguingly, a clinical isolate showing enhanced virulence requires the system for growth. BfmRS is therefore tightly linked to the intractability of infections with the pathogen and represents a key potential therapeutic target. Despite its fundamental importance, we lack an understanding of how the large BfmRS regulon controls broad- range drug resistance and pathogenicity, and what signals the system senses. The objective of the proposed studies is to understand how A. baumannii uses a single control circuit to simultaneously modulate resistance and virulence. Our central hypothesis is that BfmRS jointly controls the barrier to both drug penetration and innate immune attack by modulating the level of key outer membrane (OM) structures in response to disruptions in envelope protein production. We will test this hypothesis by pursuing three Aims, which build on our extensive preliminary data defining the BfmRS regulon and its chemical-genetic profile, as well as the phosphorylation cascade it uses for signaling. In Aim 1 we will test the model that BfmRS controls the bacterial interface with both antibiotics and innate immune effectors by modulating the OM barrier. In Aim 2, we will identify the antibiotic-induced and intrinsic stress signals that are sensed by BfmRS. In Aim 3, we will define the relationship between variability in BfmRS activity, growth-dependence, and virulence across diverse patient isolates as a test of the model that variation in BfmRS signaling level is a driver of enhanced virulence in invasive strains. This work will elucidate the mechanisms by which a unique regulatory system controls both resistance and pathogenicity in a critically important nosocomial microbe. These results will inform strategies for potentiating antibiotic and immune action for killing extensively drug-resistant bacteria.