The Gram-negative bacterium Acinetobacter baumannii is a leading cause ofnosocomial infections in humans. The emergence of A. baumannii strains resistant tomost, and in some cases, all available antibiotics has made the treatment of theseinfections exceedingly difficult. This underlies the importance of finding new ways tocombat these infections. This proposal addresses regulation of a high-frequencyphenotypic switch in A. baumannii that controls virulence. The goals of this proposal areto understand regulatory mechanism responsible for this switch andidentify/characterize a quorum sensing signal that stimulates this switch.