The respective roles of BfmRS and PmrA in stress responses and antibiotic resistance in Acinetobacter baumannii

Abstract

The Gram-negative bacterium Acinetobacter baumannii is considered one of the most serious nosocomial pathogens worldwide. Predominantly responsible for ventilator-associated pneumonia, this insidious pathogen typically infects critically ill individuals and patients in long-term care. A. baumannii harbors a myriad of resistance mechanisms to aid its survival in both the environment and the host. Remarkably, A. baumannii does not encode an RpoS sigma factor homolog that is used to control the general stress response by the majority of Gram-negative bacteria. Instead, A. baumannii relies on an intricate regulatory network involving numerous two-component regulatory systems to respond to stress. Two key players that contribute to A. baumannii's "persist and resist" survival mechanisms are the BfmRS and PmrAB two-component regulatory systems. We have demonstrated that the response regulator BfmR can directly activate numerous stress-related pathways. We also provide evidence that the sensor kinase BfmS acts as a phosphatase to negatively regulate BfmR activity. Overall, we show that the BfmRS system harbors the characteristics of a master regulator by controlling the osmotic stress response, the oxidative stress response, the misfolded protein response, csu pili/fimbriae production, capsule polysaccharide biosynthesis, siderophore biosynthesis and transport, type IV pili production, and antibiotic resistance. Additionally, the PmrAB two-component system contributes to antibiotic resistance in A. baumannii. Clinically relevant mutations that arise in the pmrCAB operon often promote resistance to the last resort antibiotic colistin. We biochemically characterized the response regulator PmrA to identify its DNA-binding domain, in addition to the potential PmrA regulon. We also provide the first structural information for the PmrA N-terminal domain. Together, these data allowed us to analyze the structural and dynamic changes in two clinically relevant PmrA point mutants that alter the function of PmrA and promote colistin resistance. Understanding these regulatory mechanisms at a molecular level will allow us to explore novel ways to develop effective antimicrobial agents to combat this serious pathogen.