Acinetobacter baumannii is a major nosocomial and community pathogen worldwide that is highly resistant to antimicrobials. A key aspect of its global success is the ability to tolerate a wide range of stresses including those imposed by commonly used hospital disinfectants or during host infection e.g. oxidative agents and heavy metals. Bacterial species often switch into a non-growing “persister” state after exposure to environmental stresses. This switching between active growth and maladapted stressed states is often mediated by an alarmone ppGpp in conjunction with alternative sigma factors of RNA polymerase such as RpoS. However, A. baumannii lacks RpoS and its molecular switches underlying this metabolic transition in stress conditions remain elusive.
In this study, we used a functional genomic approach to understand the molecular stress responses of A. baumannii. Transposon directed insertion-site sequencing (TraDIS) was used to assay the fitness of >100,000 random Tn5 mutants of A. baumannii in the presence of copper (Cu) or zinc (Zn) stress. Genes and pathways conditionally essential in these stresses were identified and gene ontology analysis revealed that they were mostly involved in efflux, and envelop biogenesis. Further, a largely uncharacterised global pleiotropic regulator that acted as a switch between Cu and Zn stress was identified: the mutant had increased fitness under copper stress whereas the opposite is true for the stress. In vivomurine and Galleria mellonella infection studies showed that bacteria lacking this regulator were no longer virulent, in a niche specific manner. In vitro assays showed the regulator mutant exhibited increased sensitivity to human serum yet promoted some virulence-associated properties like biofilm formation and capsule production. Our results collectively provide the detailed insight into the role of a key regulator in stress protection and virulence in A. baumannii and establishes a roadmap for how it coordinates the stress response for this important pathogen.