Host-mediated zinc sequestration during Acinetobacter baumannii infection

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PROJECT SUMMARY Acinetobacter baumannii is an important nosocomial pathogen that causes a range of diseases, including respiratory and urinary tract infections, meningitis, endocarditis, wound infections, and bacteremia. In fact, A. baumannii is now responsible for up to 20% of all intensive care unit infections in some regions of the world with pneumonia being the most common presentation. The clinical significance of A. baumannii has been propelled by this organism’s rapid acquisition of resistance to virtually all antibiotics. The identification of novel targets for therapeutic intervention is critical to our ability to protect the public health from this emerging infectious threat. A promising area of therapeutic development exploits the idea that all bacterial pathogens require nutrient metal to cause infection. This approach mimics host-mediated metal sequestration, which is a potent defense against infection in a process termed “nutritional immunity”. Paradoxically, dietary restriction of metal exacerbates infection underscoring how alterations in metal abundance can profoundly impact the outcome of host-pathogen interactions. The host protein calprotectin (CP) is one of the most important contributors to immune-mediated metal restriction and CP protects against infection through the chelation of nutrient metals, including zinc (Zn). Using CP as a probe, we have uncovered a genetic locus within A. baumannii that is important for survival during conditions of CP-dependent Zn starvation. This locus encodes a member of the conserved COG0523 family of GTPases that we have named Zur-induced GTPase A (ZigA). We have also found a second COG0523 member within the A. baumannii genome that we refer to as 0934. ZigA and 0934 have biological properties consistent with metallochaperones that insert Zn into client proteins. Our preliminary data indicate that 0934 and ZigA bind and regulate client metalloenzymes involved in cell wall synthesis and recycling. Based on these fundamental discoveries, we hypothesize that A. baumannii colonization of the lung leads to massive accumulation of CP which starves A. baumannii for nutrient Zn. We further hypothesize that ZigA and 0934 regulate cell wall remodeling during Zn starvation to accommodate insertion of nutrient Zn transporters into the cell envelope that are necessary to combat host- or dietary-imposed Zn starvation. To test this central model, we propose a series of experiments aimed at understanding the mechanism and pathophysiological consequence of the A. baumannii response to dietary and host-imposed Zn deprivation during pneumonia. In these studies, we will (i) define the molecular interactions of ZigA and 0934 with their metalloprotein clients, (ii) interrogate the physiological function of ZigA and 0934 in A. baumannii, and (iii) determine the importance of ZigA and 0934 in response to Zn starvation during the pathogenesis of pneumonia. This research will establish a new paradigm for how bacteria respond to dietary- or immune-mediated Zn restriction during infection through metallochaperone-dependent regulation of cell wall homeostasis.