Copper Resistance of the Emerging Pathogen Acinetobacter baumannii

Abstract

Acinetobacter baumannii is an important emerging pathogen that is capable of causing many types of severe infection, especially in immunocompromised hosts. Since A. baumannii can rapidly acquire antibiotic resistance genes, many infections are on the verge of being untreatable, and novel therapies are desperately needed. To investigate the potential utility of copper-based antibacterial strategies against Acinetobacter infections, we characterized copper resistance in a panel of recent clinical A. baumannii isolates. Exposure to increasing concentrations of copper in liquid culture and on solid surfaces resulted in dose-dependent and strain-dependent effects; levels of copper resistance varied broadly across isolates, possibly resulting from identified genotypic variation among strains. Examination of the growth-phase-dependent effect of copper on A. baumannii revealed that resistance to copper increased dramatically in stationary phase. Moreover, A. baumannii biofilms were more resistant to copper than planktonic cells but were still susceptible to copper toxicity. Exposure of bacteria to subinhibitory concentrations of copper allowed them to better adapt to and grow in high concentrations of copper; this copper tolerance response is likely achieved via increased expression of copper resistance mechanisms. Indeed, genomic analysis revealed numerous putative copper resistance proteins that share amino acid homology to known proteins in Escherichia coli and Pseudomonas aeruginosa. Transcriptional analysis revealed significant upregulation of these putative copper resistance genes following brief copper exposure. Future characterization of copper resistance mechanisms may aid in the search for novel antibiotics against Acinetobacter and other highly antibiotic-resistant pathogens. IMPORTANCEAcinetobacter baumannii causes many types of severe nosocomial infections; unfortunately, some isolates have acquired resistance to almost every available antibiotic, and treatment options are incredibly limited. Copper is an essential nutrient but becomes toxic at high concentrations. The inherent antimicrobial properties of copper give it potential for use in novel therapeutics against drug-resistant pathogens. We show that A. baumannii clinical isolates are sensitive to copper in vitro, both in liquid and on solid metal surfaces. Since bacterial resistance to copper is mediated though mechanisms of efflux and detoxification, we identified genes encoding putative copper-related proteins in A. baumannii and showed that expression of some of these genes is regulated by the copper concentration. We propose that the antimicrobial effects of copper may be beneficial in the development of future therapeutics that target multidrug-resistant bacteria.

Zinc stress induces copper depletion in Acinetobacter baumannii

“…1b). The limited impact of 1000 μM Cu on the growth of A. baumannii corroborates observations from a previous study which examined copper stress in a nutrient limited media [58]. Treatment with both Zn + Cu had a synergistic impact on A. baumannii ATCC 17978 growth (Fig.…”

Iron Efflux by PmtA Is Critical for Oxidative Stress Resistance and Contributes Significantly to Group A Streptococcus Virulence

“…In addition, the host recruits NRAMP-1 transporters to the phagosomal membrane to shuttle iron away from phagocytosed pathogens to limit their growth within phagosomes (44)(45)(46). However, recent evidence indicates that hosts also deploy defense strategies to increase intracellular Cu(II) and Zn(II) levels and impose metal toxicity on microbial pathogens (8,9,11,12,47). However, given the significance of iron to bacterial physiology and the presence of redundant host nutritional defense mechanisms to impose iron limitation both in the extracellular compartment and within phagosomes, our observations that GAS possesses machinery to counter iron toxicity were surprising.…”

“…Host innate immune cells, such as neutrophils and macrophages, accumulate copper and zinc around phagocytosed bacteria and impose metal toxicity (8)(9)(10)(11)(12). Conversely, bacteria employ metal efflux pumps to reduce the intracellular metal concentration and negate host-induced metal toxicity (8,11,12). In contrast with copper and zinc, relatively little is known regarding the possible role of iron toxicity as a host nutritional immune mechanism during infection.…”

“…Emerging evidence indicates that the host also employs metal toxicity at microbial colonization surfaces, predominantly within phagosomes, to mediate microbial killing and control bacterial infection (8,9). Host innate immune cells, such as neutrophils and macrophages, accumulate copper and zinc around phagocytosed bacteria and impose metal toxicity (8)(9)(10)(11)(12). Conversely, bacteria employ metal efflux pumps to reduce the intracellular metal concentration and negate host-induced metal toxicity (8,11,12).…”

“…Host innate immune cells, such as neutrophils and macrophages, accumulate copper and zinc around phagocytosed bacteria and impose metal toxicity (8-12). Conversely, bacteria employ metal efflux pumps to reduce the intracellular metal concentration and negate host-induced metal toxicity (8,11,12). In …”