Zinc (Zn) and copper (Cu) are essential for optimal innate immune function and nutritional deficiency in either metal leads to increased susceptibility to bacterial infection. Recently, the decreased survival of bacterial pathogens with impaired Cu and/or Zn detoxification systems in phagocytes and animal models of infection has been reported. Consequently, a model has emerged in which the host utilises Cu and/or Zn intoxication to reduce the intracellular survival of pathogens within phagocytes. This review describes and assesses the potential role for Cu and Zn intoxication in innate immune function and their direct bactericidal function.

Streptococcus pneumoniae requires manganese for colonization of the human host, but the underlying molecular basis for this requirement has not been elucidated. Recently, it was shown that zinc could compromise manganese uptake and that zinc levels increased during infection by S. pneumoniae in all the niches that it colonized. Here we show, by quantitative means, that extracellular zinc acts in a dose dependent manner to competitively inhibit manganese uptake by S. pneumoniae, with an EC₅₀ of 30.2 µM for zinc in cation-defined media. By exploiting the ability to directly manipulate S. pneumoniae accumulation of manganese, we analyzed the connection between manganese and superoxide dismutase (SodA), a primary source of protection for S. pneumoniae against oxidative stress. We show that manganese starvation led to a decrease in sodA transcription indicating that expression of sodA was regulated through an unknown manganese responsive pathway. Intriguingly, examination of recombinant SodA revealed that the enzyme was potentially a cambialistic superoxide dismutase with an iron/manganese cofactor. SodA was also shown to provide the majority of protection against oxidative stress as a S. pneumoniae ΔsodA mutant strain was found to be hypersensitive to oxidative stress, despite having wild-type manganese levels, indicating that the metal ion alone was not sufficiently protective. Collectively, these results provide a quantitative assessment of the competitive effect of zinc upon manganese uptake and provide a molecular basis for how extracellular zinc exerts a ‘toxic’ effect on bacterial pathogens, such as S. pneumoniae.

Background. Zinc plays an important role in human immunity, and it is known that zinc deficiency in the host is linked to increased susceptibility to bacterial infection. In this study, we investigate the role of zinc efflux in the pathogenesis of Streptococcus pyogenes (group A Streptococcus [GAS]), a human pathogen responsible for superficial infections, such as pharyngitis and impetigo, and severe invasive infections. Methods. The clinically important M1T1 wild-type strain was used in this study, and isogenic mutants were constructed with deletions in the czcD gene (Spy0653; which encodes a putative zinc efflux pump) and adjacent gczA gene (Spy0654; which encodes a putative zinc-dependent activator of czcD). Wild-type, isogenic mutants and complemented strains were tested for resistance against zinc stress, intracellular zinc accumulation, and virulence. Results. Both czcD and gczA mutants exhibited increased sensitivity to zinc. Transcriptional analyses indicate that GczA upregulates czcD in response to zinc. Both mutants displayed increased susceptibility to human neutrophil killing and reduced virulence in a murine infection model. Furthermore, we showed that neutrophils mobilize zinc in response to GAS. Conclusions. These data indicate that the innate immune system may use zinc as an antimicrobial agent and that zinc efflux is an important contributor to GAS pathogenesis.

Neutrophils release free zinc to eliminate the phagocytosed bacterial pathogen Streptococcus pyogenes (Group A Streptococcus; GAS). In this study, we investigated the mechanisms underpinning zinc toxicity towards this human pathogen, responsible for diseases ranging from pharyngitis and impetigo, to severe invasive infections. Using the globally-disseminated M1T1 GAS strain, we demonstrate that zinc stress impairs glucose metabolism through the inhibition of the glycolytic enzymes phosphofructokinase and glyceraldehyde-3-phosphate dehydrogenase. In the presence of zinc, a metabolic shift to the tagatose-6-phosphate pathway allows conversion of D-galactose to dihydroxyacetone phosphate and glyceraldehyde phosphate, partially bypassing impaired glycolytic enzymes to generate pyruvate. Additionally, zinc inhibition of phosphoglucomutase results in decreased capsule biosynthesis. These data indicate that zinc exerts it toxicity via mechanisms that inhibit both GAS central carbon metabolism and virulence pathways.

Cadmium is a transition metal ion that is highly toxic in biological systems. Although relatively rare in the Earth’s crust, anthropogenic release of cadmium since industrialization has increased biogeochemical cycling and the abundance of the ion in the biosphere. Despite this, the molecular basis of its toxicity remains unclear. Here we combine metal-accumulation assays, high-resolution structural data and biochemical analyses to show that cadmium toxicity, in Streptococcus pneumoniae, occurs via perturbation of first row transition metal ion homeostasis. We show that cadmium uptake reduces the millimolar cellular accumulation of manganese and zinc, and thereby increases sensitivity to oxidative stress. Despite this, high cellular concentrations of cadmium (~17 mM) are tolerated, with negligible impact on growth or sensitivity to oxidative stress, when manganese and glutathione are abundant. Collectively, this work provides insight into the molecular basis of cadmium toxicity in prokaryotes, and the connection between cadmium accumulation and oxidative stress.

Catheter-associated urinary tract infection (CAUTI) is the most common nosocomial infection in the United States. UropathogenicEscherichia coli(UPEC), the most common cause of CAUTI, can form biofilms on indwelling catheters. Here, we identify and characterize novel factors that affect biofilm formation by UPEC strains that cause CAUTI. Sixty-five CAUTI UPEC isolates were characterized for phenotypic markers of urovirulence, including agglutination and biofilm formation. One isolate,E. coliMS2027, was uniquely proficient at biofilm growth despite the absence of adhesins known to promote this phenotype. Mini-Tn5mutagenesis ofE. coliMS2027 identified several mutants with altered biofilm growth. Mutants containing insertions in genes involved in O antigen synthesis (rmlCandmanB) and capsule synthesis (kpsM) possessed enhanced biofilm phenotypes. Three independent mutants deficient in biofilm growth contained an insertion in a gene locus homologous to the type 3 chaperone-usher class fimbrial genes ofKlebsiella pneumoniae. These type 3 fimbrial genes (mrkABCDF), which were located on a conjugative plasmid, were cloned fromE. coliMS2027 and could complement the biofilm-deficient transconjugants when reintroduced on a plasmid. Primers targeting themrkBchaperone-encoding gene revealed its presence in CAUTI strains ofCitrobacter koseri,Citrobacter freundii,Klebsiella pneumoniae, andKlebsiella oxytoca. All of thesemrkB-positive strains caused type 3 fimbria-specific agglutination of tannic acid-treated red blood cells. This is the first description of type 3 fimbriae inE. coli,C. koseri, andC. freundii. Our data suggest that type 3 fimbriae may contribute to biofilm formation by different gram-negative nosocomial pathogens.