Role of oxidants in microbial pathophysiology

Periplasmic superoxide dismutase protects Salmonella from products of phagocyte NADPH-oxidase and nitric oxide synthase

Superoxide dismutase (SOD) catalyzes the conversion of superoxide radical to hydrogen peroxide. Periplasmic localization of bacterial Cu,Zn-SOD has suggested a role of this enzyme in defense against extracellular phagocyte-derived reactive oxygen species. Sequence analysis of regions flanking the Salmonella typhimurium sodC gene encoding Cu,Zn-SOD demonstrates significant homology to lambda phage proteins, reflecting possible bacteriophage-mediated horizontal gene transfer of this determinant among pathogenic bacteria. Salmonella deficient in Cu,Zn-SOD has reduced survival in macrophages and attenuated virulence in mice, which can be restored by abrogation of either the phagocyte respiratory burst or inducible nitric oxide synthase. Moreover, a sodC mutant is extremely susceptible to the combination of superoxide and nitric oxide. These observations suggest that SOD protects periplasmic or inner membrane targets by diverting superoxide and limiting peroxynitrite formation, and they demonstrate the ability of the respiratory burst and nitric oxide synthase to synergistically kill microbial pathogens in vivo.

Ascorbate recycling in human neutrophils: induction by bacteria

Ascorbate (vitamin C) recycling occurs when extracellular ascorbate is oxidized, transported as dehydroascorbic acid, and reduced intracellularly to ascorbate. We investigated microorganism induction of ascorbate recycling in human neutrophils and in microorganisms themselves. Ascorbate recycling was determined by measuring intracellular ascorbate accumulation. Ascorbate recycling in neutrophils was induced by both Gram-positive and Gram-negative pathogenic bacteria, and the fungal pathogen Candida albicans. Induction of recycling resulted in as high as a 30-fold increase in intracellular ascorbate compared with neutrophils not exposed to microorganisms. Recycling occurred at physiologic concentrations of extracellular ascorbate within 20 min, occurred over a 100-fold range of effector/target ratios, and depended on oxidation of extracellular ascorbate to dehydroascorbic acid. Ascorbate recycling did not occur in bacteria nor in C. albicans. Ascorbate did not enter microorganisms, and dehydroascorbic acid entry was less than could be accounted for by diffusion. Because microorganism lysates reduced dehydroascorbic acid to ascorbate, ascorbate recycling was absent because of negligible entry of the substrate dehydroascorbic acid. Because ascorbate recycling occurs in human neutrophils but not in microorganisms, it may represent a eukaryotic defense mechanism against oxidants with possible clinical implications.

Contribution of the Mn-cofactored superoxide dismutase (SodA) to the virulence of Yersinia enterocolitica serotype O8

Enteric pathogens harbor a set of enzymes (e.g., superoxide dismutases [SOD]) for detoxification of endogenous and exogenous reactive oxygen species which are encountered during infection. To analyze the role of the Mn-cofactored SOD (SodA) in the pathogenicity of yersiniae, we cloned the sodA gene of Yersinia enterocolitica serotype O8 by complementation of an Escherichia coli sodA sodB mutant and subsequently constructed an isogenic mutant by allelic exchange. Sequence analysis revealed an open reading frame that enabled the deduction of a sequence of 207 amino acids with 85% identity to SodA of E. coli. In a mouse infection model, the sodA null mutant was strongly attenuated in comparison to its parental strain. After intravenous infection, the survival and multiplication of the mutant in the spleen and liver were markedly reduced. In contrast, inactivation of sodA had only minor effects on survival and multiplication in the gut and Peyer's patches, as could be demonstrated in the orogastric infection model. The reduction in virulence was accompanied by a low but significant increase of susceptibility of the soda mutant to bacterial killing by polymorphonuclear leukocytes (PMN) and an alteration of the intracellular chemiluminescence response of PMN. These results suggest that the resistance of Y. enterocolitica to exogenous oxygen radicals produced by phagocytes involves the Mn-cofactored SOD. The important role of sodA for the pathogenicity of Y. enterocolitica could also be due to detoxification of endogenous, metabolically produced oxygen radicals which are encountered by extracellular enteric pathogens during the invasion of the host.

Cloning and disruption of the antigenic catalase gene of Aspergillus fumigatus

Aspergillus fumigatus possesses two catalases (described as fast and slow on the basis of their electrophoretic mobility). The slow catalase has been recognized as a diagnostic antigen for aspergillosis in immunocompetent patients. The antigenic catalase has been purified. The enzyme is a tetrameric protein composed of 90-kDa subunits. The corresponding cat1 gene was cloned, and sequencing data show that the cat1 gene codes for a 728-amino-acid polypeptide. A recombinant protein expressed in Pichia pastoris is enzymatically active and has biochemical and antigenic properties that are similar to those of the wild-type catalase. Molecular experiments reveal that CAT1 contains a signal peptide and a propeptide of 15 and 12 amino acid residues, respectively. cat1-disrupted mutants that were unable to produce the slow catalase were as sensitive to H2O2 and polymorphonuclear cells as the wild-type strain. In addition, there was no difference in pathogenicity between the cat1 mutant and its parental cat1+ strain in a murine model of aspergillosis.

Lethal outcome of uterine infection in pregnant but not in nonpregnant rats and increased death rate with inhibition of nitric oxide

PROBLEM

Limited information is available on potential differences in sensitivity to urogenital infections between pregnant and nonpregnant hosts.

METHOD OF STUDY

In this study, we evaluated Escherichia coli infectious complications in pregnant and nonpregnant rats and the effect of nitric oxide (NO) inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), on the outcome of an experimental uterine infection.

RESULTS

Of the infected pregnant animals, 31% were found dead in 24-48 hr. The death rate was increased 2-fold (66%) with L-NAME treatment. No deaths occurred in nonpregnant animals with or without L-NAME treatment. The rate of uterine infection in pregnant animals was about 10-fold higher than in nonpregnant animals.

CONCLUSION

We propose that infectious complications of pregnancy may be related to gestation-dependent sensitivity to the pathogenic microorganism and the host NO status.

SlyA, a transcriptional regulator of Salmonella typhimurium, is required for resistance to oxidative stress and is expressed in the intracellular environment of macrophages

Appropriate regulation of genes enables Salmonella typhimurium to adapt to the intracellular environment of the host. The Salmonella slyA gene is in a family of transcriptional regulators that may play an important role in this adaptation. We have previously shown that slyA mutant Salmonella strains are profoundly attenuated for virulence and do not survive in macrophages. In this study, we demonstrate that the expression of multiple Salmonella proteins is regulated by SlyA during stationary phase and during infection of macrophages. Both of these conditions also induced the expression of a slyA::lacZ transcriptional fusion. Expression of the slyA::lacZ transcriptional fusion increased 15-fold in stationary phase and was not dependent on the stationary-phase sigma factor, RpoS. slyA mutant Salmonella strains were sensitive to oxidative products of the respiratory burst, including hydrogen peroxide and the products of the redox cycling compound paraquat, but not to nitric oxide donors. These results suggest that the SlyA regulon is activated during infection of the host and is required for resistance to toxic oxidative products of the reticuloendothelial system.

Macrophages in resistance to candidiasis

Candida albicans, an increasingly common opportunistic pathogenic fungus, frequently causes disease in immunodeficient but not immunocompetent hosts. Clarifying the role of the phagocytic cells that participate in resistance to candidiasis not only is basic to understanding how the host copes with this dimorphic pathogen but also will expedite the development of innovative prophylactic and therapeutic approaches for treating the multiple clinical presentations that candidiasis encompasses. In this review, we present evidence that a diverse population of mononuclear phagocytes, in different states of activation and differentiation and from a variety of host species, can phagocytize C. albicans blastoconidia via an array of opsonic and nonopsonic mechanisms and can kill C. albicans blastoconidia and hyphae by means of oxygen-dependent and -independent mechanisms. Reactive nitrogen intermediates should now be added to the well-established candidacidal reactive oxygen intermediates of macrophages. Furthermore, what were thought to be two independent pathways, i.e., nitric oxide and superoxide anion, have now been shown to combine to form a potent macrophage candidacidal molecule, peroxynitrite. In contrast to monocytes and neutrophils, which are important in resistance to early stages of C. albicans infections, more differentiated macrophages activated by cytokines such as gamma interferon participate in the acquired resistance of hosts with C. albicans-specific, cell-mediated immunity. Evidence presented in this review demonstrates that mononuclear phagocytes, in some instances in the absence of other professional phagocytes such as neutrophils, play an import role in resistance to systemic and mucosal candidiasis.