Periplasmic copper-zinc superoxide dismutase of Legionella pneumophila: role in stationary-phase survival

Oxygen-driven divergence of marine group II archaea reflected by transitions of superoxide dismutases

IMPORTANCE

Reactive oxygen species (ROS), including superoxide anion, is a series of substances that cause oxidative stress for all organisms. Marine group II (MGII) archaea are mainly live in the surface seawater and exposed to considerable ROS. Therefore, it is important to understand the antioxidant capacity of MGII. Our research found that Fe/Mn- superoxide dismutase (Fe/MnSOD) may be more suitable for MGII to resist oxidative damage, and the changes in oxygen concentrations and SOD metallic cofactors play an important role in the selection of SOD by the 17 clades of MGII, which in turn affects the species differentiation of MGII. Overall, this study provides insight into the co-evolutionary history of these uncultivated marine archaea with the earth system.

L. pneumophila resists its self-harming metabolite HGA via secreted factors and collective peroxide scavenging

IMPORTANCE

Before environmental opportunistic pathogens can infect humans, they must first successfully grow and compete with other microbes in nature, often via secreted antimicrobials. We previously discovered that the bacterium Legionella pneumophila, the causative agent of Legionnaires' disease, can compete with other microbes via a secreted molecule called HGA. Curiously, L. pneumophila strains that produce HGA is not wholly immune to its toxicity, making it a mystery how these bacteria can withstand the "friendly fire" of potentially self-targeting antimicrobials during inter-bacterial battles. Here, we identify several strategies that allow the high-density bacterial populations that secrete HGA to tolerate its effects. Our study clarifies how HGA works. It also points to some explanations of why it is difficult to disinfect L. pneumophila from the built environment and prevent disease outbreaks.

Metal manipulators and regulators in human pathogens: A comprehensive review on microbial redox copper metalloenzymes "multicopper oxidases and superoxide dismutases"

The chemistry of metal ions with human pathogens is essential for their survival, energy generation, redox signaling, and niche dominance. To regulate and manipulate the metal ions, various enzymes and metal chelators are present in pathogenic bacteria. Metalloenzymes incorporate transition metal such as iron, zinc, cobalt, and copper in their reaction centers to perform essential metabolic functions; however, iron and copper have gained more importance. Multicopper oxidases have the ability to perform redox reaction on phenolic substrates with the help of copper ions. They have been reported from Enterobacteriaceae, namely Salmonella enterica, Escherichia coli, and Yersinia enterocolitica, but their role in virulence is still poorly understood. Similarly, superoxide dismutases participate in reducing oxidative stress and allow the survival of pathogens. Their role in virulence and survival is well established in Salmonella typhimurium and Mycobacterium tuberculosis. Further, to ensure survival against stress, like metal starvation or metal toxicity, redox metalloenzymes and metal transportation systems of pathogens actively participate in metal homeostasis. Recently, the omics and protein structure biology studies have helped to predict new targets for regulation the colonization potential of the pathogenic strains. The current review is focused on the major roles of redox metalloenzymes, especially MCOs and SODs of human pathogenic bacteria.

Dynamics of Legionella Community Interactions in Response to Temperature and Disinfection Treatment: 7 Years of Investigation

In man-made water distribution systems, Legionella community interactions remain unknown, due to their ability to change from sessile to planktonic states or live in viable but non-culturable forms, in response to anthropic and environmental stress. During 7 years of hospital Legionella surveillance, in 191 hot water positive samples, the interactions among the Legionella species, temperature, and disinfection treatment were evaluated. Legionella was isolated following ISO 11731:2017, and identification was performed by mip gene sequencing and sequence-based typing (SBT) for L. anisa or L. rubrilucens and L. pneumophila, respectively. The species with the higher frequency of isolation was L. pneumophila serogroup 1 (78.53%; 4865.36 ± 25,479.11 cfu/L), followed by L. anisa (54.45%; 558.79 ± 2637.41 cfu/L) and L. rubrilucens (21.99%; 307.73 ± 1574.95 cfu/L), which were sometimes present together. Spearman's rho correlation test was conducted among the species with respect to temperature and disinfectant (H₂O₂/Ag⁺). The results showed a generally positive interaction among these species sharing the same environment, except for competition between L. anisa and L. rubrilucens. High temperature (48.83 ± 2.59 °C) and disinfection treatment (11.58 ± 4.99 mg/L) affected the presence of these species. An exception was observed with L. anisa, which showed disinfection treatment resistance. For the purposes of environmental surveillance, it is fundamental to better understand the interactions and dynamic of the Legionella community in man-made water systems in order to choose the proper physical or chemical treatments. The simultaneous presence of different Legionella species could result in an increased resistance to high temperature and disinfectant treatment, leading to changes in contamination level and species diversity.

Old dogs, new tricks: New insights into the iron/manganese superoxide dismutase family

Superoxide dismutases (SODs) are ancient enzymes of widespread importance present in all domains of life. Many insights have been gained into these important enzymes over the 50 years since their initial description, but recent studies in the context of microbial pathogenesis have resulted in findings that challenge long established dogmas. The repertoire of SODs that bacterial pathogens encode is diverse both in number and in metal dependencies, including copper, copper and zinc, manganese, iron, and cambialistic enzymes. Other bacteria also possess nickel dependent SODs. Compartmentalization of SODs only partially explains their diversity. The need for pathogens to maintain SOD activity across distinct hostile environments encountered during infection, including those limited for essential metals, is also a driver of repertoire diversity. SOD research using pathogenic microbes has also revealed the apparent biochemical ease with which metal specificity can change within the most common family of SODs. Collectively, these studies are revealing the dynamic nature of SOD evolution, both that of individual SOD enzymes that can change their metal specificity to adapt to fluctuating cellular metal availability, and of a cell's repertoire of SOD isozymes that can be differentially expressed to adapt to fluctuating environmental metal availability in a niche.

Metagenomic recovery of two distinct comammox Nitrospira from the terrestrial subsurface

The recently discovered comammox process encompasses both nitrification steps, the aerobic oxidation of ammonia and nitrite, in a single organism. All known comammox bacteria are affiliated with Nitrospira sublineage II and can be grouped into two distinct clades, referred to as A and B, based on ammonia monooxygenase phylogeny. In this study, we report high-quality draft genomes of two novel comammox Nitrospira from the terrestrial subsurface, representing one clade A and one clade B comammox organism. The two metagenome-assembled genomes were compared with other representatives of Nitrospira sublineage II, including both canonical and comammox Nitrospira. Phylogenomic analyses confirmed the affiliation of the two novel Nitrospira with comammox clades A and B respectively. Based on phylogenetic distance and pairwise average nucleotide identity values, both comammox Nitrospira were classified as novel species. Genomic comparison revealed high conservation of key metabolic features in sublineage II Nitrospira, including respiratory complexes I-V and the machineries for nitrite oxidation and carbon fixation via the reductive tricarboxylic acid cycle. In addition, the presence of the enzymatic repertoire for formate and hydrogen oxidation in the Rifle clades A and B comammox genomes, respectively, suggest a broader distribution of these metabolic features than previously anticipated.

Deletion of oxyR in Legionella pneumophila causes growth defect on agar

The intracellular pathogen Legionella pneumophila (Lp) is a strict aerobe, surviving and replicating in environments where it frequently encounters reactive oxygen species (ROS), such as the nutrient-poor water environment and its replicative niche inside host cells. In many proteobacteria, the LysR-type regulator OxyR controls the oxidative stress response; however, the importance of the OxyR homologue in Lp is still unclear. Therefore, we undertook the characterization of phenotypes associated with the deletion of oxyR in Lp. Contrary to the wild type, the oxyR deletion mutant exhibits a severe growth defect on charcoal - yeast extract (CYE) agar lacking α-ketoglutarate supplementation. Growth in AYE broth (CYE without agar and charcoal), in amoeba and in human cultured macrophages, and survival in water is unaffected by the deletion. Supplementing CYE agar with antioxidants that neutralize ROS or introducing the oxyR gene in trans rescues the observed growth defect. Moreover, the mutant grows as well as the wild type on CYE plates made with agarose instead of agar, suggesting that a compound present in the latter is responsible for the growth defect phenotype.

Application of Hydrogen Peroxide as an Innovative Method of Treatment for Legionella Control in a Hospital Water Network

Objectives: To evaluate the effectiveness of hydrogen peroxide (HP) use as a disinfectant in the hospital water network for the control of Legionella spp. colonization. Methods: Following the detection of high levels of Legionella contamination in a 136-bed general hospital water network, an HP treatment of the hot water supply (25 mg/L) was adopted. During a period of 34 months, the effectiveness of HP on Legionella colonization was assessed. Legionella was isolated in accordance with ISO-11731 and identification was carried out by sequencing of the mip gene. Results: Before HP treatment, L. pneumophila sg 2–15 was isolated in all sites with a mean count of 9950 ± 8279 cfu/L. After one-month of HP treatment, we observed the disappearance of L. pneumophila 2–15, however other Legionella species previously not seen were found; Legionellapneumophila 1 was isolated in one out of four sampling sites (2000 cfu/L) and other non-pneumophila species were present in all sites (mean load 3000 ± 2887 cfu/L). Starting from September 2013, HP treatment was modified by adding food-grade polyphosphates, and in the following months, we observed a progressive reduction of the mean load of all species (p < 0.05), resulting in substantial disappearance of Legionella colonization. Conclusion: Hydrogen peroxide demonstrated good efficacy in controlling Legionella. Although in the initial phases of treatment it appeared unable to eliminate all Legionella species, by maintaining HP levels at 25 mg/L and adding food-grade polyphosphates, a progressive and complete control of colonization was obtained.

Transcriptional Profiling of Coxiella burnetii Reveals Extensive Cell Wall Remodeling in the Small Cell Variant Developmental Form

A hallmark of Coxiella burnetii, the bacterial cause of human Q fever, is a biphasic developmental cycle that generates biologically, ultrastructurally, and compositionally distinct large cell variant (LCV) and small cell variant (SCV) forms. LCVs are replicating, exponential phase forms while SCVs are non-replicating, stationary phase forms. The SCV has several properties, such as a condensed nucleoid and an unusual cell envelope, suspected of conferring enhanced environmental stability. To identify genetic determinants of the LCV to SCV transition, we profiled the C. burnetii transcriptome at 3 (early LCV), 5 (late LCV), 7 (intermediate forms), 14 (early SCV), and 21 days (late SCV) post-infection of Vero epithelial cells. Relative to early LCV, genes downregulated in the SCV were primarily involved in intermediary metabolism. Upregulated SCV genes included those involved in oxidative stress responses, arginine acquisition, and cell wall remodeling. A striking transcriptional signature of the SCV was induction (>7-fold) of five genes encoding predicted L,D transpeptidases that catalyze nonclassical 3-3 peptide cross-links in peptidoglycan (PG), a modification that can influence several biological traits in bacteria. Accordingly, of cross-links identified, muropeptide analysis showed PG of SCV with 46% 3-3 cross-links as opposed to 16% 3-3 cross-links for LCV. Moreover, electron microscopy revealed SCV with an unusually dense cell wall/outer membrane complex as compared to LCV with its clearly distinguishable periplasm and inner and outer membranes. Collectively, these results indicate the SCV produces a unique transcriptome with a major component directed towards remodeling a PG layer that likely contributes to Coxiella's environmental resistance.

Cloning, expression, and characterization of a Coxiella burnetii Cu/Zn Superoxide dismutase

Background

Periplasmically localized copper-zinc co-factored superoxide dismutase (SodC) enzymes have been identified in a wide range of Gram-negative bacteria and are proposed to protect bacteria from exogenously produced toxic oxygen radicals, which indicates the potential significance of a Coxiella burnetii SodC.

Results

Assays for SOD activity demonstrated that the cloned C. burnetii insert codes for a SOD that was active over a wide range of pH and inhibitable with 5 mM H₂O₂ and 1 mM sodium diethyldithiocarbamate, a characteristic of Cu/ZnSODs that distinguishes them from Fe or Mn SODs. The sodC was expressed by C. burnetii, has a molecular weight of approximately 18 kDa, which is consistent with the predicted molecular weight, and localized towards the periphery of C. burnetii. Over expression of the C. burnetii sodC in an E. coli sodC mutant restored resistance to H₂O₂ killing to wild type levels.

Conclusions

We have demonstrated that C. burnetii does express a Cu/ZnSOD that is functional at low pH, appears to be excreted, and was able to restore H₂O₂ resistance in an E. coli sodC mutant. Taken together, these results indicate that the C. burnetii Cu/ZnSOD is a potentially important virulence factor.

The Legionella pneumophila kai operon is implicated in stress response and confers fitness in competitive environments

Legionella pneumophila uses aquatic protozoa as replication niche and protection from harsh environments. Although L. pneumophila is not known to have a circadian clock, it encodes homologues of the KaiBC proteins of Cyanobacteria that regulate circadian gene expression. We show that L. pneumophila kaiB, kaiC and the downstream gene lpp1114, are transcribed as a unit under the control of the stress sigma factor RpoS. KaiC and KaiB of L. pneumophila do not interact as evidenced by yeast and bacterial two-hybrid analyses. Fusion of the C-terminal residues of cyanobacterial KaiB to Legionella KaiB restores their interaction. In contrast, KaiC of L. pneumophila conserved autophosphorylation activity, but KaiB does not trigger the dephosphorylation of KaiC like in Cyanobacteria. The crystal structure of L. pneumophila KaiB suggests that it is an oxidoreductase-like protein with a typical thioredoxin fold. Indeed, mutant analyses revealed that the kai operon-encoded proteins increase fitness of L. pneumophila in competitive environments, and confer higher resistance to oxidative and sodium stress. The phylogenetic analysis indicates that L. pneumophila KaiBC resemble Synechosystis KaiC2B2 and not circadian KaiB1C1. Thus, the L. pneumophila Kai proteins do not encode a circadian clock, but enhance stress resistance and adaption to changes in the environments.

SodA is a major metabolic antioxidant in Brucella abortus 2308 that plays a significant, but limited, role in the virulence of this strain in the mouse model

The gene designated BAB1_0591 in the Brucella abortus 2308 genome sequence encodes the manganese-cofactored superoxide dismutase SodA. An isogenic sodA mutant derived from B. abortus 2308, designated JB12, displays a small colony phenotype, increased sensitivity in vitro to endogenous superoxide generators, hydrogen peroxide and exposure to acidic pH, and a lag in growth when cultured in rich and minimal media that can be rescued by the addition of all 20 amino acids to the growth medium. B. abortus JB12 exhibits significant attenuation in both cultured murine macrophages and experimentally infected mice, but this attenuation is limited to the early stages of infection. Addition of the NADPH oxidase inhibitor apocynin to infected macrophages does not alleviate the attenuation exhibited by JB12, suggesting that the basis for the attenuation of the B. abortus sodA mutant is not an increased sensitivity to exogenous superoxide generated through the oxidative burst of host phagocytes. It is possible, however, that the increased sensitivity of the B. abortus sodA mutant to acid makes it less resistant than the parental strain to killing by the low pH encountered during the early stages of the development of the brucella-containing vacuoles in macrophages. These experimental findings support the proposed role for SodA as a major cytoplasmic antioxidant in brucella. Although this enzyme provides a clear benefit to B. abortus 2308 during the early stages of infection in macrophages and mice, SodA appears to be dispensable once the brucellae have established an infection.

Legionella pneumophila transcriptional response to chlorine treatment

Legionella pneumophila is a ubiquitous environmental microorganism found in freshwater that can cause an acute form of pneumonia known as Legionnaires' disease. Despite widespread use of chlorine to ensure drinking water quality and awareness that L. pneumophila may escape these treatments, little is known about its effects on L. pneumophila. The aim of this study was to investigate the L. pneumophila transcriptional response induced by chlorine treatment. Transcriptome analysis, using DNA arrays, showed that a sublethal dose of chlorine induces a differential expression of 391 genes involved in stress response, virulence, general metabolism, information pathways and transport. Many of the stress response genes were significantly upregulated, whereas a significant number of virulence genes were repressed. In particular, exposure of L. pneumophila to chlorine induced the expression of cellular antioxidant proteins, stress proteins and transcriptional regulators. In addition, glutathione S-transferase specific activity was enhanced following chlorine treatment. Our results clearly indicate that chlorine induces expression of proteins involved in cellular defence mechanisms against oxidative stress that might be involved in adaptation or resistance to chlorine treatment.

Virulence properties of the legionella pneumophila cell envelope

The bacterial envelope plays a crucial role in the pathogenesis of infectious diseases. In this review, we summarize the current knowledge of the structure and molecular composition of the Legionella pneumophila cell envelope. We describe lipopolysaccharides biosynthesis and the biological activities of membrane and periplasmic proteins and discuss their decisive functions during the pathogen–host interaction. In addition to adherence, invasion, and intracellular survival of L. pneumophila, special emphasis is laid on iron acquisition, detoxification, key elicitors of the immune response and the diverse functions of outer membrane vesicles. The critical analysis of the literature reveals that the dynamics and phenotypic plasticity of the Legionella cell surface during the different metabolic stages require more attention in the future.

Iso-superoxide dismutase in Deinococcus grandis, a UV resistant bacterium

Deinococcus grandis possesses two types of superoxide dismutase (SOD, E. C. 1.15.1.1.) that show distinct electrophoretic behavior, one that migrates slowly and the other that migrates rapidly (SOD-1 and SOD-2, respectively). In this study, SOD-1 was uniformly and abundantly detected, regardless of growth phase, whereas SOD-2 was not detected during early growth, but was detectable from the exponential growth phase. In addition, a substantial increase in SOD-2 was observed in cells that were treated with potassium superoxide or UV, which suggests that SOD-2 is an inducible protein produced in response to stressful environments. Insensitivity of SOD-1 to both H(2)O(2) and cyanide treatment suggests that SOD-1 is MnSOD. However, SOD-2 would be FeSOD, since it lost activity in response to H(2)O(2) treatment, but not to cyanide. Localization studies of D. grandis iso-SODs in sucrose-shocked cells suggest that SOD-1 is a membrane-associated enzyme, whereas SOD-2 is a cytosolic enzyme. In conclusion, SOD-1 seems to be an essential constitutive enzyme for viability and SOD-2 appears to be an inducible enzyme that is probably critical for survival upon UV irradiation and oxidative stress.

An ortholog of OxyR in Legionella pneumophila is expressed postexponentially and negatively regulates the alkyl hydroperoxide reductase (ahpC2D) operon

Legionella pneumophila expresses two peroxide-scavenging alkyl hydroperoxide reductase systems (AhpC1 and AhpC2D) that are expressed differentially during the bacterial growth cycle. Functional loss of the postexponentially expressed AhpC1 system is compensated for by increased expression of the exponentially expressed AhpC2D system. In this study, we used an acrylamide capture of DNA-bound complexes (ACDC) technique and mass spectrometry to identify proteins that bind to the promoter region of the ahpC2D operon. The major protein captured was an ortholog of OxyR (OxyR(Lp)). Genetic studies indicated that oxyR(Lp) was an essential gene expressed postexponentially and only partially complemented an Escherichia coli oxyR mutant (GS077). Gel shift assays confirmed specific binding of OxyR(Lp) to ahpC2D promoter sequences, but not to promoters of ahpC1 or oxyR(Lp); however, OxyR(Lp) weakly bound to E. coli OxyR-regulated promoters (katG, oxyR, and ahpCF). DNase I protection studies showed that the OxyR(Lp) binding motif spanned the promoter and transcriptional start sequences of ahpC2 and that the protected region was unchanged by treatments with reducing agents or hydrogen peroxide (H(2)O(2)). Moreover, the OxyR(Lp) (pBADLpoxyR)-mediated repression of an ahpC2-gfp reporter construct in E. coli GS077 (the oxyR mutant) was not reversed by H(2)O(2) challenge. Alignments with other OxyR proteins revealed several amino acid substitutions predicted to ablate thiol oxidation or conformational changes required for activation. We suggest these mutations have locked OxyR(Lp) in an active DNA-binding conformation, which has permitted a divergence of function from a regulator of oxidative stress to a cell cycle regulator, perhaps controlling gene expression during postexponential differentiation.

A role for Haemophilus ducreyi Cu,ZnSOD in resistance to heme toxicity

The Cu,Zn superoxide dismutase (Cu,ZnSOD) from Haemophilus ducreyi is the only enzyme of this class which binds a heme molecule at its dimer interface. To explore the role of the enzyme in this heme-obligate bacterium, a sodC mutant was created by insertional inactivation. No difference in growth rate was observed during heme limitation. In contrast, under heme rich conditions growth of the sodC mutant was impaired compared to the wild type strain. This growth defect was abolished by supplementation of exogenous catalase. Genetic complementation of the sodC mutant in trans demonstrated that the enzymatic property or the heme-binding activity of the protein could repair the growth defect of the sodC mutant. These results indicate that Cu,ZnSOD protects Haemophilus ducreyi from heme toxicity.

Enhancing survival of Escherichia coli by increasing the periplasmic expression of Cu,Zn superoxide dismutase from Saccharomyces cerevisiae

The gene for the Cu,Zn superoxide dismutase (Cu,ZnSOD) from Saccharomyces cerevisiae was cloned and expressed in Escherichia coli LMG194. The sod gene sequence obtained is 465 bp and encodes 154 amino acid residues. The sod gene sequence was cloned into the E. coli periplasmic expression vector pBAD/gIIIA, yielding pBAD-1. E. coli was transformed using the constructed plasmid pBAD-1 and induced by adding 0.02% L: -arabinose to express Cu,ZnSOD protein. The results indicated that Cu,ZnSOD enzyme activity in the periplasmic space was about fivefold to sixfold higher in the recombinant E. coli strains bearing the sod gene than in the control strains. The yields of Cu,ZnSOD were about threefold higher at 48 h than at 24 h in the recombinant E. coli cells. Significantly higher survival of strains was obtained in cells bearing the sod gene than in the control cells when the cells were treated by heat shock and superoxide-generating agents, such as paraquat and menadione.

Characterization of SodC, a periplasmic superoxide dismutase from Burkholderia cenocepacia

Burkholderia cenocepacia is a gram-negative, non-spore-forming bacillus and a member of the Burkholderia cepacia complex. B. cenocepacia can survive intracellularly in phagocytic cells and can produce at least one superoxide dismutase (SOD). The inability of O2- to cross the cytoplasmic membrane, coupled with the periplasmic location of Cu,ZnSODs, suggests that periplasmic SODs protect bacteria from superoxide that has an exogenous origin (for example, when cells are faced with reactive oxygen intermediates generated by host cells in response to infection). In this study, we identified the sodC gene encoding a Cu,ZnSOD in B. cenocepacia and demonstrated that a sodC null mutant was not sensitive to a H2O2, 3-morpholinosydnonimine, or paraquat challenge but was killed by exogenous superoxide generated by the xanthine/xanthine oxidase method. The sodC mutant also exhibited a growth defect in liquid medium compared to the parental strain, which could be complemented in trans. The mutant was killed more rapidly than the parental strain was killed in murine macrophage-like cell line RAW 264.7, but killing was eliminated when macrophages were treated with an NADPH oxidase inhibitor. We also confirmed that SodC is periplasmic and identified the metal cofactor. B. cenocepacia SodC was resistant to inhibition by H2O2 and was unusually resistant to KCN for a Cu,ZnSOD. Together, these observations establish that B. cenocepacia produces a periplasmic Cu,ZnSOD that protects this bacterium from exogenously generated O2- and contributes to intracellular survival of this bacterium in macrophages.

Compensatory functions of two alkyl hydroperoxide reductases in the oxidative defense system of Legionella pneumophila

Legionella pneumophila expresses two catalase-peroxidase enzymes that exhibit strong peroxidatic but weak catalatic activities, suggesting that other enzymes participate in decomposition of hydrogen peroxide (H2O2). Comparative genomics revealed that L. pneumophila and its close relative Coxiella burnetii each contain two peroxide-scavenging alkyl hydroperoxide reductase (AhpC) systems: AhpC1, which is similar to the Helicobacter pylori AhpC system, and AhpC2 AhpD (AhpC2D), which is similar to the AhpC AhpD system of Mycobacterium tuberculosis. To establish a catalatic function for these two systems, we expressed L. pneumophila ahpC1 or ahpC2 in a catalase/peroxidase mutant of Escherichia coli and demonstrated restoration of H2O2 resistance by a disk diffusion assay. ahpC1::Km and ahpC2D::Km chromosomal deletion mutants were two- to eightfold more sensitive to H2O2, tert-butyl hydroperoxide, cumene hydroperoxide, and paraquat than the wild-type L. pneumophila, a phenotype that could be restored by trans-complementation. Reciprocal strategies to construct double mutants were unsuccessful. Mutant strains were not enfeebled for growth in vitro or in a U937 cell infection model. Green fluorescence protein reporter assays revealed expression to be dependent on the stage of growth, with ahpC1 appearing after the exponential phase and ahpC2 appearing during early exponential phase. Quantitative real-time PCR showed that ahpC1 mRNA levels were approximately 7- to 10-fold higher than ahpC2D mRNA levels. However, expression of ahpC2D was significantly increased in the ahpC1 mutant, whereas ahpC1 expression was unchanged in the ahpC2D mutant. These results indicate that AhpC1 or AhpC2D (or both) provide an essential hydrogen peroxide-scavenging function to L. pneumophila and that the compensatory activity of the ahpC2D system is most likely induced in response to oxidative stress.

Activation of superoxide dismutases: putting the metal to the pedal

Superoxide dismutases (SOD) are important anti-oxidant enzymes that guard against superoxide toxicity. Various SOD enzymes have been characterized that employ either a copper, manganese, iron or nickel co-factor to carry out the disproportionation of superoxide. This review focuses on the copper and manganese forms, with particular emphasis on how the metal is inserted in vivo into the active site of SOD. Copper and manganese SODs diverge greatly in sequence and also in the metal insertion process. The intracellular copper SODs of eukaryotes (SOD1) can obtain copper post-translationally, by way of interactions with the CCS copper chaperone. CCS also oxidizes an intrasubunit disulfide in SOD1. Adventitious oxidation of the disulfide can lead to gross misfolding of immature forms of SOD1, particularly with SOD1 mutants linked to amyotrophic lateral sclerosis. In the case of mitochondrial MnSOD of eukaryotes (SOD2), metal insertion cannot occur post-translationally, but requires new synthesis and mitochondrial import of the SOD2 polypeptide. SOD2 can also bind iron in vivo, but is inactive with iron. Such metal ion mis-incorporation with SOD2 can become prevalent upon disruption of mitochondrial metal homeostasis. Accurate and regulated metallation of copper and manganese SOD molecules is vital to cell survival in an oxygenated environment.

Application of comparative phylogenomics to study the evolution of Yersinia enterocolitica and to identify genetic differences relating to pathogenicity

Yersinia enterocolitica, an important cause of human gastroenteritis generally caused by the consumption of livestock, has traditionally been categorized into three groups with respect to pathogenicity, i.e., nonpathogenic (biotype 1A), low pathogenicity (biotypes 2 to 5), and highly pathogenic (biotype 1B). However, genetic differences that explain variation in pathogenesis and whether different biotypes are associated with specific nonhuman hosts are largely unknown. In this study, we applied comparative phylogenomics (whole-genome comparisons of microbes with DNA microarrays combined with Bayesian phylogenies) to investigate a diverse collection of 94 strains of Y. enterocolitica consisting of 35 human, 35 pig, 15 sheep, and 9 cattle isolates from nonpathogenic, low-pathogenicity, and highly pathogenic biotypes. Analysis confirmed three distinct statistically supported clusters composed of a nonpathogenic clade, a low-pathogenicity clade, and a highly pathogenic clade. Genetic differences revealed 125 predicted coding sequences (CDSs) present in all highly pathogenic strains but absent from the other clades. These included several previously uncharacterized CDSs that may encode novel virulence determinants including a hemolysin, a metalloprotease, and a type III secretion effector protein. Additionally, 27 CDSs were identified which were present in all 47 low-pathogenicity strains and Y. enterocolitica 8081 but absent from all nonpathogenic 1A isolates. Analysis of the core gene set for Y. enterocolitica revealed that 20.8% of the genes were shared by all of the strains, confirming this species as highly heterogeneous, adding to the case for the existence of three subspecies of Y. enterocolitica. Further analysis revealed that Y. enterocolitica does not cluster according to source (host).

Superoxide dismutase and catalase activities in Photobacterium damselae ssp. piscicida

The ability of a set of Photobacterium damselae ssp. piscicida strains isolated from different fish species to produce different superoxide dismutase (SOD) and catalase enzymes was determined. Unlike other bacterial pathogens, P. damselae ssp. piscicida is not able to produce different isoforms of SOD or catalase containing different metal cofactors when cultured under oxidative stress induced by hydrogen peroxide or methyl viologen, or under iron depleted conditions. However, iron content of the growth medium influenced the levels of SOD and catalase activity in cells, these levels decreasing with iron availability of the medium. Comparison of virulent and non-virulent strains of P. damselae ssp. piscicida showed similar contents of SOD, but higher levels of catalase were detected in cells of the virulent strain. Incubation of bacteria with sole, Solea senegalensis (Kaup), phagocytes has shown that survival rates range from 19% to 62%, these rates being higher for the virulent strain. The increased levels of catalase activity detected in the virulent strain indicates a possible role for this enzyme in bacterial survival.

Posttranslational modifications in Cu,Zn-superoxide dismutase and mutations associated with amyotrophic lateral sclerosis

Activation of the enzyme Cu,Zn-superoxide dismutase (SOD1) involves several posttranslational modifications including copper and zinc binding, as well as formation of the intramolecular disulfide bond. The copper chaperone for SOD1, CCS, is responsible for intracellular copper loading in SOD1 under most physiological conditions. Recent in vitro and in vivo assays reveal that CCS not only delivers copper to SOD1 under stringent copper limitation, but it also facilitates the stepwise conversion of the disulfide-reduced immature SOD1 to the active disulfide-containing enzyme. The two new functions attributed to CCS, (i.e., O(2)-dependent sulfhydryl oxidase- and disulfide isomerase-like activities) indicate that this protein has attributes of the larger class of molecular chaperones. The CCS-dependent activation of SOD1 is dependent upon oxygen availability, suggesting that the cell only loads copper and activates this enzyme when O(2)-based oxidative stress is present. Thiol/disulfide status as well as metallation state of SOD1 significantly affects its structure and protein aggregation, which are relevant in pathologies of a neurodegenerative disease, amyotrophic lateral sclerosis (ALS). The authors review here a mechanism for posttranslational activation of SOD1 and discuss models for ALS in which the most immature forms of the SOD1 polypeptide exhibits propensity to form toxic aggregates.

Both inducible nitric oxide synthase and NADPH oxidase contribute to the control of virulent phase I Coxiella burnetii infections

Host control of Coxiella burnetii infections is believed to be mediated primarily by activated monocytes/macrophages. The activation of macrophages by cytokines leads to the production of reactive oxygen intermediates (ROI) and reactive nitrogen intermediates (RNI) that have potent antimicrobial activities. The contributions of ROI and RNI to the inhibition of C. burnetii replication were examined in vitro by the use of murine macrophage-like cell lines and primary mouse macrophages. A gamma interferon (IFN-gamma) treatment of infected cell lines and primary macrophages resulted in an increased production of nitric oxide (NO) and hydrogen peroxide (H2O2) and a significant inhibition of C. burnetii replication. The inhibition of replication was reversed in the murine cell line J774.16 upon the addition of either the inducible nitric oxide synthase (iNOS) inhibitor NG-monomethyl-L-arginine (NGMMLA) or the H2O2 scavenger catalase. IFN-gamma-treated primary macrophages from iNOS-/- and p47phox-/- mice significantly inhibited replication but were less efficient at controlling infection than IFN-gamma-treated wild-type macrophages. To investigate the contributions of ROI and RNI to resistance to infection, we performed in vivo studies, using C57BL/6 wild-type mice and knockout mice lacking iNOS or p47phox. Both iNOS-/- and p47phox-/- mice were attenuated in the ability to control C. burnetii infection compared to wild-type mice. Together, these results strongly support a role for both RNI and ROI in the host control of C. burnetii infection.

Legionella pneumophila catalase-peroxidases are required for proper trafficking and growth in primary macrophages

Legionella pneumophila, a parasite of aquatic amoebae and pathogen of pulmonary macrophages, replicates intracellularly, utilizing a type IV secretion system to subvert the trafficking of Legionella-containing phagosomes. Defense against host-derived reactive oxygen species has been proposed as critical for intracellular replication. Virulence traits of null mutants in katA and katB, encoding the two Legionella catalase-peroxidases, were analyzed to evaluate the hypothesis that L. pneumophila must decompose hydrogen peroxide to establish a replication niche in macrophages. Phagosomes containing katA or katB mutant Legionella colocalize with LAMP-1, a late endosomal-lysosomal marker, at twice the frequency of those of wild-type strain JR32 and show a decreased frequency of bacterial replication, in similarity to phenotypes of mutants with mutations in dotA and dotB, encoding components of the Type IV secretion system. Quantitative similarity of the katA/B phenotypes indicates that each contributes to virulence traits largely independently of intracellular compartmentalization (KatA in the periplasm and KatB in the cytosol). These data support a model in which KatA and KatB maintain a critically low level of H(2)O(2) compatible with proper phagosome trafficking mediated by the type IV secretion apparatus. During these studies, we observed that dotA and dotB mutations in wild-type strain Lp02 had no effect on intracellular multiplication in the amoeba Acanthamoeba castellanii, indicating that certain dotA/B functions in Lp02 are dispensable in that experimental model. We also observed that wild-type JR32, unlike Lp02, shows minimal contact-dependent cytotoxicity, suggesting that cytotoxicity of JR32 is not a prerequisite for formation of replication-competent Legionella phagosomes in macrophages.

Bacterial [Cu,Zn]-cofactored superoxide dismutase protects opsonized, encapsulated Neisseria meningitidis from phagocytosis by human monocytes/macrophages

Superoxide dismutase cofactored by copper and zinc ([Cu,Zn]-SOD) contributes to the protection of opsonized serogroup B Neisseria meningitidis against phagocytosis by human monocytes/macrophages, with sodC mutant organisms being endocytosed in significantly higher numbers than are wild-type organisms. The influence of [Cu,Zn]-SOD was found to be exerted at the stage of phagocytosis, rather than at earlier (modulating surface association) or later (intracellular killing) stages.

Function of oxygen resistance proteins in the anaerobic, sulfate-reducing bacterium Desulfovibrio vulgaris hildenborough

Two mutant strains of Desulfovibrio vulgaris Hildenborough lacking either the sod gene for periplasmic superoxide dismutase or the rbr gene for rubrerythrin, a cytoplasmic hydrogen peroxide (H(2)O(2)) reductase, were constructed. Their resistance to oxidative stress was compared to that of the wild-type and of a sor mutant lacking the gene for the cytoplasmic superoxide reductase. The sor mutant was more sensitive to exposure to air or to internally or externally generated superoxide than was the sod mutant, which was in turn more sensitive than the wild-type strain. No obvious oxidative stress phenotype was found for the rbr mutant, indicating that H(2)O(2) resistance may also be conferred by two other rbr genes in the D. vulgaris genome. Inhibition of Sod activity by azide and H(2)O(2), but not by cyanide, indicated it to be an iron-containing Sod. The positions of Fe-Sod and Sor were mapped by two-dimensional gel electrophoresis (2DE). A strong decrease of Sor in continuously aerated cells, indicated by 2DE, may be a critical factor in causing cell death of D. vulgaris. Thus, Sor plays a key role in oxygen defense of D. vulgaris under fully aerobic conditions, when superoxide is generated mostly in the cytoplasm. Fe-Sod may be more important under microaerophilic conditions, when the periplasm contains oxygen-sensitive, superoxide-producing targets.

Differential accumulation of Salmonella[Cu, Zn] superoxide dismutases SodCI and SodCII in intracellular bacteria: correlation with their relative contribution to pathogenicity

Most Salmonella enterica strains have two peri-plasmic [Cu, Zn] superoxide dismutases, SodCI and SodCII, encoded by prophage and chromosomal genes respectively. Both enzymes are thought to play a role in Salmonella pathogenicity by intercepting reactive oxygen species produced by the host's innate immune response. To examine the apparent redundancy, we have compared the levels of epitope-tagged SodCI and SodCII proteins in bacteria growing in vitro, as well as inside tissue culture cells and in mouse tissues. Concomitantly, we have measured the abilities of mutants of either or both sodC genes to proliferate in infected mice in competition assays. Our results show a striking variation in the relative abundance of the two proteins in different environments. In vitro, both proteins accumulate when bacteria enter stationary phase; however, the increase is much sharper and conspicuous for SodCII than for SodCI. In contrast, SodCI vastly predominates in intracellular bacteria where SodCII levels are negligible. In agreement with these findings, most, if not all, of the contribution of [Cu, Zn] superoxide dismutase activity to murine salmonellosis can be ascribed to the SodCI protein. Overall the results of this work suggest that the duplicate sodC genes of Salmonella have evolved to respond to different sets of conditions encountered by bacteria inside the host and in the environment.

The role of two periplasmic copper- and zinc-cofactored superoxide dismutases in the virulence of Salmonella choleraesuis

Periplasmic copper- and zinc-cofactored superoxide dismutases ([Cu,Zn]-SODs, SodC) of several Gram-negative pathogens can protect against superoxide-radical-mediated host defences, and thus contribute to virulence. This role has been previously defined for one [Cu,Zn]-SOD in various Salmonella serovars. Following the recent discovery of a second periplasmic [Cu,Zn]-SOD in Salmonella, the effect of knockout mutations in one or both of the original sodC-1 and the new sodC-2 on the virulence of the porcine pathogen Salmonella choleraesuis is investigated here. In comparison to wild-type, while sodC mutants--whether single or double--showed no impairment in growth, they all showed equally enhanced sensitivity to superoxide and a dramatically increased sensitivity to the combination of superoxide and nitric oxide in vitro. This observation had its correlate in experimental infection both ex vivo and in vivo. Mutation of sodC significantly impaired survival of S. choleraesuis in interferon gamma-stimulated murine macrophages compared to wild-type organisms, and all S. choleraesuis sodC mutants persisted in significantly lower numbers than wild-type in BALB/c (Ity(s)) and C3H/HeN (Ity(r)) mice after experimental infection, but in no experimental system were sodC-1 sodC-2 double mutants more attenuated than either single mutant. These data suggest that both [Cu,Zn]-SODs are needed to protect bacterial periplasmic or membrane components. While SodC plays a role in S. choleraesuis virulence, the data presented here suggest that this is through overcoming a threshold effect, probably achieved by acquisition of sodC-1 on a bacteriophage. Loss of either sodC gene confers maximum vulnerability to superoxide on S. choleraesuis.

A superoxide dismutase C mutant of Haemophilus ducreyi is virulent in human volunteers

Haemophilus ducreyi produces a periplasmic copper-zinc superoxide dismutase (Cu-Zn SOD), which is thought to protect the organism from exogenous reactive oxygen species generated by neutrophils during an inflammatory response. We had previously identified the gene, sodC, responsible for the production and secretion of Cu-Zn SOD and constructed an isogenic H. ducreyi strain with a mutation in the sodC gene (35000HP-sodC-cat). Compared to the parent, the mutant does not survive in the presence of exogenous superoxide (L. R. San Mateo, M. Hobbs, and T. H. Kawula, Mol. Microbiol. 27:391-404, 1998) and is impaired in the swine model of H. ducreyi infection (L. R. San Mateo, K. L. Toffer, P. E. Orndorff, and T. H. Kawula, Infect. Immun. 67:5345-5351, 1999). To test whether Cu-Zn SOD is important for bacterial survival in vivo, six human volunteers were experimentally infected with 35000HP and 35000HP-sodC-cat and observed for papule and pustule formation. Papules developed at similar rates at sites inoculated with the mutant or parent. The pustule formation rates were 75% (95% confidence intervals [CI], 43 to 95%) at 12 parent-inoculated sites and 67% (95% CI, 41 to 88%) at 18 mutant-inoculated sites (P = 0.47). There was no significant difference in levels of H. ducreyi recovery from mutant- and parent-inoculated biopsy sites. These results suggest that expression of Cu-Zn SOD does not play a major role in the survival of this pathogen in the initial stages of experimental infection of humans.

Occurrence of two superoxide dismutases in Aeromonas hydrophila: molecular cloning and differential expression of the sodA and sodB genes

Aeromonas spp., considered as emerging opportunistic pathogens, belong to the family Vibrionaceae. Among the criteria currently used for their classification is the presence of a single FeSOD (iron-containing superoxide dismutase), which distinguishes them from Enterobacteriacea. In this paper the cloning of the sodA and sodB genes encoding two different SODs in Aeromonas hydrophila ATCC 7966 is reported. The sodB gene encoded an FeSOD (196 amino acids, 21.5 kDa), was constitutively expressed and showed 75% homology with the E. coli FeSOD. The sodA gene encoded a protein of 206 amino acids (22.5 kDa) with MnSOD (manganese-containing SOD) activity and showed 55% homology with the Escherichia coli MnSOD. The MnSOD of A. hydrophila was detected only during the stationary phase of growth under high aeration or when induced by lack of iron. Nevertheless, paraquat had no detectable effect on its production. The amino-terminal part of the Mn-containing protein contained a putative signal sequence which could permit a periplasmic localization.

Contribution of Mn-cofactored superoxide dismutase (SodA) to the virulence of Streptococcus agalactiae

Superoxide dismutases convert superoxide anions to molecular oxygen and hydrogen peroxide, which, in turn, is metabolized by catalases and/or peroxidases. These enzymes constitute one of the major defense mechanisms of cells against oxidative stress and hence play a role in the pathogenesis of certain bacteria. We previously demonstrated that group B streptococci (GBS) possess a single Mn-cofactored superoxide dismutase (SodA). To analyze the role of this enzyme in the pathogenicity of GBS, we constructed a sodA-disrupted mutant of Streptococcus agalactiae NEM316 by allelic exchange. This mutant was subsequently cis complemented by integration into the chromosome of pAT113/Sp harboring the wild-type sodA gene. The SOD specific activity detected by gel analysis in cell extracts confirmed that active SODs were present in the parental and complemented strains but absent in the sodA mutant. The growth rates of these strains in standing cultures were comparable, but the sodA mutant was extremely susceptible to the oxidative stress generated by addition of paraquat or hydrogen peroxide to the culture medium and exhibited a higher mutation frequency in the presence of rifampin. In mouse bone marrow-derived macrophages, the sodA mutant showed an increased susceptibility to bacterial killing by macrophages. In a mouse infection model, after intravenous injection the survival of the sodA mutant in the blood and the brain was markedly reduced in comparison to that of the parental and complemented strains whereas only minor effects on survival in the liver and the spleen were observed. These results suggest that SodA plays a role in GBS pathogenesis.

Cu,Zn superoxide dismutase of Mycobacterium tuberculosis contributes to survival in activated macrophages that are generating an oxidative burst

Macrophages produce reactive oxygen species and reactive nitrogen species that have potent antimicrobial activity. Resistance to killing by macrophages is critical to the virulence of Mycobacterium tuberculosis. M. tuberculosis has two genes encoding superoxide dismutase proteins, sodA and sodC. SodC is a Cu,Zn superoxide dismutase responsible for only a minor portion of the superoxide dismutase activity of M. tuberculosis. However, SodC has a lipoprotein binding motif, which suggests that it may be anchored in the membrane to protect M. tuberculosis from reactive oxygen intermediates at the bacterial surface. To examine the role of the Cu,Zn superoxide dismutase in protecting M. tuberculosis from the toxic effects of exogenously generated reactive oxygen species, we constructed a null mutation in the sodC gene. In this report, we show that the M. tuberculosis sodC mutant is readily killed by superoxide generated externally, while the isogenic parental M. tuberculosis is unaffected under these conditions. Furthermore, the sodC mutant has enhanced susceptibility to killing by gamma interferon (IFN-gamma)-activated murine peritoneal macrophages producing oxidative burst products but is unaffected by macrophages not activated by IFN-gamma or by macrophages from respiratory burst-deficient mice. These observations establish that the Cu,Zn superoxide dismutase contributes to the resistance of M. tuberculosis against oxidative burst products generated by activated macrophages.

Identification of Legionella pneumophila rcp, a pagP-like gene that confers resistance to cationic antimicrobial peptides and promotes intracellular infection

In the course of characterizing a locus involved in heme utilization, we identified a Legionella pneumophila gene predicted to encode a protein with homology to the product of the Salmonella enterica serovar Typhimurium pagP gene. In Salmonella, pagP increases resistance to the bactericidal effects of cationic antimicrobial peptides (CAMPs). Mutants with insertions in the L. pneumophila pagP-like gene were generated and showed decreased resistance to different structural classes of CAMPs compared to the wild type; hence, this gene was designated rcp for resistance to cationic antimicrobial peptides. Furthermore, Legionella CAMP resistance was induced by growth in low-magnesium medium. To determine whether rcp had any role in intracellular survival, mutants were tested in the two most relevant host cells for Legionnaires' disease, i.e., amoebae and macrophages. These mutants exhibited a 1,000-fold-decreased recovery during a Hartmannella vermiformis coculture. Complementation of the infectivity defect could be achieved by introduction of a plasmid containing the intact rcp gene. Mutations in rcp consistently reduced both the numbers of bacteria recovered during intracellular infection and their cytopathic capacity for U937 macrophages. The rcp mutant was also more defective for lung colonization of A/J mice. Growth of rcp mutants in buffered yeast extract broth was identical to that of the wild type, indicating that the observed differences in numbers of bacteria recovered from host cells were not due to a generalized growth defect. However, in low-Mg(2+) medium, the rcp mutant was impaired in stationary-phase survival. This is the first demonstration of a pagP-like gene, involved in resistance to CAMPs, being required for intracellular infection and virulence.

Catalase-peroxidases of Legionella pneumophila: cloning of the katA gene and studies of KatA function

Legionella pneumophila, the causative organism of Legionnaires' pneumonia, contains two enzymes with catalatic and peroxidatic activity, KatA and KatB. To address the issue of redundant, overlapping, or discrete in vivo functions of highly homologous catalase-peroxidases, the gene for katA was cloned and its function was studied in L. pneumophila and Escherichia coli and compared with prior studies of katB in this laboratory. katA is induced during exponential growth and is the predominant peroxidase in stationary phase. When katA is inactivated, L. pneumophila is more sensitive to exogenous hydrogen peroxide and less virulent in the THP-1 macrophage cell line, similar to katB. Catalatic-peroxidatic activity with different peroxidatic cosubstrates is comparable for KatA and KatB, but KatA is five times more active towards dianisidine. In contrast with these examples of redundant or overlapping function, stationary-phase survival is decreased by 100- to 10,000-fold when katA is inactivated, while no change from wild type is seen for the katB null. The principal clue for understanding this discrete in vivo function was the demonstration that KatA is periplasmic and KatB is cytosolic. This stationary-phase phenotype suggests that targets sensitive to hydrogen peroxide are present outside the cytosol in stationary phase or that the peroxidatic activity of KatA is critical for stationary-phase redox reactions in the periplasm, perhaps disulfide bond formation. Since starvation-induced stationary phase is a prerequisite to acquisition of virulence by L. pneumophila, further studies on the function and regulation of katA in stationary phase may give insights on the mechanisms of infectivity of this pathogen.

Purification and characterization of iron superoxide dismutase and copper-zinc superoxide dismutase from Acanthamoeba castellanii

Two superoxide dismutases (SOD I and SOD II) were purified from Acanthamoeba castellanii and characterized for several biochemical properties. Analysis of the primary structure and inhibition studies revealed that SOD I is iron SOD (Fe-SOD), with a molecular mass of 50 kDa, and SOD II is copper-zinc SOD (Cu,Zn-SOD), with a molecular mass of 38 kDa. Both enzymes have a homodimeric structure consisting of 2 identical subunits, each with a molecular mass of 26 and 19 kDa for SOD I and SOD II, respectively. The isoelectric points of SOD I and SOD II were 6.4 and 3.5, respectively, and there were no isoenzyme forms detected. Both enzymes show a broad optimal pH of 7.0-11.0. Because no differences were observed in the apparent molecular weight of SOD I after addition of the reducing agent 2-mercaptoethanol, the subunits do not appear to be linked covalently by disulfide bonds. However, the subunits of SOD II were covalently linked by intra- and interdisulfide bonds. Western blot analyses showed that the 2 enzymes have different antigenicity. Both enzymes occur as cytoplasmic and detergent-extractable fractions. These enzymes may be potential virulence factors of A. castellanii by acting both as antioxidants and antiinflammatory agents. These enzymes may be attractive targets for chemotherapy and immunodiagnosis of acanthamoebiasis.

Expression and role of superoxide dismutases (SOD) in pathogenic bacteria

This review will be limited to the expression and roles of the family of metalloenzymes superoxide dismutases in pathogenic bacteria. Only animal pathogens will be described, with particular emphasis on those causing disease in man.

Production of two different catalase-peroxidases by Deinococcus radiophilus

The production of two kinds of catalase-peroxidase, viz. catalase-2 and catalase-3 of Deinococcus radiophilus varied depending upon growth phases and oxidative stress. A gradual increase in total catalase activity occurred during exponential and stationary phase. Electrophoretic resolution of these catalases in Deinococcal cell extracts revealed the uniform occurrence of catalase-2 and the appearance of catalase-3 only during the late exponential and stationary phase. A substantial increase in total catalase was observed in either hydrogen peroxide- or UV-treated cells. Monitoring of D. radiophilus catalase activity in the oxidative stressed and non-treated cells by gel electrophoresis followed by densitometry revealed the several-fold increase in catalase-3, which is above the constant level of catalase-2. The occurrence of catalase-3 and catalase-2 revealed by fractionation of sucrose-shocked cells suggests that catalase-3 is a cytosolic inducible enzyme whereas catalase-2 is the membrane-associated constitutive enzyme.

Cu,Zn superoxide dismutase structure from a microbial pathogen establishes a class with a conserved dimer interface

Macrophages and neutrophils protect animals from microbial infection in part by issuing a burst of toxic superoxide radicals when challenged. To counteract this onslaught, many Gram-negative bacterial pathogens possess periplasmic Cu,Zn superoxide dismutases (SODs), which act on superoxide to yield molecular oxygen and hydrogen peroxide. We have solved the X-ray crystal structure of the Cu,Zn SOD from Actinobacillus pleuropneumoniae, a major porcine pathogen, by molecular replacement at 1.9 A resolution. The structure reveals that the dimeric bacterial enzymes form a structurally homologous class defined by a water-mediated dimer interface, and share with all Cu,Zn SODs the Greek-key beta-barrel subunit fold with copper and zinc ions located at the base of a deep loop-enclosed active-site channel. Our structure-based sequence alignment of the bacterial enzymes explains the monomeric nature of at least two of these, and suggests that there may be at least one additional structural class for the bacterial SODs. Two metal-mediated crystal contacts yielded our C222(1) crystals, and the geometry of these sites could be engineered into proteins recalcitrant to crystallization in their native form. This work highlights structural differences between eukaryotic and prokaryotic Cu,Zn SODs, as well as similarities and differences among prokaryotic SODs, and lays the groundwork for development of antimicrobial drugs that specifically target periplasmic Cu,Zn SODs of bacterial pathogens.

Legionella pneumophila pathogesesis: a fateful journey from amoebae to macrophages

Legionella pneumophila first commanded attention in 1976, when investigators from the Centers for Disease Control and Prevention identified it as the culprit in a massive outbreak of pneumonia that struck individuals attending an American Legion convention (). It is now clear that this gram-negative bacterium flourishes naturally in fresh water as a parasite of amoebae, but it can also replicate within alveolar macrophages. L. pneumophila pathogenesis is discussed using the following model as a framework. When ingested by phagocytes, stationary-phase L. pneumophila bacteria establish phagosomes which are completely isolated from the endosomal pathway but are surrounded by endoplasmic reticulum. Within this protected vacuole, L. pneumophila converts to a replicative form that is acid tolerant but no longer expresses several virulence traits, including factors that block membrane fusion. As a consequence, the pathogen vacuoles merge with lysosomes, which provide a nutrient-rich replication niche. Once the amino acid supply is depleted, progeny accumulate the second messenger guanosine 3',5'-bispyrophosphate (ppGpp), which coordinates entry into the stationary phase with expression of traits that promote transmission to a new phagocyte. A number of factors contribute to L. pneumophila virulence, including type II and type IV secretion systems, a pore-forming toxin, type IV pili, flagella, and numerous other factors currently under investigation. Because of its resemblance to certain aspects of Mycobacterium, Toxoplasma, Leishmania, and Coxiella pathogenesis, a detailed description of the mechanism used by L. pneumophila to manipulate and exploit phagocyte membrane traffic may suggest novel strategies for treating a variety of infectious diseases. Knowledge of L. pneumophila ecology may also inform efforts to combat the emergence of new opportunistic macrophage pathogens.

Increased expression of periplasmic Cu,Zn superoxide dismutase enhances survival of Escherichia coli invasive strains within nonphagocytic cells

We have studied the influence of periplasmic Cu,Zn superoxide dismutase on the intracellular survival of Escherichia coli strains able to invade epithelial cells by the expression of the inv gene from Yersinia pseudotuberculosis but unable to multiply intracellularly. Intracellular viability assays, confirmed by electron microscopy observations, showed that invasive strains of E. coli engineered to increase Cu,Zn superoxide dismutase production are much more resistant to intracellular killing than strains containing only the chromosomal sodC copy. However, we have found only a slight difference in survival within HeLa cells between a sodC-null mutant and its isogenic wild-type strain. Such a small difference in survival correlates with the very low expression of this enzyme in the wild-type strain. We have also observed that acid- and oxidative stress-sensitive E. coli HB101(pRI203) is more rapidly killed in epithelial cells than E. coli GC4468(pRI203). The high mortality of E. coli HB101(pRI203), independent of the acidification of the endosome, is abolished by the overexpression of sodC. Our data suggest that oxyradicals are involved in the mechanisms of bacterial killing within epithelial cells and that high-level production of periplasmic Cu,Zn superoxide dismutase provides bacteria with an effective protection against oxidative damage. We propose that Cu,Zn superoxide dismutase could offer an important selective advantage in survival within host cells to bacteria expressing high levels of this enzyme.

Identification of copper-zinc superoxide dismutase gene from enteroaggregative Escherichia coli

We describe here the identification of sodC gene from enteroaggregative Escherichia coli (EAggEC). A 294 bp gene-specific fragment was amplified from the organism by DNA as well as RT-PCR using primers from bacterial sodC sequences. The metal co-factor present in the protein was confirmed by running samples in native gels and inhibiting with 2 mM potassium cyanide. However, the nonpathogenic E. coli possesses the gene but does not express it. Thus, the presence of copper-zinc superoxide dismutase encoded by sodC was demonstrated for the first time in EAggEC, which means it could be a novel candidate for a virulence marker.

The Legionella pneumophila rpoS gene is required for growth within Acanthamoeba castellanii

To investigate regulatory networks in Legionella pneumophila, the gene encoding the homolog of the Escherichia coli stress and stationary-phase sigma factor RpoS was identified by complementation of an E. coli rpoS mutation. An open reading frame that is approximately 60% identical to the E. coli rpoS gene was identified. Western blot analysis showed that the level of L. pneumophila RpoS increased in stationary phase. An insertion mutation was constructed in the rpoS gene on the chromosome of L. pneumophila, and the ability of this mutant strain to survive various stress conditions was assayed and compared with results for the wild-type strain. Both the mutant and wild-type strains were more resistant to stress when in stationary phase than when in the logarithmic phase of growth. This finding indicates that L. pneumophila RpoS is not required for a stationary-phase-dependent resistance to stress. Although the mutant strain was able to kill HL-60- and THP-1-derived macrophages, it could not replicate within a protozoan host, Acanthamoeba castellanii. These data suggest that L. pneumophila possesses a growth phase-dependent resistance to stress that is independent of RpoS control and that RpoS likely regulates genes that enable it to survive in the environment within protozoa. Our data indicate that the role of rpoS in L. pneumophila is very different from what has previously been reported for E. coli rpoS.

Identification of a novel periplasmic catalase-peroxidase KatA of Legionella pneumophila

A gene katA that encodes a novel catalase-peroxidase was cloned from the chromosome of Legionella pneumophila. The nucleotide sequence revealed that KatA was highly homologous to members of the bacterial bifunctional catalase-peroxidase family. In addition, KatA has a N-terminal signal sequence and was considered to be present in the periplasm of the bacterium.