Mechanism of the bactericidal action of myeloperoxidase: increased permeability of the Escherichia coli cell envelope

The effects of neutrophil-generated hypochlorous acid and other hypohalous acids on host and pathogens

Neutrophils are predominant immune cells that protect the human body against infections by deploying sophisticated antimicrobial strategies including phagocytosis of bacteria and neutrophil extracellular trap (NET) formation. Here, we provide an overview of the mechanisms by which neutrophils kill exogenous pathogens before we focus on one particular weapon in their arsenal: the generation of the oxidizing hypohalous acids HOCl, HOBr and HOSCN during the so-called oxidative burst by the enzyme myeloperoxidase. We look at the effects of these hypohalous acids on biological systems in general and proteins in particular and turn our attention to bacterial strategies to survive HOCl stress. HOCl is a strong inducer of protein aggregation, which bacteria can counteract by chaperone-like holdases that bind unfolding proteins without the need for energy in the form of ATP. These chaperones are activated by HOCl through thiol oxidation (Hsp33) or N-chlorination of basic amino acid side-chains (RidA and CnoX) and contribute to bacterial survival during HOCl stress. However, neutrophil-generated hypohalous acids also affect the host system. Recent studies have shown that plasma proteins act not only as sinks for HOCl, but get actively transformed into modulators of the cellular immune response through N-chlorination. N-chlorinated serum albumin can prevent aggregation of proteins, stimulate immune cells, and act as a pro-survival factor for immune cells in the presence of cytotoxic antigens. Finally, we take a look at the emerging role of HOCl as a potential signaling molecule, particularly its role in neutrophil extracellular trap formation.

The Mechanism of Chlorine Damage Using Enhanced Green Fluorescent Protein-Expressing Escherichia coli

This study investigated how chlorine inactivates and damages Escherichia coli cells. E. coli that had transformed to express enhanced green fluorescent protein (EGFP) at the cytoplasm was treated with chlorine. Damage to the cell membrane and cell wall was analyzed by measuring the fluorescence intensity of the leaked EGFP, then accounting for the fluorescence deterioration. At pH 7, E. coli was lethally damaged after treatment with chlorine, but significant leakage of EGFP was not observed. In contrast, significant leakage of EGFP was observed at pH 9, even though E. coli was not as inactivated as it was at pH 7. Flow cytometry was used to confirm the fluorescence intensity of the remaining EGFP inside the cells. No significant fluorescence loss was observed in the cells at pH 7. However, at pH 9, the fluorescence intensity in the cells decreased, indicating leakage of EGFP. These results suggest that hypochlorous acid inactivates E. coli without damaging its cell membrane and cell wall, whereas the hypochlorite ion inactivates E. coli by damaging its cell membrane and cell wall. It was possible to confirm the chlorine damage mechanism on E. coli by measuring the fluorescence intensity of the leaked EGFP.

Multiplex quantitative SILAC for analysis of archaeal proteomes: a case study of oxidative stress responses

Stable isotope labelling of amino acids in cell culture (SILAC) is a quantitative proteomic method that can illuminate new pathways used by cells to adapt to different lifestyles and niches. Archaea, while thriving in extreme environments and accounting for ∼20%-40% of the Earth's biomass, have not been analyzed with the full potential of SILAC. Here, we report SILAC for quantitative comparison of archaeal proteomes, using Haloferax volcanii as a model. A double auxotroph was generated that allowed for complete incorporation of ¹³ C/¹⁵ N-lysine and ¹³ C-arginine such that each peptide derived from trypsin digestion was labelled. This strain was found amenable to multiplex SILAC by case study of responses to oxidative stress by hypochlorite. A total of 2565 proteins was identified by LC-MS/MS analysis (q-value ≤ 0.01) that accounted for 64% of the theoretical proteome. Of these, 176 proteins were altered at least 1.5-fold (p-value < 0.05) in abundance during hypochlorite stress. Many of the differential proteins were of unknown function. Those of known function included transcription factor homologs related to oxidative stress by 3D-homology modelling and orthologous group comparisons. Thus, SILAC is found to be an ideal method for quantitative proteomics of archaea that holds promise to unravel gene function.

Toxicology of ambient particulate matter

It is becoming increasingly clear that inhalation exposure to particulate matter (PM) can lead to or exacerbate various diseases, which are not limited to the lung but extend to the cardiovascular system and possibly other organs and tissues. Epidemiological studies have provided strong evidence for associations with chronic obstructive pulmonary disease (COPD), asthma, bronchitis and cardiovascular disease, while the evidence for a link with lung cancer is less strong. Novel research has provided first hints that exposure to PM might lead to diabetes and central nervous system (CNS) pathology. In the current review, an overview is presented of the toxicological basis for adverse health effects that have been linked to PM inhalation. Oxidative stress and inflammation are discussed as central processes driving adverse effects; in addition, profibrotic and allergic processes are implicated in PM-related diseases. Effects of PM on key cell types considered as regulators of inflammatory, fibrotic and allergic mechanisms are described.

Comparisons of resistance of CF and non-CF pathogens to hydrogen peroxide and hypochlorous acid oxidants in vitro

Background

Cystic fibrosis (CF) lung disease has a unique profile of pathogens predominated by Pseudomonas aeruginosa (PsA) and Staphylococcus aureus (SA). These microorganisms must overcome host immune defense to colonize the CF lungs. Polymorphonuclear neutrophils are a major component of the host defense against bacterial infection. A crucial microbicidal mechanism is the production of oxidants including hydrogen peroxide (H₂O₂) and hypochlorous acid (HOCl) by neutrophils to achieve efficient bacterial killing. To determine to what degrees various CF pathogens resist the oxidants relative to non-CF pathogens, we compared the susceptibility of PsA, SA, Burkholderia cepacia (BC), Klebsiella pneumoniae (KP), and Escherichia coli (EC) to various concentrations of H₂O₂ or HOCl, in vitro. The comparative oxidant-resistant profiles were established. Oxidant-induced damage to ATP production and cell membrane integrity of the microbes were quantitatively assessed. Correlation of membrane permeability and ATP levels with bacterial viability was statistically evaluated.

Results

PsA was relatively resistant to both H₂O₂ (LD₅₀ = 1.5 mM) and HOCl (LD₅₀ = 0.035 mM). SA was susceptible to H₂O₂ (LD₅₀ = 0.1 mM) but resistant to HOCl (LD₅₀ = 0.035 mM). Interestingly, KP was extremely resistant to high doses of H₂O₂ (LD₅₀ = 2.5-5.0 mM) but was very sensitive to low doses of HOCl (LD₅₀ = 0.015 mM). BC was intermediate to resist both oxidants: H₂O₂ (LD₅₀ = 0.3-0.4 mM) and HOCl (LD₅₀ = 0.025 mM). EC displayed the least resistance to H₂O₂ (LD₅₀ = 0.2-0.3 mM) and HOCl (LD₅₀ = 0.015 mM). The identified profile of H₂O₂-resistance was KP > PsA > BC > EC > SA and the profile of HOCl-resistance PsA > SA > BC > EC > KP. Moreover, both oxidants affected ATP production and membrane integrity of the cells. However, the effects varied among the tested organisms and, the oxidant-mediated damage correlated differentially with the bacterial viability.

Conclusions

The order of HOCl-resistance identified herein best fits the clinical profile of CF infections. Even though oxidants are able to disrupt ATP production and cell membrane integrity, the degrees of damage vary among the organisms and correlate differentially with their viability.

Neutrophil bleaching of GFP-expressing staphylococci: probing the intraphagosomal fate of individual bacteria

Successful host defense against bacteria such as Staphylococcus aureus (SA) depends on a prompt response by circulating polymorphonuclear leukocytes (PMN). Stimulated PMN create in their phagosomes an environment inhospitable to most ingested bacteria. Granules that fuse with the phagosome deliver an array of catalytic and noncatalytic antimicrobial peptides, while activation of the NADPH oxidase at the phagosomal membrane generates reactive oxygen species within the phagosome, including hypochlorous acid (HOCl), formed by the oxidation of chloride by the granule protein myeloperoxidase in the presence of H(2)O(2). In this study, we used SA-expressing cytosolic GFP to provide a novel probe of the fate of SA in human PMN. PMN bleaching of GFP in SA required phagocytosis, active myeloperoxidase, H(2)O(2) from the NADPH oxidase, and chloride. Not all ingested SA were bleached, and the number of cocci within PMN-retaining fluorescent GFP closely correlated with the number of viable bacteria remaining intracellularly. The percent of intracellular fluorescent and viable SA increased at higher multiplicity of infection and when SA presented to PMN had been harvested from the stationary phase of growth. These studies demonstrate that the loss of GFP fluorescence in ingested SA provides a sensitive experimental probe for monitoring biochemical events within individual phagosomes and for identifying subpopulations of SA that resist intracellular PMN cytotoxicity. Defining the molecular basis of SA survival within PMN should provide important insights into bacterial and host properties that limit PMN antistaphylococcal action and thus contribute to the pathogenesis of staphylococcal infection.

Nucleic acid fluorochromes and flow cytometry prove useful in assessing the effect of chlorination on drinking water bacteria

Flow cytometry (FCM), combined with staining using two fluorochromes (propidium iodide, PI, or SYBR Green II RNA gel stain, SYBR-II), was used to assess nucleic acid injuries to chlorinated drinking water bacteria. Highly fluorescent SYBR-II-stained bacteria were converted to bacteria with low fluorescence after chlorination. PI staining of bacteria exposed to different doses of chlorine showed membrane permeabilisation ([Cl2] < 0.2 mg L(-1)) and nucleic acid damage at higher doses ([Cl2] > 0.3 mg L(-1)). Above a threshold dose (between 1.5 and 3 mg Cl2 L(-1)), nucleic acids appeared severely damaged and incapable of being stained by PI or SYBR-II. These results constitute evidence that FCM is a promising tool for assessing drinking water bacteria injuries and for controlling chlorine disinfection efficiency much more rapidly than the standard sensitive but time-consuming heterotrophic plate count method.

Potential Role of Methionine Sulfoxide in the Inactivation of the Chaperone GroEL by Hypochlorous Acid (HOCl) and Peroxynitrite (ONOO–)

GroEL is an Escherichia coli molecular chaperone that functions in vivo to fold newly synthesized polypeptides as well as to bind and refold denatured proteins during stress. This protein is a suitable model for its eukaryotic homolog, heat shock protein 60 (Hsp60), due to the high number of conserved amino acid sequences and similar function. Here, we will provide evidence that GroEL is rather insensitive to oxidants produced endogenously during metabolism, such as nitric oxide (.NO) or hydrogen peroxide (H(2)O(2)), but is modified and inactivated by efficiently reactive species generated by phagocytes, such as peroxynitrite (ONOO(-)) and hypochlorous acid (HOCl). For the exposure of 17.5 microm GroEL to 100-250 microm HOCl, the major pathway of inactivation was through the oxidation of methionine to methionine sulfoxide, established through mass spectrometric detection of methionine sulfoxide and the reactivation of a significant fraction of inactivated GroEL by the enzyme methionine sulfoxide reductase B/A (MsrB/A). In addition to the oxidation of methionine, HOCl caused the conversion of cysteine to cysteic acid and this product may account for the remainder of inactivated GroEL not recoverable through MsrB/A. In contrast, HOCl produced only negligible yields of 3-chlorotyrosine. A remarkable finding was the conversion of Met(111) and Met(114) to Met sulfone, which suggests a rather low reduction potential of these 2 residues in GroEL. The high sensitivity of GroEL toward HOCl and ONOO(-) suggests that this protein may be a target for bacterial killing by phagocytes.

Heat inactivation of Escherichia coli O157:H7 in apple juice exposed to chlorine

Exposure of Escherichia coli O157:H7 to chlorine before heat treatment results in increased production of heat shock proteins. Current heating regimens for pasteurizing apple cider do not account for chlorine exposure in the wash water. This research determined the effect of sublethal chlorine treatment on thermal inactivation of E. coli O157:H7. D58-values were calculated for stationary-phase cells exposed to 0.6 mg/liter of total available chlorine and unchlorinated cells in commercial shelf-stable apple juice (pH 3.6). D58-values for unchlorinated and chlorine-exposed cells in buffer were 5.45 and 1.65 min, respectively (P < 0.01). Death curves of chlorine-exposed and unchlorinated cells in apple juice were not completely linear. Unchlorinated cells heated in apple juice exhibit a 3-min delay before onset of linear inactivation. Chlorine treatment eliminated this shoulder, indicating an overall loss of thermotolerance. The linear portion of each curve represented a small fraction of the total population. D58-values calculated from these populations are 0.77 min for unexposed cells and 1.19 min for chlorine-exposed cells (P = 0.05). This indicates that a subpopulation of chorine-treated cells is possibly more resistant to heat because of chlorine treatment. The effect of chlorine treatment, however, is insignificant when compared with the effect of losing the shoulder. This is illustrated by the time required to kill the initial 90% of the cell population. This is observed to be 3.14 min for unchlorinated versus 0.3 min for chlorine-exposed cells (P < 0.001). These observations indicate that current heat treatments need not be adjusted for the effect of chlorine treatment.

Inactivation of myocardial dihydrolipoamide dehydrogenase by myeloperoxidase systems: effect of halides, nitrite and thiol compounds

Dihydrolipoamide dehydrogenase (LADH) lipoamide reductase activity decreased whereas enzyme diaphorase activity increased after LADH treatment with myeloperoxidase (MPO) dependent systems (MPO/H2O2/halide, MPO/NADH/halide and MPO/H2O2/nitrite systems. LADH inactivation was a function of the composition of the inactivating system and the incubation time. Chloride, iodide, bromide, and the thiocyanate anions were effective complements of the MPO/H2O2 system. NaOCl inactivated LADH, thus supporting hypochlorous acid (HOCl) as putative agent of the MPO/H2O2/NaCl system. NaOCl and the MPO/H2O2/NaCl system oxidized LADH thiols and NaOCl also oxidized LADH methionine and tyrosine residues. LADH inactivation by the MPO/NADH/halide systems was prevented by catalase and enhanced by superoxide dismutase, in close agreement with H2O2 production by the LADH/NADH system. Similar effects were obtained with lactoperoxidase and horse-radish peroxidase supplemented systems. L-cysteine, N-acetylcysteine, penicillamine, N-(2-mercaptopropionylglycine), Captopril and taurine protected LADH against MPO systems and NaOCl. The effect of the MPO/H2O2/NaNO2 system was prevented by MPO inhibitors (sodium azide, isoniazid, salicylhydroxamic acid) and also by L-cysteine, L-methionine, L-tryptophan, L-tyrosine, L-histidine and reduced glutathione. The summarized observations support the hypothesis that peroxidase-generated "reactive species" oxidize essential thiol groups at LADH catalytic site.

The effects of different (pseudo)halide substrates on peroxidase-mediated killing of Actinobacillus actinomycetemcomitans

Actinobacillus actinomycetemcomitans is a Gram-negative bacterium which has an important role in localized juvenile and in progressive periodontitis. It is sensitive to killing by the myeloperoxidase (MP)-hydrogen peroxide (H2O2)-chloride system which is part of the innate host defense mediated by polymorphonuclear leukocytes. Since it has been recently suggested that thiocyanate, instead of chloride, could serve as a main substrate for MP as for lactoperoxidase (LP) and salivary peroxidase, we investigated in this study the effect of both LP and MP systems on A. actinomycetemcomitans with different (pseudo)halide substrates, thiocyanate, chloride and iodide. The concentrations of the substrates were physiological for oral fluids, as was the concentration range of H2O2. Both peroxidases produced end products with identical antibacterial activity with thiocyanate and iodide. The oxidation of iodide resulted in the highest antimicrobial efficiency followed by chloride and thiocyanate. Addition of thiocyanate into either MP-H2O2-chloride or MP/LP-H2O2-iodide system abolished the bactericidal activity of the oxidized halide. However, the chloride did not affect the bactericidality of the MP-H2O2-iodide system, but when all 3 (pseudo)halide substrates were present no antimicrobial effect was recorded. Our study shows that the presence of thiocyanate in physiological amounts is able to prevent the bactericidal activity of halide-peroxidase systems in low H2O2 concentrations. These results explain why thiocyanate-peroxidase systems of either innate origin (saliva, crevicular fluid) or introduced by commercial oral hygiene products are most probably ineffective against A. actinomycetemcomitans in vivo. Further studies of halide/thiocyanate ratio are needed to develop products which are also effective against oral anaerobes.

Hypochlorous acid stress in Escherichia coli: resistance, DNA damage, and comparison with hydrogen peroxide stress

We have investigated the mechanisms of killing of Escherichia coli by HOCl by identifying protective functions. HOCl challenges were performed on cultures arrested in stationary phase and in exponential phase. Resistance to HOCl in both cases was largely mediated by genes involved in resistance to hydrogen peroxide (H2O2). In stationary phase, a mutation in rpoS, which controls the expression of starvation genes including those which protect against oxidative stress, renders the cells hypersensitive to killing by HOCl. RpoS-regulated genes responsible for this sensitivity were dps, which encodes a DNA-binding protein, and, to a lesser extent, katE and katG, encoding catalases; all three are involved in resistance to H2O2. In exponential phase, induction of the oxyR regulon, an adaptive response to H2O2, protected against HOCl exposure, and the oxyR2 constitutive mutant is more resistant than the wild-type strain. The genes involved in this oxyR-dependent resistance have not yet been identified, but they differ from those primarily involved in resistance to H2O2, including katG, ahp, and dps. Pretreatment with HOCl conferred resistance to H2O2 in an OxyR-independent manner, suggesting a specific adaptive response to HOCl. fur mutants, which have an intracellular iron overload, were more sensitive to HOCl, supporting the generation of hydroxyl radicals upon HOCl exposure via a Fenton-type reaction. Mutations in recombinational repair genes (recA or recB) increased sensitivity to HOCl, indicative of DNA strand breaks. Sensitivity was visible in the wild type only at concentrations above 0.6 mg/liter, but it was observed at much lower concentrations in dps recA mutants.

Hypochlorous acid activates the heat shock and soxRS systems of Escherichia coli

A series of plasmids, containing fusions of different stress promoters to lux reporter genes, was used in an attempt to monitor the defense circuits activated upon exposure of Escherichia coli to sublethal doses of free chlorine. A significant level of activation was exhibited by promoters of three heat shock genes (grpE, dnaK, and lon), in an rpoH-dependent manner. The promoter of micF, a gene under the control of the soxRS regulon, was also strongly induced, but not in a soxR mutant. This induction was not affected by sodA and sodB mutations, implying that it did not involve oxygen radical activity. Free-chlorine activation of both heat shock and soxRS regulons required an exposure of less then I s in duration. The oxyR or the SOS regulons were apparently not induced by free chlorine (as judged by lack of activation of katG and recA, respectively), and neither was the universal stress (uspA) protein.

Loss of DNA-membrane interactions and cessation of DNA synthesis in myeloperoxidase-treated Escherichia coli

Neutrophils and monocytes employ a diverse array of antimicrobial effector systems to support their host defense functions. The mechanisms of action of most of these systems are incompletely understood. The present report indicates that microbicidal activity by a neutrophil-derived antimicrobial system, consisting of myeloperoxidase, enzymatically generated hydrogen peroxide, and chloride ion, is accompanied by prompt cessation of DNA synthesis in Escherichia coli, as determined by markedly reduced incorporation of [3H]thymidine into trichloracetic acid-precipitable material. Simultaneously, the myeloperoxidase system mediates a decline in the ability of E. coli membranes to bind hemimethylated DNA sequences containing the E. coli chromosomal origin of replication (oriC). Binding of oriC to the E. coli membrane is an essential element of orderly chromosomal DNA replication. Comparable early changes in DNA synthesis and DNA-membrane interactions were not observed with alternative oxidant or antibiotic-mediated microbicidal systems. It is proposed that oxidants generated by the myeloperoxidase system modify the E. coli membrane in such a fashion that oriC binding is markedly impaired. As a consequence chromosomal DNA replication is impaired and organisms can no longer replicate.

Interaction of human defensins with Escherichia coli. Mechanism of bactericidal activity

Defensins are small, cysteine-rich antimicrobial peptides that are abundant in human, rabbit, and guinea pig neutrophils (PMN). Three defensins (human neutrophil peptide defensin [HNP]-1, HNP-2, and HNP-3) constitute between 30 and 50% of the total protein in azurophil granules of human PMN. We examined the mechanism of HNP-mediated bactericidal activity against Escherichia coli ML-35 (i-, y-, z+) and its pBR322-transformed derivative, E. coli ML-35p. Under conditions that supported bactericidal activity, HNP-1 sequentially permeabilized the outer membrane (OM) and inner membrane (IM) of E. coli. Coincident with these events, bacterial synthesis of DNA, RNA, and protein ceased and the colony count fell. Although these events were closely coupled under standard assay conditions, OM permeabilization was partially dissociated from IM permeabilization when experiments were performed with E. coli that had been plasmolyzed by mannitol. Under such conditions, the rate and extent of bacterial death more closely paralled loss of IM integrity than OM permeabilization. Electron microscopy of E. coli that had been killed by defensins revealed the presence of striking electron-dense deposits in the periplasmic space and affixed to the OM. Overall, these studies show that HNP-mediated bactericidal activity against E. coli ML-35 is associated with sequential permeabilization of the OM and IM, and that inner membrane permeabilization appears to be the lethal event.

Leukocytic oxygen activation and microbicidal oxidative toxins

Following a brief introduction of cellular response to stimulation comprising leukocyte activation, three major areas are discussed: (1) the neutrophil oxidase; (2) myeloperoxidase (MPO)-dependent oxidative microbicidal reactions; and (3) MPO-independent oxidative reactions. Topics included in section (A) are current views on the activation mechanism, redox composition, structural and topographic organization of the oxidase, and its respiratory products. In section (B), emphasis is placed on recent research on cidal mechanisms of HOCl, including the oxidative biochemistry of active chlorine compounds, identification of sites of lesions in bacteria, and attendant metabolic consequences. In section (C), we review the (bio)chemistry of H2O2 and .OH microbicidal reactions, with particular attention being given to addressing the controversial issue of probe methods to identify .OH radical and critical assessment of the recent proposal that MPO-independent killing arises from site-specific metal-catalyzed Fenton-type chemistry.

Candidacidal activity of myeloperoxidase: characterization of myeloperoxidase-yeast complex formation

We have previously demonstrated the ability of human neutrophil myeloperoxidase to bind to cell wall mannan polysaccharide isolated from Candida albicans. This binding capacity provides for association of the enzyme with target yeast which is essential for efficient candidacidal activity. In this report, we further consider the role of the mannan-binding property of myeloperoxidase in the candidacidal activity of the enzyme. Solubilized mannan antagonizes binding of the enzyme to yeast, suggesting that mannan may be a primary component of the fungal cell wall which serves as a target for binding of myeloperoxidase. Myeloperoxidase is shown to form complexes with both solubilized mannan and Candida yeast, with Kds of 0.97 x 10(-5) M and 1.2 x 10(-5) M, respectively. The interaction between myeloperoxidase and mannan does not allow the enzyme to readily dissociate from the surface of target yeast. As a result, the enzyme may be unable to dissociate from dead yeast to become available for binding to additional fungal targets.

Hypochlorous acid-promoted loss of metabolic energy in Escherichia coli

Oxidation of Escherichia coli by hypochlorous acid (HOCl) or chloramine (NH2Cl) gives rise to massive hydrolysis of cytosolic nucleotide phosphoanhydride bonds, although no immediate change occurs in either the nucleotide pool size or the concentrations of extracellular end products of AMP catabolism. Titrimetric curves of the extent of hydrolysis coincide with curves for loss of cell viability, e.g., reduction in the adenylate energy charge from 0.8 to 0.1-0.2 accompanies loss of 99% of the bacterial CFU. The oxidative damage caused by HOCl is irreversible within 100 ms of exposure of the organism, although nucleotide phosphate bond hydrolysis requires several minutes to reach completion. Neither HOCl nor NH2Cl reacts directly with nucleotides to hydrolyze phosphoanhydride bonds. Loss of viability is also accompanied by inhibition of induction of beta-galactosidase. The proton motive force, determined from the distribution of 14C-radiolabeled lipophilic ions, declines with incremental addition of HOCl after loss of respiratory function; severalfold more oxidant is required for the dissipation of the proton motive force than for loss of viability. These observations establish a causal link between loss of metabolic energy and cellular death and indicate that the mechanisms of oxidant-induced nucleotide phosphate bond hydrolysis are indirect and that they probably involve damage to the energy-transducing and transport proteins located in the bacterial plasma membrane.

Killing of Actinobacillus actinomycetemcomitans by the human neutrophil myeloperoxidase-hydrogen peroxide-chloride system

Actinobacillus actinomycetemcomitans is a facultative gram-negative coccobacillus associated with periodontal disease and nonoral infections. This organism is resistant to serum bactericidal mechanisms but is nevertheless killed by human neutrophils under aerobic and anaerobic conditions. Most of the killing attributable to oxidative mechanisms is inhibited by sodium cyanide, which suggests that the myeloperoxidase-hydrogen peroxide-chloride (MPO-H2O2-Cl-) system may be a key factor in the oxidative killing process. In this report, we examine whether the isolated MPO-H2O2-Cl- system is bactericidal against A. actinomycetemcomitans. We found that three major chromatographic forms of MPO were capable of killing A. actinomycetemcomitans at sublethal concentrations of H2O2 and that both catalase-positive and catalase-negative strains of this organism were sensitive to killing by the MPO-H2O2-Cl- system. We conclude that the isolated MPO-H2O2-Cl- system is bactericidal for A. actinomycetemcomitans independent of other neutrophil granule constituents and may be an important component of the oxygen-dependent bactericidal activity of the neutrophil with respect to this periodontopathic organism.

Effects of the putative neutrophil-generated toxin, hypochlorous acid, on membrane permeability and transport systems of Escherichia coli

Titrimetric addition of hypochlorous acid (HOCl) or chloramine (NH2Cl) to suspensions of Escherichia coli decreases their ability to accumulate 14C-labeled glutamine, proline, thiomethylgalactoside, and leucine in a manner that approximately coincides with loss of cell viability; quantitative differences in cellular response are observed with the two oxidants. Inhibition of beta-galactosidase activity in E. coli ML-35, a strain lacking functional lactose permease, is complex and also depends upon the identity of the oxidant. Membrane proton conductivities and glycerol permeabilities are unchanged by addition of HOCl or NH2Cl in excess of that required for inactivation. The combined results are interpreted to indicate that the locus of HOCl attack is the cell envelope, that HOCl inactivation does not occur by loss of membrane structural integrity, that loss of transport function can be identified with either selective oxidative inhibition of the transport proteins or loss of cellular metabolic energy, and that different mechanisms of inactivation may exist for HOCl and NH2Cl.

Bactericidal action of eosinophils from normal human blood

The ability of normal human eosinophils to ingest and kill Staphylococcus aureus and Escherichia coli was investigated and compared with the reactions shown by neutrophils from the same donors. The rate of phagocytosis of S. aureus by eosinophils was 50% of that shown by neutrophils. Unlike neutrophils, eosinophils were not able to kill ingested S. aureus at low bacterium/phagocyte ratios. The degree of S. aureus killing increased with increasing ratios, being equal to that of neutrophils when bacterium/phagocyte ratios of about 15 were used. This was probably due to a better triggering of the eosinophil oxidase system at high bacterium/phagocyte ratios. The early kinetics of the association of bacteria with eosinophils, the perforation of the bacterial envelope and the inactivation of bacterial proteins, was monitored in the ML-35 mutant strain of E. coli. The association of E. coli with eosinophils was 70% of that with neutrophils. Eosinophils had only 25% of the capacity of neutrophils to perforate the E. coli envelope. E. coli loses its colony-forming ability when the bacterial envelope has been perforated, indicating that eosinophils also kill E. coli more slowly than do neutrophils. This was confirmed with a plating assay for colony formation. The perforation of E. coli is independent of peroxidase-mediated reactions. Hence, the defective bactericidal action of eosinophils is probably not related to the differences between myeloperoxidase and eosinophil peroxidase. On the other hand, the inactivation of bacterial proteins is peroxidase dependent and was also seen to occur to a lesser extent in eosinophils compared with neutrophils. We conclude that eosinophils ingest E. coli but only slowly perforate (kill) these bacteria and barely inactivate the bacterial enzymes. In contrast, neutrophils quickly ingest and perforate (kill) E. coli and quickly inactivate the bacterial enzymes.

Inhibition of neutrophil killing of Candida albicans pseudohyphae by substances which quench hypochlorous acid and chloramines

Using a microtiter plate killing assay, we investigated the in vitro killing of Candida albicans by human neutrophils and by hypochlorous acid/hypochlorite ion (HOCl/OCl-) or chloramine solutions to evaluate the inhibition of this process by quenchers of these oxidants. Methionine, tryptophan, and alanine were able to effectively inhibit neutrophil killing of candida pseudohyphae. These substances were capable of quenching the oxidant activity of NaOCl, monochloramine (NH2Cl), and to a lesser extent, taurine chloramine. NaOCl and NH2Cl were able to kill C. albicans in the absence of inhibitors in concentrations of less than 5 microns M, whereas greater than 100 microns M taurine chloramine was required for killing. Methionine and tryptophan were capable of markedly inhibiting killing by all three oxidants, whereas alanine affected only killing by NaOCl. The oxidant activity of NaOCl was more readily quenched by opsonized or unopsonized Candida yeast than was the oxidant activity of either NH2Cl or taurine chloramine. These results suggest that some substances which quench the oxidizing activity of the products of the neutrophil myeloperoxidase system can inhibit the killing of C. albicans by these cells.

Antibacterial effect of lactoperoxidase and myeloperoxidase against Bacillus cereus

An oral periodontopathic bacterium, Bacillus cereus, was inhibited both by lactoperoxidase (LP) and myeloperoxidase (MP) antimicrobial systems. With the LP-SCN--H2O2 system, the growth inhibition was directly proportional to the amount of OSCN- ions present. The OSCN-, which is the principal oxidation product of the LP (or MP)-SCN--H2O2 system at neutral pH, is a normal component of human saliva. The oxidation products of both peroxidase systems inhibited the growth of the bacteria. This inhibition was associated with reduced extracellular release of collagenase activity from the cells. With LP, the antimicrobial efficiency of the oxidizable substrates was SCN- greater than I-, and with MP, the efficiency was I- greater than Cl- greater than SCN-, respectively. LP did not oxidize Cl-.

A wild and an attenuated strain of Francisella tularensis differ in susceptibility to hypochlorous acid: a possible explanation of their different handling by polymorphonuclear leukocytes

We have previously reported that a wild strain of Francisella tularensis is much less efficiently killed by human polymorphonuclear leukocytes than is an attenuated strain. In the present study, the killing of the attenuated strain was found to be strictly oxygen dependent. The wild and the attenuated strains both induced a respiratory burst in the leukocytes. The difference between the strains in susceptibility to agents produced at the burst could be explained by a difference in susceptibility to hypochlorous acid.

Effect of growth phase and cell envelope structure on susceptibility of Salmonella typhimurium to the lactoperoxidase-thiocyanate-hydrogen peroxide system

The lactoperoxidase-thiocyanate-hydrogen peroxide system was found to have both bacteriostatic and bactericidal activities against strains of Salmonella typhimurium. The bactericidal activity was clearly dependent on the permeability of the bacterial cell envelope. The deep rough mutant TA1535, with the most permeable cell envelope, was killed both at neutral and acid pH, whereas very little or no killing was observed with the intact cells of the parent strain hisG46. The delta gal mutant, TA1530, representing an intermediate in cell envelope permeability, was inhibited to a much lesser extent than TA1535. Bacteria in log phase of growth were more sensitive to the bactericidal effects than were those in stationary phase. Growth phase had little influence on the bacteriostatic effects. The hisG46 strain produced significant quantities of acid in the presence of glucose. This acid production was inhibited by the lactoperoxidase-thiocyanate-hydrogen peroxide system, and, in contrast to results obtained with several strains of streptococci, this inhibition was not reversed by addition of a reducing agent (2-mercaptoethanol).

A new method for assessment of serum-induced damage to E. coli

A simple and rapid spectrophotometric assay for the kinetic evaluation of serum-induced damage to E. coli is described, based on changes in the optical density (OD) of a bacterial suspension. Exposure of antibody-coated E. coli to human absorbed serum results in a diphasic response, namely an increase in OD, which reaches a maximum at about 17 min and is followed by a progressive decrease in OD until a minimum value is reached after 45 min. The increase and the decrease in OD are related to bacterial death and bacterial lysis, respectively.

Assessment of chlorination by human neutrophils

On phagocytosing a microorganism, the neutrophil (polymorphonuclear leukocyte, PMN) consumes oxygen at a sharply elevated rate1. The oxygen is used to kill the microorganism, presumably being used to produce a potent oxidizing agent or agents. Candidates for these bactericidal agents are singlet oxygen, hydroxyl radical and chlorinating agents (that is, species containing 'active' Cl in a formal +1 oxidation state: HOCl, Cl2, N-chloroamides, and so on)1-5. We now report a semiquantitative assay for PMN-generated active chlorine based on its trapping with 1,3,5-trimethoxybenzene (TMB). Using this assay, we have found that at least 28% of the oxygen consumed by stimulated normal human PMNs is converted to active chlorinating agents.

Peroxidase-thiocyanate-peroxide antibacterial system does not damage DNA

The hypothiocyanite ion (OSCN(-)) is a normal component of human saliva. It is a highly reactive oxidizing agent, and at concentrations above the values normally found in human saliva, it inhibits the growth and metabolism of oral bacteria. This finding has led to the suggestion that antibacterial properties of human saliva might be enhanced in vivo by appropriate supplements which elevate OSCN(-) concentrations. Since DNA is sensitive to oxidizing agents (hydrogen peroxide attacks nucleosides), high concentrations of OSCN(-) in human saliva might damage DNA and produce deleterious effects on the oral mucosa. In the present study, the effect of high OSCN(-) concentrations on several mutagen-sensitive Salmonella typhimurium strains was determined. These strains are used to detect base-pair substitutions and frameshift mutations. We also studied the effects of OSCN(-) on a Saccharomyces cerevisiae (yeast) strain commonly employed as a test cell for evaluating the potential of a compound to produce gene conversion, mitotic crossing-over, or reverse mutation. By recording the UV spectra of mixtures of calf thymus DNA and OSCN(-), we explored the possible in vitro reactions of this oxidizing agent with eucaryotic genetic material. Our results show that, at concentrations above 10 muM, OSCN(-) is toxic for the tested Salmonella typhimurium strains. The mutant strains with defects in cell wall lipopolysaccharides are killed more readily by OSCN(-) than is the strain lacking these defects. However, OSCN(-) was not mutagenic for any of the tested strains. Saccharomyces cerevisiae was not affected by OSCN(-) even at concentrations above 800 muM. Calf thymus DNA was not oxidized by OSCN(-). We conclude that the elevated concentrations of OSCN(-) required to produce antibacterial effects in the human mouth pose no threat to the genetic material of host tissues.

Monocyte and granulocyte-mediated tumor cell destruction. A role for the hydrogen peroxide-myeloperoxidase-chloride system

Human monocytes stimulated with phorbol myristate acetate were able to destroy a T lymphoblast cell target (CEM). Stimulated human granulocytes were also capable of mediating CEM cytotoxicity to a comparable degree as the monocyte. CEM destruction was dependent on the pH and the effector cell number. Both monocyte or granulocyte mediated cytotoxicity were inhibited by the addition of catalase, whereas superoxide dismutase had no inhibitory effect. In addition, CEM were protected from cytolysis by the effector cells by the myeloperoxidase inhibitors, azide and cyanide, or by performing the experiment under halide-free conditions. Glucose oxidase, an enzyme system capable of generating hydrogen peroxide, did not mediate CEM cytotoxicity, while the addition of purified myeloperoxidase dramatically enhanced cytolysis. Hypochlorous acid scavengers prevented CEM destruction by the glucose oxidase-myeloperoxidase-chloride system but neither hydroxyl radical nor singlet oxygen scavengers had any protective effect. These hypochlorous acid scavengers were also successful in inhibiting monocyte or granulocyte-mediated CEM cytotoxicity. Based on these observations we propose that human monocytes or granulocytes can utilize the hydrogen peroxide-myeloperoxidase-chloride system to generate hypochlorous acid or species of similar reactivity as a potential mediator of CEM destruction.

The killing of newborn larvae of Trichinella spiralis by eosinophil peroxidase in vitro

Helminth infections in mammals are characterized by a high level of eosinophils in parasitized tissues and blood, and it has recently been suggested that these cells have a direct parasiticidal effect. Newborn larvae of Trichinella spiralis can be killed within 20 min by incubation at room temperature in a cell-free system, including purified human eosinophil peroxidase (EPO), H2O2 and chloride at pH 5.5. Killing was measured by microscopic observation of the larvae. The larvicidal effect was dependent on each component of the EPO/H2O2/Cl- system and could be prevented by using SO4(2-) instead of Cl-. Killing was totally inhibited by sodium azide and catalase, and substantially by bovine serum albumin, a protein that is an effective scavenger for HOCl. Since larvae could also be killed directly by HOCl under these conditions and EPO is able to oxidize Cl- to hypochlorous acid, it is very likely that the larvicidal effect of the EPO system is due to formation of hypochlorous acid. It is proposed that in vivo, the combination of EPO, which is exocytosed onto the surface of the parasite, and H2O2, which is generated by stimulated eosinophils, is responsible for the larvicidal effect.