Outer membrane mutants of Salmonella typhimurium LT2 have lipopolysaccharide-dependent resistance to the bactericidal activity of anaerobic human neutrophils

Periplasmic methionine sulfoxide reductase (MsrP)-a secondary factor in stress survival and virulence of Salmonella Typhimurium

Among others, methionine residues are highly susceptible to host-generated oxidants. Repair of oxidized methionine (Met-SO) residues to methionine (Met) by methionine sulfoxide reductases (Msrs) play a chief role in stress survival of bacterial pathogens, including Salmonella Typhimurium. Periplasmic proteins, involved in many important cellular functions, are highly susceptible to host-generated oxidants. According to location in cell, two types of Msrs, cytoplasmic and periplasmic are present in S. Typhimurium. Owing to its localization, periplasmic Msr (MsrP) might play a crucial role in defending the host-generated oxidants. Here, we have assessed the role of MsrP in combating oxidative stress and colonization of S. Typhimurium. ΔmsrP (mutant strain) grew normally in in-vitro media. In comparison to S. Typhimurium (wild type), mutant strain showed mild hypersensitivity to HOCl and chloramine-T (ChT). Following exposure to HOCl, mutant strain showed almost similar protein carbonyl levels (a marker of protein oxidation) as compared to S. Typhimurium strain. Additionally, ΔmsrP strain showed higher susceptibility to neutrophils than the parent strain. Further, the mutant strain showed very mild defects in survival in mice spleen and liver as compared to wild-type strain. In a nutshell, our results indicate that MsrP plays only a secondary role in combating oxidative stress and colonization of S. Typhimurium.

Malate synthase contributes to the survival of Salmonella Typhimurium against nutrient and oxidative stress conditions

To survive and replicate in the host, S. Typhimurium have evolved several metabolic pathways. The glyoxylate shunt is one such pathway that can utilize acetate for the synthesis of glucose and other biomolecules. This pathway is a bypass of the TCA cycle in which CO₂ generating steps are omitted. Two enzymes involved in the glyoxylate cycle are isocitrate lyase (ICL) and malate synthase (MS). We determined the contribution of MS in the survival of S. Typhimurium under carbon limiting and oxidative stress conditions. The ms gene deletion strain (∆ms strain) grew normally in LB media but failed to grow in M9 minimal media supplemented with acetate as a sole carbon source. However, the ∆ms strain showed hypersensitivity (p < 0.05) to hypochlorite. Further, ∆ms strain has been significantly more susceptible to neutrophils. Interestingly, several folds induction of ms gene was observed following incubation of S. Typhimurium with neutrophils. Further, ∆ms strain showed defective colonization in poultry spleen and liver. In short, our data demonstrate that the MS contributes to the virulence of S. Typhimurium by aiding its survival under carbon starvation and oxidative stress conditions.

Methionine sulfoxide reductase A (MsrA) contributes to Salmonella Typhimurium survival against oxidative attack of neutrophils

Salmonella Typhimurium (ST) must evade neutrophil assault for infection establishment in the host. Myeloperoxidase generated HOCl is the key antimicrobial agent produced by the neutrophils; and methionine (Met) residues are the primary targets of this oxidant. Oxidation of Mets leads to methionine sulfoxide (Met-SO) formation and consequently compromises the protein function(s). Methionine sulfoxide reductase A (MsrA) reductively repairs Met-SO to Mets. In this manner, MsrA maintains the function(s) of key proteins which are important for virulence of ST and enhance the survival of this bacterium under oxidative stress. We constructed msrA gene deletion strain (ΔmsrA). The primers located in the flanking regions to ΔmsrA gene amplified 850 and 300 bp amplicons in ST and ΔmsrA strains, respectively. The ΔmsrA strain grew normally in in vitro broth culture. However, ΔmsrA strain showed high susceptibility (p<0.001) to very low concentrations of HOCl which was restored (at least in part) by plasmid based complementation. ΔmsrA strain was hypersensitive (than ST) to the granules isolated from neutrophils. Further, the ΔmsrA strain was significantly (p<0.05) more susceptible to neutrophil mediated killing.

LPS structure and PhoQ activity are important for Salmonella Typhimurium virulence in the Galleria mellonella infection model [corrected]

The larvae of the wax moth, Galleria mellonella, have been used experimentally to host a range of bacterial and fungal pathogens. In this study we evaluated the suitability of G. mellonella as an alternative animal model of Salmonella infection. Using a range of inoculum doses we established that the LD₅₀ of SalmonellaTyphimurium strain NCTC 12023 was 3.6 × 10³ bacteria per larva. Further, a set of isogenic mutant strains depleted of known virulence factors was tested to identify determinants essential for S. Typhimurium pathogenesis. Mutants depleted of one or both of the type III secretion systems encoded by Salmonella Pathogenicity Islands 1 and 2 showed no virulence defect. In contrast, we observed reduced pathogenic potential of a phoQ mutant indicating an important role for the PhoPQ two-component signal transduction system. Lipopolysaccharide (LPS) structure was also shown to influence Salmonella virulence in G. mellonella. A waaL(rfaL) mutant, which lacks the entire O-antigen (OAg), was virtually avirulent, while a wzz(ST)/wzz(fepE) double mutant expressing only a very short OAg was highly attenuated for virulence. Furthermore, shortly after infection both LPS mutant strains showed decreased replication when compared to the wild type in a flow cytometry-based competitive index assay. In this study we successfully established a G. mellonella model of S. Typhimurium infection. By identifying PhoQ and LPS OAg length as key determinants of virulence in the wax moth larvae we proved that there is an overlap between this and other animal model systems, thus confirming that the G. mellonella infection model is suitable for assessing aspects of Salmonella virulence function.

Acute-phase concentrations of lipopolysaccharide (LPS)-binding protein inhibit innate immune cell activation by different LPS chemotypes via different mechanisms

The chain length of bacterial lipopolysaccharide (LPS) is a crucial factor for host-pathogen interaction during bacterial infection. While rough (R)-type and smooth (S)-type LPSs have been shown to differ in their ability to interact with the bactericidal/permeability-increasing protein, little is known about the differential mode of interaction with the acute-phase reactant LPS-binding protein (LBP). At lower concentrations, LBP catalyzes the binding of LPS to CD14 and enhances LPS-induced cellular activation via Toll-like receptor 4. In humans, however, concentrations of LBP in serum increase during an acute-phase response, and these LBP concentrations exhibit inhibitory effects in terms of cellular activation. The mechanisms of inhibition of LPS effects by LBP are not completely understood. Here, we report that human high-dose LBP (hd-LBP) suppresses binding of both R-type and S-type LPS to CD14 and inhibits LPS-induced nuclear translocation of NF-kappaB, although cellular uptake of R-type LPS was found to be increased by hd-LBP. In contrast, we found that hd-LBP enhanced the binding and uptake of S-type LPS only under serum-free conditions, whereas in the presence of serum, hd-LBP inhibited cellular binding and uptake. This inhibitory effect of serum could be mimicked by the addition of purified high-density lipoprotein (HDL) to serum-free medium, indicating an LBP-mediated transfer of preferentially S-type LPS to plasma lipoproteins such as HDL. A complete understanding of the host's mechanisms to modulate the proinflammatory effects of LPS will most likely help in the understanding of inflammation and infection and may lead to novel therapeutic intervention strategies.

Regulation of Salmonella typhimurium lipopolysaccharide O antigen chain length is required for virulence; identification of FepE as a second Wzz

Wzz proteins regulate the degree of polymerization of the O antigen (Oag) subunits in lipopolysaccharide (LPS) biosynthesis. Although the pathogenic relevance of Oag is well recognized, the significance of Oag chain length regulation is not well defined. In this report, Salmonella typhimurium was shown to possess two functional wzz genes resulting in a bimodal Oag length distribution. In addition to the previously described wzzST that results in long (L) modal length LPS with 16-35 Oag repeat units (RUs), we now report that wzzfepE, a homologue of Escherichia coli fepE, is responsible for the production of very long (VL) modal length LPS Oag, estimated to contain> 100 Oag RUs. Analysis of a series of isogenic S. typhimurium C5 mutants found that the presence of either wzz gene (and hence either modal length) was sufficient for complement resistance and virulence in the mouse model of infection, suggesting a degree of redundancy in the role of these two wzz genes and their respective Oag modal lengths. In contrast, the wzzST/wzzfepE double mutant, with relatively short, random-length Oag, displayed enhanced susceptibility to complement and was highly attenuated in the mouse. This clearly demonstrates the molecular genetic basis for the longer LPS Oag chains previously identified as the basis of complement resistance in Salmonella. The presence of wzzfepE homologues in the genomic sequences of strains of Escherichia coli, Shigella flexneri and multiple serovars of Salmonella suggests that bimodality of LPS Oag is a common phenomenon in the Enterobacteriaceae.

The polysaccharide portion of lipopolysaccharide regulates antigen-specific T-cell activation via effects on macrophage-mediated antigen processing

The lipopolysaccharide (LPS) structure of Salmonella typhimurium has been correlated with the virulence of wild-type strain LT2. Mutants of LT2 with truncated polysaccharide portions of LPS are less virulent than strains with a complete LPS structure. Polyclonal T cells and monoclonal T-cell hybridomas were more reactive to heat-killed rough mutants than to heat-killed smooth strains, as measured by interleukin-2 (IL-2) production. Using a large panel of strains with truncated LPS molecules, we found that T-cell reactivity decreased with certain lengths of polysaccharide. The decreased response was not due to differential phagocytic uptake, IL-12 production, or major histocompatibility complex class II surface expression by macrophages. Also, LT2 did not mediate any global suppression since addition of LT2 did not diminish the response of T cells specific for antigens unrelated to Salmonella. In an experiment in which processing times were varied, we found that antigens from rough strains were processed and presented more quickly than those associated with smooth strains. At longer processing times, epitopes from LT2 were presented well. We hypothesize that the slower antigen processing and presentation of wild-type Salmonella may be caused by masking of surface antigens by the longer polysaccharide portion of smooth LPS. This blocking of effective antigen presentation may contribute to the virulence of Salmonella.

Origins and development of peptide antibiotic research. From extracts to abstracts to contracts

That cationic proteins might be factors on the antimicrobial defenses of mammalian hosts and are apparently associated with the cytoplasmic granules of phagocytic leukocytes first became evident on the late nineteenth century. It remained, however, for development of sophisticated microanalytic techniques in microbiology, cell biology and protein biochemistry to place these hypotheses in the realm of established theory. This article is a brief summary of significant steps in the development of this theory. It also attempts to outline the firmly established scope and significance of these developments both for the theory of immunity to infection in the different phyla and for the now global quest for new antibiotics.

The effects of magainin 2, cecropin, mastoparan and melittin on Brucella abortus

The effect of the alpha-helical polycationic peptides magainin 2, melittin, mastoparan and cecropin on the viability of Brucella abortus 544 (type species), B. abortus S19 (vaccine strain) and B. abortus S2308 (vaccine challenge strain) was determined. Rough mutants of these strains and the rough candidate vaccine strain B. abortus RB51 were also tested. S. typhimurium was used as a control. The peptides did not affect the viability of B. abortus smooth strains but some of the peptides affected viability of the rough strains. Magainin 2 at a concentration of 100 micrograms ml-1 did not reduce the viability of the rough B. abortus strains. Cecropin at a concentration of 15 micrograms ml-1 reduced the viability of the rough strains by approximately 10-fold. Mastoparan at a concentration of 50 micrograms ml-1 reduced the viability of the rough strains by approximately 100-fold. Melittin at a concentration of 20 micrograms ml-1 reduced the viability of the rough strains of B. abortus by approximately 1000-fold. The brucellae were significantly more resistant to all the cationic peptides than was S. typhimurium.

Susceptibility of lipopolysaccharide-responsive and -hyporesponsive ItyS Mice to infection with rough mutants of Salmonella typhimurium

The R5 (chemotype Rb) but not the R10 (chemotype Rd) mutant of murine pathogen Salmonella typhimurium 395MS was extremely virulent in intraperitoneal infections of C57BL/10ScCr mice carrying the ityS and lpsD alleles. C57BL/6J (ityS lpsN) and C3H/HeJ (ityR lpsD) mice showed a much higher resistance to the R5 mutant. Further studies were performed with peritoneal macrophages in vitro in order to elucidate susceptibility in lipopolysaccharide (LPS)-hyporesponsive mice carrying ItyS. The intracellular killing capacity of the ItyS LpsD macrophages was lower than that of the ItyS LpsN macrophages for the R5 mutant and may partly explain the increased susceptibility of the ItyS LpsD mice. The deep rough mutant, R10, was rapidly killed intracellularly by the ItyS LpsD macrophages. Processing of the bacteria in macrophages that had phagocytosed R5 or R10 bacteria was followed for up to 18 days by endotoxin measurements (limulus assay) and immunostaining, with monoclonal antibodies to various parts of the LPS molecule being used. Only 0.1% or less of the macrophage-associated bacteria remained alive after 48 h of incubation, and none were alive on day 7. Although immunostaining showed that LPS was present in both the LpsD and LpsN macrophages during the whole incubation period of 18 days, endotoxin activity in the LpsD macrophages on day 7 was lower than that in the LpsN macrophages, indicating that qualitative modifications of the chemical composition or physical state of the LPS molecule occurred. The interleukin-6 response in the ItyS LpsD macrophages was delayed and of shorter duration compared with that in the ItyS LpsN macrophages. The results suggest that the difference between the LPS-hyporesponsive and -responsive ItyS mice in susceptibility to infection with the R5 mutant was due to the lower activation state of the LpsD macrophages during infection, leading to a lower intracellular bactericidal systems of the macrophages. A rapid killing of the bacterium should restrict the infection and may partly compensate for a diminished inflammatory response. The persistence of LPS within the cells is discussed.

Comparison of the sensitivities of Salmonella typhimurium oxyR and katG mutants to killing by human neutrophils

The respiratory burst of neutrophils is believed to kill bacteria by generating oxidative species, such as superoxide anion, hydrogen peroxide, and oxidized halogen species. The oxyR gene of Salmonella typhimurium controls a regulon induced by oxidative stress, such as exposure to hydrogen peroxide. Some researchers have suggested that oxyR may play a key role in bacterial survival following phagocytosis. We have tested this possibility by comparing the survival, following exposure to human neutrophils, of isogenic strains bearing different oxyR alleles. Neither inactivation of the oxyR gene nor constitutive overexpression of the oxyR-regulated proteins (oxyR1 allele) greatly alters bacterial resistance to neutrophils. The katG gene, encoding the oxyR-regulated enzyme hydroperoxidase I, was also without effect on survival following exposure to neutrophils. We conclude that the oxyR response does not play a significant role in the resistance of S. typhimurium to phagocytic killing in vitro.

Bactericidal activities of rat defensins and synthetic rabbit defensins on Staphylococci, Klebsiella pneumoniae (Chedid, 277, and 8N3), Pseudomonas aeruginosa (mucoid and nonmucoid strains), Salmonella typhimurium (Ra, Rc, Rd, and Re of LPS mutants) and Escherichia coli

Rat defensins were purified and tested for in vitro bactericidal assay against gram-positive and gram-negative bacteria. Staphylococcus aureus (209P, Cowan I, Smith diffuse and Smith compact) were resistant to defensins, whereas Staphylococcus epidermidis, Staphylococcus saprophyticus, Micrococcus lysodeikticus and Bacillus subtilis were less sensitive. Gram-negative bacteria, such as Pseudomonas aeruginosa (mucoid and K) and Klebsiella pneumoniae (Chedid, 277, and 8N3 which were heavily capsulated, moderately capsulated and noncapsulated, respectively) were all very sensitive to defensins and killed within 20 min. Escherichia coli was moderately sensitive and the rough mutants of lipopolysaccharide (LPS) of Salmonella typhimurium LT2, such as Ra, Rc, Rd, and Re were equally sensitive to defensins, being killed within 40 min. Lysozyme did not show any bactericidal activity except against M. lysodeikticus and B. subtilis, whereas it enhanced the bactericidal activity of defensins against P. aeruginosa, E. coli, and K. pneumoniae and suppressed the killing activity of defensins against S. typhimurium and S. aureus. With regard to the three synthetic rabbit defensins, NP1, NP4, and NP5, NP1 showed strong bactericidal activity against K. pneumoniae 277, comparable to that of rat defensins. Neither NP4 nor NP5 showed any bactericidal activity, while NP5 rather enhanced the bactericidal activity of NP1 against K. pneumoniae 277.

Rapid membrane permeabilization and inhibition of vital functions of gram-negative bacteria by bactenecins

Bactenecins are a class of arginine-rich antibacterial peptides of bovine neutrophil granules. Two bactenecins with approximate molecular weights of 5,000 and 7,000 designated Bac5 and Bac7, respectively, exert in vitro a potent bactericidal activity toward several gram-negative bacteria (R. Gennaro, B. Skerlavaj, and D. Romeo, Infect. Immun. 57:3142-3146, 1989). We have now found that this activity shows an inverse relationship to the ionic strength of the medium and is inhibited by divalent cations and greatly potentiated by lactoferrin. Under conditions supporting marked bactericidal activity, the two peptides cause a rapid increase in the permeability of both the outer and inner membranes of Escherichia coli, as shown by unmasking of periplasmic beta-lactamase and of cytoplasmic beta-galactosidase. In addition, the two bactenecins inhibit the respiration of E. coli and Klebsiella pneumoniae but not of Bac5- and Bac7-resistant Staphylococcus aureus. Furthermore, they induce a drop in ATP content in E. coli, K. pneumoniae, and Salmonella typhimurium and a marked decrease in the rates of transport and incorporation of [3H]leucine and [3H]uridine into E. coli protein and RNA, respectively. In general, all these effects become evident within 1 to 2 min and reach their maximal expression within about 5 min. Overall, these data strongly suggest that the decrease in bacterial viability is causally related to the increase in membrane permeability and the subsequent fall in respiration-linked proton motive force, with the attendant loss of cellular metabolites and macromolecular biosynthesis ability.

O antigen and lipid A phosphoryl groups in resistance of Salmonella typhimurium LT-2 to nonoxidative killing in human polymorphonuclear neutrophils

We have compared the intraleukocytic survival of isogenic strains of Salmonella typhimurium, whose outer membrane lipopolysaccharide differed in O antigen and lipid A composition and whose susceptibility to nonoxidative antimicrobial granule proteins of human polymorphonuclear neutrophilis (PMN) could be established. We found that the order of resistance to the bactericidal activity of intact PMN of the three bacterial strains utilized closely resembled their ordered resistance to the purified human cationic antimicrobial 57,000-dalton protein (CAP57). LT-2, a smooth wild-type strain, was far more resistant than SH9178, its rough (Rb LPS) mutant. It was most significant that SH7426, a polymyxin B-resistant pmrA mutant of SH9178, not only was substantially more resistant to CAP57 and to intraphagocytic killing than SH9178 but also came close to being as resistant as LT-2. These experiments confirm earlier work that showed the importance of the glycosyl groups of O antigens of S. typhimurium for their resistance to O2-independent antimicrobial phagocytosis by PMN. The surprising result was that a rough strain, very susceptible to bactericide, became substantially more resistant when a mutation led to its lipid A phosphoryl groups being 100% substituted with amino pentoses. Yet unresolved is whether the protection is due to the loss of negative charges on the lipid A, the substitution of sugar molecules in vulnerable loci in the outer membrane, or both.

Oxygen-independent antimicrobial action in sphingosine-treated neutrophils

Sphingosine is reported to inhibit the oxidative burst and superoxide anion production of human polymorphonuclear neutrophils (PMN) phagocytosing in atmospheric oxygen (Wilson et al., 1986). We have confirmed its effect on superoxide production and examined the antimicrobial phagocytic capacity of PMN treated with sphingosine, comparing them with PMN, untreated but phagocytosing either under anaerobic conditions or in atmospheric oxygen. Sphingosine just like anaerobiosis partially inhibited, but did not eliminate, the bactericidal activity of PMN when compared to non-treated aerobic cells. In fact, sphingosine-treated PMN mimicked killing of Staphylococcus aureus (S. aureus) and Serratia marcescens (S. marcescens) due to anaerobic PMN. Moreover, our results with Salmonella typhimurium and sphingosine-treated cells duplicated results this laboratory published previously about comparative killing of Salmonella in aerobic versus anaerobic neutrophils. In these studies sphingosine-treated PMN took up bacteria as avidly as untreated PMN and retained their viability, as assessed by trypan blue exclusion. While sphingosine should not be completely substituted for anaerobic studies, it is a convenient screening reagent for the study of non-oxidative killing mechanisms of PMN. Results achieved with anaerobic and with sphingosine-treated cells suggest that O2-independent antimicrobial action is substantially more powerful than has been generally acknowledged.

Nonoxidative microbicidal activity in normal human alveolar and peritoneal macrophages

Although Toxoplasma gondii multiplies within normal murine alveolar and peritoneal macrophages, it is killed by normal rat alveolar and peritoneal macrophages. The killing by rat macrophages is by a nonoxidative mechanism. Studies on normal human alveolar macrophages have reported disparate results in regard to their ability to inhibit or kill T. gondii. We considered it of interest to explore further the effect of normal human alveolar and peritoneal macrophages on T. gondii. Unstimulated alveolar macrophages from each of seven individuals demonstrated a marked ability to kill or inhibit multiplication of T. gondii in vitro (e.g., the number of parasites per 100 alveolar macrophages was 31 at time zero and 2 at 18 h, whereas this value increased from 37 at time zero to 183 at 18 h in murine macrophages assayed in parallel). In quantitative assays of superoxide, alveolar macrophages released a substantial amount of superoxide when exposed to phorbol myristate acetate or to candidae. In contrast, alveolar macrophages incubated with T. gondii released no more superoxide than when in medium alone. Scavengers of superoxide anions, hydrogen peroxide, singlet oxygen, and hydroxyl radicals failed to inhibit killing of T. gondii by alveolar macrophages. Peritoneal macrophages from each of six normal women undergoing laparoscopy killed T. gondii in vitro; results of quantitative superoxide assays and scavenger experiments demonstrated that no oxidative burst was triggered in these macrophages by exposure to T. gondii. These data indicate that normal human alveolar and peritoneal macrophages can kill an intracellular parasite by nonoxidative mechanisms and suggest that these mechanisms are important in inhibition or killing of other opportunistic intracellular pathogens.

Oxidative and nonoxidative killing of Actinobacillus actinomycetemcomitans by human neutrophils

Actinobacillus actinomycetemcomitans is a facultative gram-negative microorganism which has been implicated as an etiologic agent in localized juvenile periodontitis and in subacute bacterial endocarditis and abscesses. Although resistant to serum bactericidal action and to oxidant injury mediated by superoxide anion (O2-) and hydrogen peroxide (H2O2), this organism is sensitive to killing by the myeloperoxidase-hydrogen peroxide-chloride system (K.T. Miyasaki, M.E. Wilson, and R.J. Genco, Infect. Immun. 53:161-165, 1986). In this study, we examined the sensitivity of A. actinomycetemcomitans to killing by intact neutrophils under aerobic conditions, under anaerobic conditions, and under aerobic conditions in the presence of the heme-protein inhibitor sodium cyanide. Intact neutrophils killed opsonized A. actinomycetemcomitans under aerobic and anaerobic conditions, and the kinetics of these reactions indicated that both oxidative and nonoxidative mechanisms were operative. Oxidative mechanisms contributed significantly, and most of the killing attributable to oxidative mechanisms was inhibited by sodium cyanide, which suggested that the myeloperoxidase-hydrogen peroxide-chloride system participated in the oxidative process. We conclude that human neutrophils are capable of killing A. actinomycetemcomitans by both oxygen-dependent and oxygen-independent pathways, and that most oxygen-dependent killing requires myeloperoxidase activity.

Neisseria gonorrhoeae survive intraleukocytic oxygen-independent antimicrobial capacities of anaerobic and aerobic granulocytes in the presence of pyocin lethal for extracellular gonococci

The resistance of a piliated, transparent variant of Neisseria gonorrhoeae FA19 to intraleukocytic killing by human polymorphonuclear neutrophils (PMN) was examined. In both aerobic and anaerobic PMN monolayers, approximately 2% of the intracellular gonococci survived for as long as 165 min. Anaerobic PMN were as effective as aerobic PMN in the intracellular killing of gonococci. Hence, O2-independent antimicrobial systems of PMN performed a significant role in the intraleukocytic killing of gonococci were intracellular was supported by the elimination of extracellular bacteria by the addition of pyocin 103 and confirmed by the fluorescent antibody staining of intact gonococci after the PMN were permeabilized to antibody with a Formalin-acetone treatment of PMN monolayers. Our data indicate that while the majority of ingested gonococci are killed by O2-independent antimicrobial systems, a small number (about 2%), survive even when care is taken to eliminate extracellular bacteria.

Defensins. Natural peptide antibiotics of human neutrophils

We extracted a granule-rich sediment from normal human neutrophils and subjected it to chromatographic, electrophoretic, and functional analysis. The extract contained three small (molecular weight less than 3,500) antibiotic peptides that were named human neutrophil peptide (HNP)-1, HNP-2, and HNP-3, and which will be referred to as "defensins." HNP 1-3, a mixture of the three defensins, killed Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli effectively in vitro when tested in 10 mM phosphate buffer containing certain nutrients, but it had little or no bactericidal activity in nutrient-free buffer. In contrast, the nutrient-free buffer supported a high degree of activity by HNP 1-3 against Cryptococcus neoformans. In addition to its antibacterial and antifungal properties, HNP 1-3 directly inactivated herpes simplex virus, Type 1. Two of the individual purified defensins, HNP-1 and HNP-2, were as microbicidal as the mixture HNP 1-3. HNP-3 was less active than the other defensins against most but not all of the microbes tested. Immunoperoxidase stains revealed HNP 1-3 to have a granular localization in the neutrophil's cytoplasm by light microscopy. Frozen thin section immunogold transmission electron microscopy showed HNP 1-3 to be localized in azurophil granules. These studies define a broad-spectrum antimicrobial system in human neutrophils. The defensin system may operate in conjunction with or independently from oxygen-dependent microbicidal processes to enable human neutrophils to inactivate and destroy potential pathogens.

Oxygen-independent intracellular and oxygen-dependent extracellular killing of Escherichia coli S15 by human polymorphonuclear leukocytes

Effective killing of bacteria by polymorphonuclear leukocytes (PMN) is generally assumed to require intracellular sequestration and, depending on the bacterial species, can be both O2-dependent or O2-independent. Killing of several strains of Salmonella typhimurium and Escherichia coli by rabbit PMN does not require O2 and is apparently due to a granule-associated bactericidal/permeability-increasing protein (BPI) present in rabbit and human PMN. In this study we examined the O2 dependence of the killing of E. coli (S15) by human PMN. Ingested and noningested E. coli were separated by centrifugation after incubation with PMN in room air or under N2. In the presence of heat-treated serum approximately 50% of E. coli (10 bacteria/PMN) were taken up by PMN and rapidly (5-15 min) killed both in room air and under N2. The remaining extracellular bacteria (approximately 50%) were killed during 30-60 min of incubation in room air but not under N2. When uptake of E. coli by PMN was increased to approximately 80% by the use of C6-depleted serum (retaining heat-labile opsonins), bacterial survival under N2 was reduced from 54 +/- 7.6% to 13 +/- 5.5%. PMN from a patient with chronic granulomatous disease killed PMN-associated but not extracellular E. coli. BPI was detected, by indirect immunofluorescence, on the surface of PMN-associated E. coli within 5 min of incubation of E. coli with PMN both in room air and under N2. In contrast, at no time was BPI detected on the surface of extracellular E. coli, indicating that the non-PMN-associated E. coli had not been previously ingested. Thus, killing of ingested E. coli S15 by human as well as rabbit PMN does not require O2 and appears to be BPI-mediated. However, when ingestion is limited, extracellular bacteria can also be killed but principally by O2-dependent mechanisms.

Killing of gram-negative bacteria by neutrophils: role of O2-independent system in intracellular killing and evidence of O2-dependent extracellular killing

For many strains of these two bacterial species, ingested bacteria are efficiently killed by O2-independent mechanisms. Intracellular killing depends not on 2de novo generation of toxic products of the respiratory burst but rather on intracellular delivery to pre-existing cytotoxic proteins. The principal O2- independent bactericidal system toward these bacteria appears to be BPI which rapidly binds to ingested bacteria and whose discrete action closely resembles the initial lesions produced by the intact neutrophil. In addition, at least for one strain of E. coli (S15), extracellular bacteria can be killed in an O2- dependent fashion.

Cationic antimicrobial proteins isolated from human neutrophil granulocytes in the presence of diisopropyl fluorophosphate

Acid (0.2 M sodium acetate, pH 4.0) extracts of granules recovered from disrupted human polymorphonuclear granulocytes (PMNs) exhibited in vitro antimicrobial activity against Salmonella typhimurium. To minimize proteolytic destruction or modification of antimicrobial proteins derived from these granules, we pretreated the PMNs with the serine protease inhibitor diisopropyl fluorophosphate. Fractionation of such extracts by carboxymethyl Sephadex and Sephadex G-75 chromatography resulted in the recovery of at least two antimicrobial, cationic proteins. These proteins differed substantially in antimicrobial activity, amino acid composition, and molecular weight (Mr, 37,000 and 57,000). As we have shown before (Shafer et al., Infect. Immun. 43:834-858), with unfractionated proteins, these two proteins exhibited diminished activity against a polymyxin B-resistant (PBr) mutant of S. typhimurium compared with their activity against the isogenic parental polymyxin B-sensitive (PBs) strain. Expression of the relevant mutation (prmA) in the PBr mutant decreases the electronegativity of lipid A, owing to increased 4-amino-4-deoxy-L-arabinosylation at the 4' phosphate residue (Vaara et al., FEBS Lett. 129:145-149). The data suggest that at least two different cationic proteins account for the antimicrobial capacity of extracts from human PMN granules. Moreover, the availability of anionic charges in the outer membrane of S. typhimurium due to free lipid A phosphates apparently dictates phenotypic levels of resistance to both of the cationic proteins extracted from human PMN granules.

Activation of polymorphonuclear leukocytes by salmonella

The interaction of polymorphonuclear leukocytes (PMN) with salmonella, as studied by chemiluminescence and phagocytosis, was very different for a number of clinical isolates. Particularly bacteria in the serogroups C1 and E4 deviated from other Salmonella. The differences were observed in the rate of activation, peak value, duration of the chemiluminescence, and in the extent of association and ingestion as studied microscopically. Old laboratory S-strains such as Salmonella typhimurium 395 MS and S. minnesota S99 , which did not associate with the PMN, showed little activation of the PMN, whereas their phagocytosis-sensitive R-mutants induced rapid activation, high peak values, and short duration of the chemiluminescence. Certain isolates belonging to the C1/E4 group induced intermediate types of reactions. The kinetics of the activation was related to the physicochemical surface properties of the bacteria. Heating the bacteria at 70 degrees C for 45 min enhanced the activation of PMN by the S-type strains conspicuously but in different ways, whereas that of R-mutants was delayed. Different clinical isolates of salmonella have shown different physico-chemical surfaces, liability to phagocytosis by PMNs and different degrees of eliciting inflammatory mediators from PMNs in vitro. The results indicate that the C1/E4 group of Salmonella has pathogenicity mechanisms different from most salmonellae.

Oxygen-independent killing of Bacteroides fragilis by granule extracts from human polymorphonuclear leukocytes

Granule proteins from human neutrophils were prepared by extraction with acetate, and their antibacterial activity against Bacteroides fragilis was determined. Activity was highly dependent on pH; greatest killing occurred at the most acid pH tested (pH 5.0). Optimum activity was observed at physiological ionic strength and low bacterial numbers. Killing was inhibited by incubation temperatures of less than 37 degrees C. Eight times more extract was required to kill 50% of stationary-phase bacteria, compared with those growing in logarithmic phase. The antibacterial effect of granule extract was destroyed by boiling, but some activity was retained after heating to 56 degrees C and 80 degrees C. Granule extract activity was tested under conditions in which oxygen-dependent antibacterial systems were inhibited. The rate and extent of killing was not affected by anaerobiosis, sodium azide, or cysteine hydrochloride. These results suggest that the activity of granule extract is independent of oxidative antibacterial systems, and therefore, under conditions that occur in anaerobic infections, potent leukocyte granule-associated mechanisms exist for the destruction of B. fragilis.

Nonoxidative antimicrobial reactions of leukocytes

Increasingly abundant evidence supports the hypothesis that PMNs and perhaps alveolar macrophages have antimicrobial mechanisms independent of the presences of molecular oxygen for effective action against an array of bacteria and against some fungi. Eosinophils have mechanisms toxic for schistosomula and Trichinella larvae. In all instances the antimicrobial substances isolated have been cationic proteins and, in PMNs, associated with the azurophil cytoplasmic granules of the PMNs. Several of these substances have thus far demonstrated no enzymic function. Two of these substances are serine proteases but in one, chymotrypsin-like protein, the antimicrobial action depends on the cationic properties of the protein and is independent of the proteolytic action of the substance. In most instances, these proteins are cationic due to relatively large proportions of arginine. In two instances, a large proportion of lysine is present. All have high proportions (about 50%) of hydrophobic amino acid. Such proteins occur in the PMNs of man, rabbit, guinea pig, rat, cow, and chicken. The present view is that they are most active against gram-negative bacteria. At least two of them-37-kd and 57-kd proteins (Shafer and Spitznagel, 1983)-act on S. typhimurium in a manner analogous to that of polymyxin B through binding to lipid A. Currently available results shows that anaerobic PMNs have substantial antimicrobial capacity. Whether this capacity is due to the O2-independent mechanisms discussed in this chapter remains to be established with greater certainty.

Identification of a sialic acid containing glycocalyx on the surface of Schistosoma mansoni

Ruthenium red fixation of adult Schistosoma mansoni revealed the existence of a negatively charged layer external to the outer bilayer, which was morphologically similar to the glycocalyx of other cell types. Regional and sexual differences were found in the extent and organisation of the surface coat, which can be correlated with interfacial free energy, adhesiveness and protection from immune effectors. Neuraminidase treatment confirmed the presence of surface sialic acid. Mechanical or skin penetrated schistosomula, maintained in vitro for 24 h were found not to have a glycocalyx and this may relate to their increased susceptibility to immune killing. Lung stage schistosomula however, did bind ruthenium red to their surface.

Bactericidal activity of a granule extract from human polymorphonuclear leukocytes against Bacteroides species

The microbicidal activity of an acetate extract of human polymorphonuclear leukocyte granules was tested against Bacteroides fragilis, Bacteroides vulgatus, Bacteroides distasonis and Bacteroides thetaiotaomicron. All strains tested were killed by the extract, and there were no significant differences between the different Bacteroides species.