Interaction of complement with serum-sensitive and serum-resistant strains of Pseudomonas aeruginosa

Subversion of the Complement System by Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen heavily implicated in chronic diseases. Immunocompromised patients that become infected with P. aeruginosa usually are afflicted with a lifelong chronic infection, leading to worsened patient outcomes. The complement system is an integral piece of the first line of defense against invading microorganisms. Gram-negative bacteria are thought to be generally susceptible to attack from complement; however, P. aeruginosa can be an exception, with certain strains being serum resistant. Various molecular mechanisms have been described that confer P. aeruginosa unique resistance to numerous aspects of the complement response. In this review, we summarize the current published literature regarding the interactions of P. aeruginosa and complement, as well as the mechanisms used by P. aeruginosa to exploit various complement deficiencies and the strategies used to disrupt or hijack normal complement activities.

Immune Response Modulation by Pseudomonas aeruginosa Persister Cells

Bacterial persister cells-a metabolically dormant subpopulation tolerant to antimicrobials-contribute to chronic infections and are thought to evade host immunity. In this work, we studied the ability of Pseudomonas aeruginosa persister cells to withstand host innate immunity. We found that persister cells resist MAC-mediated killing by the complement system despite being bound by complement protein C3b at levels similar to regular vegetative cells, in part due to reduced bound C5b, and are engulfed at a lower rate (10- to 100-fold), even following opsonization. Once engulfed, persister cells resist killing and, contrary to regular vegetative cells which induce a M1 favored (CD80⁺/CD86⁺/CD206^-, high levels of CXCL-8, IL-6, and TNF-α) macrophage polarization, they initially induce a M2 favored macrophage polarization (CD80⁺/CD86⁺/CD206⁺, high levels of IL-10, and intermediate levels of CXCL-8, IL-6, and TNF-α), which is skewed toward M1 favored polarization (high levels of CXCL-8 and IL-6, lower levels of IL-10) by 24 h of infection, once persister cells awaken. Overall, our findings further establish the ability of persister cells to evade the innate host response and to contribute chronic infections. IMPORTANCE Bacterial cells have a subpopulation-persister cells-that have a low metabolism. Persister cells survive antimicrobial treatment and can regrow to cause chronic and recurrent infections. Currently little is known as to whether the human immune system recognizes and responds to the presence of persister cells. In this work, we studied the ability of persister cells from Pseudomonas aeruginosa to resist the host defense system (innate immunity). We found that this subpopulation is recognized by the defense system, but it is not killed. The lack of killing likely stems from hindering the immune response regulation, resulting in a failure to distinguish whether a pathogen is present. Findings from this work increase the overall knowledge as to how chronic infections are resilient.

Pseudomonas aeruginosa and the Complement System: A Review of the Evasion Strategies

The increasing emergence of multidrug resistant isolates of P. aeruginosa causes major problems in hospitals worldwide. This concern is particularly significant in bloodstream infections that progress rapidly, with a high number of deaths within the first hours and without time to select the most appropriate treatment. In fact, despite improvements in antimicrobial therapy and hospital care, P. aeruginosa bacteremia remains fatal in about 30% of cases. The complement system is a main defensive mechanism in blood against this pathogen. This system can mark bacteria for phagocytosis or directly lyse it via the insertion of a membrane attack complex in the bacterial membrane. P. aeruginosa exploits different strategies to resist complement attack. In this review for the special issue on “bacterial pathogens associated with bacteriemia”, we present an overview of the interactions between P. aeruginosa and the complement components and strategies used by this pathogen to prevent recognition and killing by the complement system. A thorough understanding of these interactions will be critical in order to develop drugs to counteract bacterial evasion mechanisms.

Immune response modulation by Pseudomonas aeruginosa persister cells

Bacterial persister cells - a metabolically dormant subpopulation tolerant to antimicrobials - contribute to chronic infections and are thought to evade host immunity. In this work, we studied the ability of Pseudomonas aeruginosa persister cells to withstand host innate immunity. We found that persister cells resist MAC-mediated killing by the complement system despite being bound by complement protein C3b at levels similar to regular vegetative cells, in part due to reduced bound C5b - and are engulfed at a lower rate (10-100 fold), even following opsonization. Once engulfed, persister cells resist killing and, contrary to regular vegetative cells which induce a M1 favored (CD80+/CD86+/CD206-, high levels of CXCL-8, IL-6, and TNF-α) macrophage polarization, they initially induce a M2 favored macrophage polarization (CD80+/CD86+/CD206+, high levels of IL-10, and intermediate levels of CXCL-8, IL-6, and TNF-α), which is skewed towards M1 favored polarization (high levels of CXCL-8 and IL-6, lower levels of IL-10) by 24 hours of infection, once persister cells awaken. Overall, our findings further establish the ability of persister cells to evade the innate host response and to contribute chronic infections.

Molecular Analysis of the Contribution of Alkaline Protease A and Elastase B to the Virulence of Pseudomonas aeruginosa Bloodstream Infections

Pseudomonas aeruginosa is a major cause of nosocomial bloodstream infections. This microorganism secretes two major proteases, alkaline protease A (AprA) and elastase B (LasB). Despite several in vitro studies having demonstrated that both purified proteases cleave a number of components of the immune system, their contribution to P. aeruginosa bloodstream infections in vivo remains poorly investigated. In this study, we used a set of isogenic mutants deficient in AprA, LasB or both to demonstrate that these exoproteases are sufficient to cleave the complement component C3, either soluble or deposited on the bacteria. Nonetheless, exoprotease-deficient mutants were as virulent as the wild-type strain in a murine model of systemic infection, in Caenorhabditis elegans and in Galleria mellonella. Consistently, the effect of the exoproteases on the opsonization of P. aeruginosa by C3 became evident four hours after the initial interaction of the complement with the microorganism and was not crucial to survival in blood. These results indicate that exoproteases AprA and LasB, although conferring the capacity to cleave C3, are not essential for the virulence of P. aeruginosa bloodstream infections.

The lytic polysaccharide monooxygenase CbpD promotes Pseudomonas aeruginosa virulence in systemic infection

The recently discovered lytic polysaccharide monooxygenases (LPMOs), which cleave polysaccharides by oxidation, have been associated with bacterial virulence, but supporting functional data is scarce. Here we show that CbpD, the LPMO of Pseudomonas aeruginosa, is a chitin-oxidizing virulence factor that promotes survival of the bacterium in human blood. The catalytic activity of CbpD was promoted by azurin and pyocyanin, two redox-active virulence factors also secreted by P. aeruginosa. Homology modeling, molecular dynamics simulations, and small angle X-ray scattering indicated that CbpD is a monomeric tri-modular enzyme with flexible linkers. Deletion of cbpD rendered P. aeruginosa unable to establish a lethal systemic infection, associated with enhanced bacterial clearance in vivo. CbpD-dependent survival of the wild-type bacterium was not attributable to dampening of pro-inflammatory responses by CbpD ex vivo or in vivo. Rather, we found that CbpD attenuates the terminal complement cascade in human serum. Studies with an active site mutant of CbpD indicated that catalytic activity is crucial for virulence function. Finally, profiling of the bacterial and splenic proteomes showed that the lack of this single enzyme resulted in substantial re-organization of the bacterial and host proteomes. LPMOs similar to CbpD occur in other pathogens and may have similar immune evasive functions.

Insights into Host-Pathogen Interactions in Biofilm-Infected Wounds Reveal Possibilities for New Treatment Strategies

Normal wound healing occurs in three phases—the inflammatory, the proliferative, and the remodeling phase. Chronic wounds are, for unknown reasons, arrested in the inflammatory phase. Bacterial biofilms may cause chronicity by arresting healing in the inflammatory state by mechanisms not fully understood. Pseudomonas aeruginosa, a common wound pathogen with remarkable abilities in avoiding host defense and developing microbial resistance by biofilm formation, is detrimental to wound healing in clinical studies. The host response towards P. aeruginosa biofilm-infection in chronic wounds and impact on wound healing is discussed and compared to our own results in a chronic murine wound model. The impact of P. aeruginosa biofilms can be described by determining alterations in the inflammatory response, growth factor profile, and count of leukocytes in blood. P. aeruginosa biofilms are capable of reducing the host response to the infection, despite a continuously sustained inflammatory reaction and resulting local tissue damage. A recent observation of in vivo synergism between immunomodulatory and antimicrobial S100A8/A9 and ciprofloxacin suggests its possible future therapeutic potential.

The Role of Pseudomonas aeruginosa Lipopolysaccharide in Bacterial Pathogenesis and Physiology

The major constituent of the outer membrane of Gram-negative bacteria is lipopolysaccharide (LPS), which is comprised of lipid A, core oligosaccharide, and O antigen, which is a long polysaccharide chain extending into the extracellular environment. Due to the localization of LPS, it is a key molecule on the bacterial cell wall that is recognized by the host to deploy an immune defence in order to neutralize invading pathogens. However, LPS also promotes bacterial survival in a host environment by protecting the bacteria from these threats. This review explores the relationship between the different LPS glycoforms of the opportunistic pathogen Pseudomonas aeruginosa and the ability of this organism to cause persistent infections, especially in the genetic disease cystic fibrosis. We also discuss the role of LPS in facilitating biofilm formation, antibiotic resistance, and how LPS may be targeted by new antimicrobial therapies.

Immunization with outer membrane proteins (OprF and OprI) and flagellin B protects mice from pulmonary infection with mucoid and nonmucoid Pseudomonas aeruginosa

BACKGROUND

Pseudomonas aeruginosa is a Gram-negative opportunistic bacterium, which considered as a common cause of nosocomial infection and life-threatening complications in immunocompromized and cystic fibrosis patients. Here, we evaluate the protective effect of recombinant vaccines composed of outer membrane proteins OprF and OprI alone or in combination with flagellin B against mucoid and nonmucoid pseudomonas infection.

METHODS

BALB/C mice were immunized subcutaneous using OprF and OprI with or without flagellin B and antibody titers were determined. Serum bactericidal and opsonophagocytosis activities of immunized and control sera were estimated against mucoid and nonmucoid pseudomonas strains. Lung tissue sections from immunized and nonimmunized mice were analyzed and the levels of peripheral neutrophils infiltration into the lung and tissue inflammation were scored.

RESULTS

Subcutaneous immunization using OprF and OprI with or without flagellin B elicited higher antibody titers against OprF, OprI, and flagellin B. The produced antibodies successfully opsonized both mucoid and nonmucoid strains with subsequent activation of the terminal pathway of complement that enhances killing of nonmucoid strains via complement-mediated lysis. Furthermore, opsonized mucoid and nonmucoid strains showed enhanced opsonophagocytosis via human peripheral neutrophils, a mechanism that kills P. aeruginosa when complement mediated lysis is not effective especially with mucoid strains. Immunized mice also showed a significant prolonged survival time, lower bacteremia, and reduced lung damage when compared with control nonimmunized mice.

CONCLUSION

Our data showed that mice immunized with OprF/OprI or OprF/OprI and flagellin B are significantly protected from infection caused by mucoid and nonmucoid strains of P. aeruginosa.

Determinants for persistence of Pseudomonas aeruginosa in hospitals: interplay between resistance, virulence and biofilm formation

Pseudomonas aeruginosa (Pa) is one of the major bacterial pathogens causing nosocomial infections. During the past few decades, multidrug-resistant (MDR) and extensively drug-resistant (XDR) lineages of Pa have emerged in hospital settings with increasing numbers. However, it remains unclear which determinants of Pa facilitated this spread. A total of 211 clinical XDR and 38 susceptible clinical Pa isolates (nonXDR), as well as 47 environmental isolates (EI), were collected at the Heidelberg University Hospital. We used RAPD PCR to identify genetic clusters. Carriage of carbapenamases (CPM) and virulence genes were analyzed by PCR, biofilm formation capacity was assessed, in vitro fitness was evaluated using competitive growth assays, and interaction with the host's immune system was analyzed using serum killing and neutrophil killing assays. XDR isolates showed significantly elevated biofilm formation (p < 0.05) and higher competitive fitness compared to nonXDR and EI isolates. Thirty percent (62/205) of the XDR isolates carried a CPM. Similarities in distribution of virulence factors, as well as biofilm formation properties, between CPM+ Pa isolates and EI and between CPM- and nonXDR isolates were detected. Molecular typing revealed two distinct genetic clusters within the XDR population, which were characterized by even higher biofilm formation. In contrast, XDR isolates were more susceptible to the immune response than nonXDR isolates. Our study provides evidence that the ability to form biofilms is an outstanding determinant for persistence and endemic spread of Pa in the hospital setting.

Complement resistance mechanisms of Klebsiella pneumoniae

The current emergence of antibiotic-resistant bacteria causes major problems in hospitals worldwide. To survive within the host, bacterial pathogens exploit several escape mechanisms to prevent detection and killing by the immune system. As a major player in immune defense, the complement system recognizes and destroys bacteria via different effector mechanisms. The complement system can label bacteria for phagocytosis or directly kill Gram-negative bacteria via insertion of a pore-forming complex in the bacterial membrane. The multi-drug resistant pathogen Klebsiella pneumoniae exploits several mechanisms to resist complement. In this review, we present an overview of strategies used by K. pneumoniae to prevent recognition and killing by the complement system. Understanding these complement evasion strategies is crucial for the development of innovative strategies to combat K. pneumoniae.

GC-MS-Based Metabolome and Metabolite Regulation in Serum-Resistant Streptococcus agalactiae

Streptococcus agalactiae causes severe systemic infections in human and fish. In the present study, we established a pathogen-plasma interaction model by which we explored how S. agalactiae evaded serum-mediated killing. We found that S. agalactiae grew faster in the presence of yellow grouper plasma than in the absence of the plasma, indicating S. agalactiae evolved a way of evading the fish immune system. To determine the events underlying this phenotype, we applied GC-MS-based metabolomics approaches to identify differential metabolomes between S. agalactiae cultured with and without yellow grouper plasma. Through bioinformatics analysis, decreased malic acid and increased adenosine were identified as the most crucial metabolites that distinguish the two groups. Meanwhile, they presented with decreased TCA cycle and elevated purine metabolism, respectively. Finally, exogenous malic acid and adenosine were used to reprogram the plasma-resistant metabolome, leading to elevated and decreased susceptibility to the plasma, respectively. Therefore, our findings reveal for the first time that S. agalactiae utilizes a metabolic trick to respond to plasma killing as a result of serum resistance, which may be reverted or enhanced by exogenous malic acid and adenosine, respectively, suggesting that the metabolic trick can be regulated by metabolites.

Pseudomonas aeruginosa Uses Dihydrolipoamide Dehydrogenase (Lpd) to Bind to the Human Terminal Pathway Regulators Vitronectin and Clusterin to Inhibit Terminal Pathway Complement Attack

The opportunistic human pathogen Pseudomonas aeruginosa controls host innate immune and complement attack. Here we identify Dihydrolipoamide dehydrogenase (Lpd), a 57 kDa moonlighting protein, as the first P. aeruginosa protein that binds the two human terminal pathway inhibitors vitronectin and clusterin. Both human regulators when bound to the bacterium inhibited effector function of the terminal complement, blocked C5b-9 deposition and protected the bacterium from complement damage. P. aeruginosa when challenged with complement active human serum depleted from vitronectin was severely damaged and bacterial survival was reduced by over 50%. Similarly, when in human serum clusterin was blocked by a mAb, bacterial survival was reduced by 44%. Thus, demonstrating that Pseudomonas benefits from attachment of each human regulator and controls complement attack. The Lpd binding site in vitronectin was localized to the C-terminal region, i.e. to residues 354-363. Thus, Lpd of P. aeruginosa is a surface exposed moonlighting protein that binds two human terminal pathway inhibitors, vitronectin and clusterin and each human inhibitor when attached protected the bacterial pathogen from the action of the terminal complement pathway. Our results showed insights into the important function of Lpd as a complement regulator binding protein that might play an important role in virulence of P. aeruginosa.

Pseudomonas aeruginosa and the host pulmonary immune response

Pseudomonas aeruginosa is a highly adaptable, opportunistic pathogen that is commonly found in the environment. It can infect a number of sites in the body and disseminate. It can cause both acute and chronic pulmonary infection and the acuity of infection and accompanying inflammatory phenotype is determined, for the most part, by the host. Although P. aeruginosa has been a successful opportunist in the context of a number of different disease states, it has been best studied in the context of cystic fibrosis (CF). The adaptability of P. aeruginosa has enabled it to adjust quickly to the CF airway, transitioning from initial colonization to chronic infection. The organism quickly expresses virulence factors that allow it to circumvent some elements of the host immune response and, even more importantly, quickly develops antimicrobial resistance. In the case of CF, chronic infection resulting in progressive lung damage, coupled with antimicrobial resistance, becomes an increasingly important issue as individuals with CF live longer. It is for these reasons that both organism- and host-targeted immunotherapies are being increasingly explored.

The role of complement in gonococcal infection of cervical epithelia

Neisseria gonorrhoeae is an exclusive human pathogen that causes the sexually transmitted disease, gonorrhea. The gonococcus has developed an exquisite repertoire of mechanisms by which it is able to evade host innate and adaptive immune responses. Our previous data indicate that the predominately asymptomatic nature ofgonococcal cervicitis may, in part, be attributed to the ability of these bacteria to subvert the normal function of complement to promote cervical disease. Herein we describe the interaction of N. gonorrhoeae with the complement alternative pathway with a particular focus on the importance of this interaction in promoting gonococcal cervicitis.

Immune evasion of the human pathogen Pseudomonas aeruginosa: elongation factor Tuf is a factor H and plasminogen binding protein

Pseudomonas aeruginosa is an opportunistic human pathogen that can cause a wide range of clinical symptoms and infections that are frequent in immunocompromised patients. In this study, we show that P. aeruginosa evades human complement attack by binding the human plasma regulators Factor H and Factor H-related protein-1 (FHR-1) to its surface. Factor H binds to intact bacteria via two sites that are located within short consensus repeat (SCR) domains 6-7 and 19-20, and FHR-1 binds within SCR domain 3-5. A P. aeruginosa Factor H binding protein was isolated using a Factor H affinity matrix, and was identified by mass spectrometry as the elongation factor Tuf. Factor H uses the same domains for binding to recombinant Tuf and to intact bacteria. Factor H bound to recombinant Tuf displayed cofactor activity for degradation of C3b. Similarly Factor H bound to intact P. aeruginosa showed complement regulatory activity and mediated C3b degradation. This acquired complement control was rather effective and acted in concert with endogenous proteases. Immunolocalization identified Tuf as a surface protein of P. aeruginosa. Tuf also bound plasminogen, and Tuf-bound plasminogen was converted by urokinase plasminogen activator to active plasmin. Thus, at the bacterial surface Tuf acts as a virulence factor and binds the human complement regulator Factor H and plasminogen. Acquisition of host effector proteins to the surface of the pathogen allows complement control and may facilitate tissue invasion.

Complement evasion of pathogens: common strategies are shared by diverse organisms

Infectious diseases represent a major health problem. Based on the limited efficacy of existing drugs and vaccines and the increasing antibiotic resistance new strategies are needed to fight infectious diseases. A better understanding of pathogen-host interaction is one important aspect to identify new virulence factors and antimicrobial and anti-inflammatory compounds utilized by pathogens represent an additional source for effective anti-inflammatory compounds. Complement forms a major defense line against invading microbes, and pathogens have learned during evolution to breach this defense line. The characterization of how pathogens evade complement attack is a rapidly developing field of current research. Pathogens mimic host surfaces and bind host complement regulators. Similarly pathogens utilize a number of complement inhibitory molecules which help to evade complement attack and which display anti-inflammatory activity. The molecular identification of these molecules, as well as the functional characterization of their roles at the pathogen-host interface is an important and emerging field of infection biology. In addition, pathogens utilize multiple sets of such regulators as redundancy and multiplicity is important for immune and complement evasion. Here we summarize the current scenarios of this emerging field which identifies multiple virulence factors and complement evasion strategies, but which at the same time reveals common mechanisms for immune and complement defense.

Antibodies against tumor cell glycolipids and proteins, but not mucins, mediate complement-dependent cytotoxicity

One of several effector mechanisms thought to contribute to Ab efficacy against cancer is complement-dependent cytotoxicity (CDC). Serological analysis of a series of clinical trials conducted over a 10-year period suggested that six vaccines containing different glycolipids induced Abs mediating CDC whereas four vaccines containing carbohydrate or peptide epitopes carried almost exclusively by mucin molecules induced Abs that did not mediate CDC. To explore this further, we have now compared cell surface reactivity using flow cytometry assays (FACS), complement-fixing ability, and CDC activity of a panel of mAbs and immune sera from these trials on the same two tumor cell lines. Abs against glycolipids GM2, globo H and Lewis Y, protein KSA (epithelial cell adhesion molecule, also known as EpCAM) and mucin Ags Tn, sialylated Tn, Thomsen Friedenreich (TF), and MUC1 all reacted comparably by FACS with tumor cells expressing these Ags. Compared with the strong complement binding and CDC with Abs against glycolipids and KSA, complement binding was diminished with Abs against mucin Ags and no CDC was detected. A major difference between these two groups of Ags is proximity to the cell membrane. Glycolipids and globular glycoproteins extend less than 100 A from the cell membrane while mucins extend up to 5000 A. Although complement activation at sites remote from the cell membrane has long been known as a mechanism for resistance from complement lysis in bacteria, it is identified here for the first time as a factor which may contribute to resistance from CDC against cancer cells.

Murine complement interactions with Pseudomonas aeruginosa and their consequences during pneumonia

Complement is necessary for defense against lung infection with Pseudomonas aeruginosa in mice. We studied in vitro interactions between complement and P. aeruginosa and in vivo effects of complement depletion to better understand this relationship. In vitro, P. aeruginosa strain UI-18 was resistant to killing by mouse serum. However, C3 opsonized the organism (via the alternative and mannose binding lectin [MBL] pathways), and C5 convertase activity on the bacterial surface was demonstrated. In vivo, compared with normal mice, complement-deficient mice experienced higher mortality and failed to sterilize their bronchoalveolar space within 24 h of inoculation. These changes did not seem to be a result of decreased inflammation because complement-deficient mice had normal neutrophil recruitment, greater lung myeloperoxidase content, and, by 24 h, a 35-fold higher level of the CXC chemokine KC. Lung static pressure-volume curves were abnormal in infected animals but were significantly more so in complement deficient mice. These data indicate that although P. aeruginosa is resistant to serum killing, C3 opsonization and C5 convertase assembly occur on its surface. This interaction in vivo plays a central role in host survival beyond just recruitment and activation of phagocytes and may serve to limit the inflammatory response to and tissue injury resulting from bacterial infection.

The role of lipooligosaccharide in Neisseria gonorrhoeae pathogenesis of cervical epithelia: lipid A serves as a C3 acceptor molecule

The use of primary, human, ecto- and endocervical epithelial cell cultures has increased our understanding of the pathogenesis of gonococcal infection in women. Primary cervical epithelial cells express complement (C') receptor type 3 (CR3) and C' proteins required for alternative pathway (AP) activity. Gonococcus -induced membrane ruffling and cellular invasion of primary cervical epithelia is mediated by CR3 and requires co-operative CR3 binding by gonococcus-bound iC3b, porin and pilus. We have extended these studies to identify the site of C3 deposition upon gonococci within the cervical microenvironment. By immunoprecipitation and ELISA we demonstrate that covalent and non-covalent associations occurred between gonococcal LOS and C' protein C3. Sialylation or LOS truncation did not alter the gonococcus-CR3 interaction. By Western blot analysis we observed comparable C3 opsonization patterns among a panel of LOS truncation mutants, sialylated wild-type gonococci, or wild-type bacteria that were not sialylated. Quantitative association/invasion assays performed in the presence or absence of LOS competimers support C3b deposition on the lipid A core structure. Our findings demonstrate a role for lipid A as a C3 acceptor site and suggest that multiple factors govern C3b deposition and its subsequent conversion to iC3b on the surface of the gonococcus within the cervical microenvironment.

Antibacterial properties of Pseudomonas aeruginosa immunotype 1 lipopolysaccharide-specific monoclonal antibody (MAb) in a murine thigh infection model: combined effects of MAb and ceftazidime

A murine monoclonal antibody (MAb) specific for the Pseudomonas aeruginosa immunotype 1 (It-1) lipopolysaccharide (LPS) O-side chain was evaluated in terms of its in vitro bactericidal opsonophagocytic activity and in vivo bacterial killing in a mouse thigh infection model. An immunoglobulin (Ig) G2a MAb Ld3-2F2, specific for It-1 LPS, mediated in vitro complement-dependent opsonophagocytic killing at a concentration of 10 microg/ml. MAb-mediated, complement-dependent killing also occurred in the absence of neutrophils at serum concentrations in excess of 20%. A remarkable synergy was observed in opsonophagocytic assays between MAb Ld3-2F2 (0.5 microg/ml) and ceftazidime (1/4 MIC). The administration of MAb Ld3-2F2 at a level of 1 microg resulted in a significant decrease in the number of bacteria in the thigh muscles of normal mice, while 100 microg of the same MAb was required for one log of reduction in the number of bacteria at the same site in neutropenic mice. The combined therapy with MAb Ld3-2F2 and ceftazidime provided a significant reduction in the density of bacteria in the thigh muscle at 9 hr post-infection in normal and neutropenic mice as compared with those after treatment alone or with no treatment (P< 0.01). These favorable in vitro and in vivo interactions of an LPS-specific IgG MAb and ceftazidime strongly support their potential for use in therapy, combined with an LPS-reactive MAb and parenteral antipseudomonas beta-lactam antibiotics in the therapy of systemic Pseudomonas infections in normal and neutropenic hosts.

Survival of avian strains of Pasteurella multocida in chicken serum

The ability of bacteria to survive in serum is considered a likely virulence determinant in diseases where the infective bacteria become septicaemic. Optimal conditions were established to test the survival of Pasteurella multocida in chicken serum. Serum was used at 90%, the inoculum was 10(3)-10(4)cfu in phosphate buffered saline pH 7.4. Survival was measured after incubation for 2-4 h; if survival was <50% the strain was considered serum susceptible. Susceptible strains were either killed or their growth was inhibited. Some resistant strains not only survived but grew rapidly in unheated serum. Thirty-five strains, all originally isolated from clinical fowl cholera, were tested; eight were susceptible, of which three were killed and five inhibited, and the remainder (27) were resistant. Ten serum-resistant P. multocida serogroup A strains were grown in hyaluronidase to remove the capsule and survival in chicken serum was re-tested. Three strains became susceptible, while seven strains remained resistant. Three serum susceptible strains were then tested in the presence of cytidine monophosphate-N-acetylneuraminic acid (CMP-NANA). This substance is present in the human serum, and is known to mask the effect of complement on Neisseria gonorrhoeae rendering susceptible strains resistant. Two of the three serum susceptible strains became resistant in the presence of CMP-NANA. Serum susceptibility/resistance was more complex than that of Escherichia coli, and the role of resistance to avian complement in the pathogenesis of fowl cholera remains to be determined.

Nitrite reductase from Pseudomonas aeruginosa released by antimicrobial agents and complement induces interleukin-8 production in bronchial epithelial cells

We have recently reported that nitrite reductase, a bifunctional enzyme located in the periplasmic space of Pseudomonas aeruginosa, could induce interleukin-8 (IL-8) generation in a variety of respiratory cells, including bronchial epithelial cells (K. Oishi et al. Infect. Immun. 65:2648-2655, 1997). In this report, we examined the mode of nitrite reductase (PNR) release from a serum-sensitive strain of live P. aeruginosa cells during in vitro treatment with four different antimicrobial agents or human complement. Bacterial killing of P. aeruginosa by antimicrobial agents induced PNR release and mediated IL-8 production in human bronchial epithelial (BET-1A) cells. Among these agents, imipenem demonstrated rapid killing of P. aeruginosa as well as rapid release of PNR and resulted in the highest IL-8 production. Complement-mediated killing of P. aeruginosa was also associated with PNR release and enhanced IL-8 production. The immunoprecipitates of the aliquots of bacterial culture containing imipenem or complement with anti-PNR immunoglobulin G (IgG) induced twofold-higher IL-8 production than did the immunoprecipitates of the aliquots of bacterial culture with a control IgG. These pieces of evidence confirmed that PNR released in the aliquots of bacterial culture was responsible for IL-8 production in the BET-1A cells. Furthermore, the culture supernatants of the BET-1A cells stimulated with aliquots of bacterial culture containing antimicrobial agents or complement similarly mediated neutrophil migration in vitro. These data support the possibility that a potent inducer of IL-8, PNR, could be released from P. aeruginosa after exposure to antimicrobial agents or complement and contributes to neutrophil migration in the airways during bronchopulmonary infections with P. aeruginosa.

Burkholderia pseudomallei activates complement and is ingested but not killed by polymorphonuclear leukocytes

The mechanism by which Burkholderia pseudomallei is resistant to lysis by human serum is unknown but may include interference with complement activation, effective opsonization, or complement-mediated lysis. We investigated the interaction of B. pseudomallei with complement in the presence and absence of specific antibody to determine potential mechanisms of serum resistance. We demonstrated rapid activation and consumption of complement by B. pseudomallei which, in the absence of specific antibody, occurred predominantly via the alternative pathway. Complement activation was associated with deposition of the opsonically active C3b and iC3b fragments on the bacterial surface. C5b-9, detected on the bacterial surface after opsonic periods of 1 to 60 min, was susceptible to elution by 1 M NaCl, indicating that resistance to complement-mediated lysis may result from deposition of the membrane attack complex in a nonmicrobicidal location. To define the role of opsonins, we investigated the ability of polymorphonuclear leukocytes (PMNL) to phagocytose B. pseudomallei. Phagocytosis of bacteria by PMNL, and the observed oxidative response, was significantly increased by opsonization of organisms with complement and/or specific antibody. Despite opsonophagocytosis by PMNL and the production of an oxidative response, no significant bacterial killing was observed.

Concordance of endotoxemia with gram-negative bacteremia in patients with gram-negative sepsis: a meta-analysis

The Limulus amebocyte lysate (LAL) assay is a sensitive method for detecting endotoxin. Using gram-negative (GN) bacteremia as the basis for comparison, concordance with endotoxemia in 45 studies could be expressed as an odds ratio. Calculation of summary odds ratios by the Mantel-Haenszel-Peto method indicated that the concordance of the results was no higher by the chromogenic LAL assay than by the gelation version, and the sensitivity was improved by only 11% (62 versus 51%). Endotoxemia was detected in 77 (68%) of 114 patients with bacteremia caused by an organism that was not a member of the family Enterobacteriaceae, whereas endotoxemia was detected in only 120 (45%) of 269 patients with bacteremia caused by a member of the family Enterobacteriaceae or an anaerobe (P < 0.001). This difference was also apparent for patients with GN bacteremia for whom a fatal outcome had been recorded. The prevalence of GN bacteremia in the tested population and the type of etiological agent are critical and previously unrecognized variables which affect the interpretation of the LAL test in patients with suspected sepsis.

Effects of the combination of lipopolysaccharide-specific monoclonal antibodies and sparfloxacin against Pseudomonas aeruginosa pneumonia in neutropenic mice

The effects of the combination of a murine monoclonal antibody (MAb) specific for the O side chain of Pseudomonas aeruginosa Fisher immunotype 1 lipopolysaccharide and sparfloxacin in a neutropenic mouse model of P. aeruginosa pneumonia were examined. Under the condition that neither MAb at a dose of 500 micrograms per mouse administered intravenously nor a suboptimal dose of oral sparfloxacin (5 mg/kg of body weight) protected mice from challenge with a fatal dose, the combination therapy with MAb and sparfloxacin caused a significant increase in the survival rate (P less than 0.001 compared with either treatment alone). The effect of the combination was closely correlated to bacterial killing in plasma and lung tissue of infected mice. In vitro, a significant MAb-dependent, complement-mediated killing of P. aeruginosa was documented in the presence of sparfloxacin at one-half the MIC, while the killing was not observed in the absence of sparfloxacin. These in vivo and in vitro data suggest the usefulness of combination therapy with a lipopolysaccharide-reactive immunoglobulin G MAb and sparfloxacin in neutropenic patients with P. aeruginosa pneumonia.

Pseudomonas aeruginosa variants isolated from patients with cystic fibrosis are killed by a bactericidal protein from human polymorphonuclear leukocytes

The susceptibility of paired mucoid and nonmucoid variants of Pseudomonas aeruginosa isolated from 13 patients with cystic fibrosis (CF) to killing by a 55,000-Da bactericidal protein (BP55) from human polymorphonuclear leukocytes was studied. Mucoid and nonmucoid variants were equally sensitive to killing by BP55 at both pH 5.6 and pH 7.2. Eleven of the isolates were resistant to the bactericidal activity of 10% normal human serum but were as sensitive as the serum-sensitive isolates to BP55. Similarly, the 15 isolates with lipopolysaccharides (LPS) containing O-polysaccharide side chains (smooth LPS) were as sensitive to BP55 as those isolates with rough LPS.P. aeruginosa isolates from patients in poor clinical condition were more likely to have LPS of the smooth type and to be resistant to killing by 10% human serum than the isolates from patients in good clinical condition. We have concluded that the susceptibility of the P. aeruginosa isolates from patients with CF to killing by BP55 does not correlate with mucoid or nonmucoid variations, with the presence or absence of smooth LPS, or with the sensitivity or resistance to killing by normal human serum.

A protective human monoclonal antibody directed to the outer core region of Pseudomonas aeruginosa lipopolysaccharide

The protective activity against experimental Pseudomonas aeruginosa infection of a human monoclonal antibody, MH-4H7, which is thought to recognize L-rhamnose and its neighboring residues in the outer core region of P. aeruginosa lipopolysaccharide and which binds to strains of Homma serotypes A, F, G, H, K, and M, was studied in normal, burned, and leukopenic mice. MH-4H7 at doses of 0.1 to 1.0 micrograms per mouse (5 to 50 micrograms/kg) was effective against serotype A, F, G, H, and K clinical isolates of P. aeruginosa tested in normal mice but not against strains of serotype M, B, E, or I. The 50% protective doses were calculated to be 0.01 and 0.1 micrograms per mouse against challenge with serotype G strains and 3 to 8 micrograms per mouse against challenge with serotype A strains. MH-4H7 promoted macrophage-mediated opsonophagocytosis of serotype A, F, G, H, and K strains but not of serotype M strains. The opsonophagocytic activity, expressed as the reduction rate of viable bacteria in the presence of MH-4H7, macrophages, and complement, was higher against serotype G strains (more than 90%) than against serotype A strains (60 to 80%) and serotype F, H, and K strains (50 to 86%). It was correlated with the protective activity but not with the binding intensity of MH-4H7 to the organisms. In addition, burned and leukopenic mice as well as normal mice infected with serotype G strains recovered from a very low dosage of MH-4H7. Thus, a monoclonal antibody directed to the outer core region of P. aeruginosa lipopolysaccharide was effective against infection with a wide range of O-serotype strains of P. aeruginosa.

Pseudomonas aeruginosa alginate in cystic fibrosis sputum and the inflammatory response

Alginate, a viscous polysaccharide from mucoid Pseudomonas aeruginosa, may interfere with the host defenses in patients with cystic fibrosis and chronic P. aeruginosa lung infection. The alginate concentration in the sol phase of expectorated sputum was quantitated by a biochemical method and a newly developed enzyme-linked immunosorbent assay. There was a high degree of correlation between the methods, and the concentration of alginate ranged from 4 to 101 micrograms/ml with a median of 35.5 micrograms/ml when measured by enzyme-linked immunosorbent assay. Alginate could not be detected in the bronchial secretions from patients without P. aeruginosa infection. In vitro investigation of alginate did not show any activation of the alternative pathway of complement, as determined by a hemolytic kinetic assay and by testing for neutrophil chemotaxis. At a high concentration, P. aeruginosa alginate caused a slight activation of the classical pathway of complement. Alginate did not cause neutrophil chemotaxis by itself but was able to reduce the neutrophil chemotactic response to N-formylmethionylleucylphenylalanine and for zymosan-activated serum. P. aeruginosa and seaweed alginates were able to prime neutrophils for increased N-formylmethionylleucylphenylalanine-induced neutrophil oxidative burst, as determined by chemiluminescence. Because of its ability to prevent attraction of neutrophils to the site of infection, lack of complement activation, and ability to enhance neutrophil oxidative burst, alginate from P. aeruginosa may contribute to the persistence and pathogenesis of chronic P. aeruginosa infection in cystic fibrosis.

Complement activation and C3 binding by serum-sensitive and serum-resistant strains of Pseudomonas aeruginosa

The relationship among complement consumption, C3 deposition, and C3 fragmentation pattern was compared for serum-sensitive (Sers) and serum-resistant (Serr) strains of Pseudomonas aeruginosa. The Sers strains, which were mucoid strains derived from patients with cystic fibrosis, had lipopolysaccharide deficient in O-antigen side chains. These organisms generally activated much less complement per organism than their Serr counterparts, characterized by the presence of lipopolysaccharide with long lipopolysaccharide O side chains. Surprisingly, however, although the Serr strains consumed more total hemolytic complement, less C3 was deposited onto the surface of these strains than onto that of the Sers strains. Maximal C3 binding required the participation of both the classical and alternative complement pathways, although classical complement pathway involvement was more important for Serr strains. Finally, while more than half of the C3 deposited on most Sers strains was in the form of C3b, most of the C3 on the Serr strains was in the form of iC3b, indicating a more rapid and extensive conversion of C3b to iC3b on the surface of these strains. Limited complement activation by Sers mucoid strains of P. aeruginosa may confer a selective survival advantage to these organisms in colonizing the airways of patients with cystic fibrosis.

Determination of serum bactericidal activity against Escherichia coli by an automated photometric method

The resistance of gram-negative bacteria to complement-mediated serum activity is supposedly an important virulence factor. However, the lack of standardization in the methods used to determine serum activity and the many definitions applied make the comparisons between studies very difficult. We developed a rapid photometric method that we compared with a classical killing one. Escherichia coli in the exponential phase of growth in brain heart infusion broth (final inoculum, 10(7) CFU/ml) at 35 degrees C was added to 50% human serum in Veronal buffer. Viable counts and automatic recording of the variations in the optical densities were obtained for 40 E. coli strains isolated from the stools of healthy adults. With the viable count method, 17 (42.5%) were susceptible (at least a 1 log CFU/ml decrease), 17 (42.5%) were resistant (a 0.6 log CFU/ml increase), 4 (10%) were intermediate (poorly growing inoculum or a decrease of less than 1 log CFU/ml), and 2 could not be classified (nonreproducible results). Agreement between both methods was observed for 87.5% of the stool strains. Eight reference strains of known susceptibilities were classified identically by both methods, leading to a final concordance rate of 89.6%. A total of 129 blood isolates were tested by the photometric method: 64 (49.6%) were resistant, 50 (38.8%) were susceptible 5 (3.9%) showed early regrowth, and 10 (7.7%) were not perfectly reproducible. Of these 129 blood isolates, 5 were also tested by the killing method: 37 (49%) were resistant, 32 (43%) were susceptible, and 6 (8%) were intermediate. The concordance rate between both assays was 89% for the blood isolates; when the minor discordances were ruled out, it was 97%. This automated method could be a useful screening tool for detecting resistance to serum in clinical trials and for studying the in vitro variations of this property.

Protective role of magnesium in the neutralization by antibodies of Chlamydia trachomatis infectivity

Neutralization of the infectivity of Chlamydia trachomatis was assessed by using polyclonal antisera and monoclonal antibodies (MAbs). Polyclonal antisera and a species-reactive MAb as well as a subspecies-specific MAb, both of which were directed toward the major outer membrane protein of C. trachomatis, reduced the number of chlamydial inclusion-forming units in an in vitro assay. Neutralization was dependent on the presence of complement. The species-specific MAb reacted with all 15 serovars by a microimmunofluorescence assay and a dot blot enzyme-linked immunosorbent assay with heat-treated elementary bodies. On the other hand, this same MAb reacted with all serovars, except those in the C complex, by the dot blot enzyme-linked immunosorbent assay with viable organisms and neutralized in vitro all 10 serovars tested, except those in the C complex. When neutralization assays were performed in a solution containing Mg2+, neutralization by both polyclonal antisera and MAbs was significantly reduced. A dose response to Mg2+ supplied as MgSO4 revealed that all concentrations tested from 50 to 800 microM had some effect. Concentrations of greater than or equal to 400 microM MgSO4 completely abolished neutralization at the lowest dilution of polyclonal antisera and species-reactive MAb tested. Although Mg2+ also blocked the neutralization effect of the subspecies-specific MAb, this neutralization was not as complete as that observed with the species-reactive MAb. Addition of Mg2+ to the assay over the initial 45 min of incubation of C. trachomatis with MAb and complement showed that the organisms could be rescued to some extent over the first 30 min of incubation, after which time neutralization of infectivity could not be reversed. C. trachomatis treated with Mg2+, the species-reactive MAb, and complement were lethal to mice in an in vivo toxicity and infectivity assay, whereas mice injected with organisms incubated with the same MAb and complement without Mg2+ survived.

Characterization of the susceptibility of Pseudomonas aeruginosa to complement-mediated killing: role of antibodies to the rough lipopolysaccharide on serum-sensitive strains

The mechanism of complement-mediated killing of seven serum-sensitive Pseudomonas aeruginosa strains was examined. All seven strains were sensitive to the bactericidal activity of 20% pooled normal human serum (PNHS) containing magnesium EGTA, which blocks the classical complement pathway (CCP), or 20% PNHS preheated to 50 degrees C for 20 min, which inactivates the alternative complement pathway, suggesting that either pathway was effective against these strains. However, for four of these strains, optimal killing required the function of both pathways. Preabsorption of PNHS with serum-sensitive strains dramatically reduced the killing activity of serum for the homologous strains when a concentration of 10% serum was used, implying a role for antibody in the activation of complement via the CCP. Affinity purification of antibodies to the rough lipopolysaccharide (LPS) on strain 144M resulted in a pool of antibodies which could restore all of the bactericidal activity and most of the C3 activation-deposition activity of serum which had been lost by preabsorption with 144M. Confirmation that the LPS was the target for these bactericidal antibodies was provided by demonstrating that exogenously added 144M LPS inhibited the killing activity of PNHS. These anti-144M LPS-specific antibodies were also bactericidal for the six other serum-sensitive strains examined, suggesting that all seven strains shared an antigenic determinant recognized by these anti-144M LPS-specific antibodies. Results from cross-absorption studies imply that there are bactericidal antibodies in PNHS directed to additional bacterial targets. These studies suggest that part of the bactericidal activity of PNHS is due to binding of antibodies to the rough LPS on serum-sensitive strains, initiating activation of the CCP, and that all seven strains examined shared this bactericidal antibody-binding site.

Role of immunoglobulin G in killing of Borrelia burgdorferi by the classical complement pathway

The antibody and complement requirements for killing of Borrelia burgdorferi 297 by normal human serum (NHS) and NHS plus immunoglobulin G (IgG) were examined. B. burgdorferi activated both the alternative and classical complement pathways in NHS. In NHS chelated with 10 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid plus 4 mM MgCl2 (Mg-EGTA) to block classical pathway activation, consumption (activation) of total hemolytic complement, complement component 3 (C3), and C9 by B. burgdorferi was observed. Furthermore, challenge of unchelated NHS with 297 cells resulted in the consumption of C4, in addition to an increase in C3 and C9 consumption over that observed in chelated serum. In spite of complement activation, B. burgdorferi was resistant to the nonspecific bactericidal activity of NHS. The addition of human anti-B. burgdorferi IgG to NHS, however, resulted in the complete killing of 297 cells. Bactericidal activity of this serum was abrogated if NHS was immunochemically depleted of C1, indicating that killing was mediated by the classical pathway. The manifestation of bactericidal activity was accompanied by a large increase in total complement and C3 consumption over that observed in NHS alone. Under similar conditions, only a minimal increase in C9 consumption was observed. No increase in total complement consumption was observed if NHS plus anti-B. burgdorferi IgG was treated with Mg-EGTA prior to challenge. The results of these experiments demonstrate that B. burgdorferi is resistant to the nonspecific bactericidal activity of NHS, in spite of classical and alternative complement pathway activation. B. burgdorferi is sensitive to serum, however, in the presence of IgG, which mediates bacterial killing through the classical complement pathway.