Membrane changes induced by exposure of Escherichia coli to human serum

Genome-Wide Identification by Transposon Insertion Sequencing of Escherichia coli K1 Genes Essential for In Vitro Growth, Gastrointestinal Colonizing Capacity, and Survival in Serum

Escherichia coli K1 strains are major causative agents of invasive disease of newborn infants. The age dependency of infection can be reproduced in neonatal rats. Colonization of the small intestine following oral administration of K1 bacteria leads rapidly to invasion of the blood circulation; bacteria that avoid capture by the mesenteric lymphatic system and evade antibacterial mechanisms in the blood may disseminate to cause organ-specific infections such as meningitis. Some E. coli K1 surface constituents, in particular the polysialic acid capsule, are known to contribute to invasive potential, but a comprehensive picture of the factors that determine the fully virulent phenotype has not emerged so far. We constructed a library and constituent sublibraries of ∼775,000 Tn5 transposon mutants of E. coli K1 strain A192PP and employed transposon-directed insertion site sequencing (TraDIS) to identify genes required for fitness for infection of 2-day-old rats. Transposon insertions were lacking in 357 genes following recovery on selective agar; these genes were considered essential for growth in nutrient-replete medium. Colonization of the midsection of the small intestine was facilitated by 167 E. coli K1 gene products. Restricted bacterial translocation across epithelial barriers precluded TraDIS analysis of gut-to-blood and blood-to-brain transits; 97 genes were required for survival in human serum. This study revealed that a large number of bacterial genes, many of which were not previously associated with systemic E. coli K1 infection, are required to realize full invasive potential.

IMPORTANCEEscherichia coli K1 strains cause life-threatening infections in newborn infants. They are acquired from the mother at birth and colonize the small intestine, from where they invade the blood and central nervous system. It is difficult to obtain information from acutely ill patients that sheds light on physiological and bacterial factors determining invasive disease. Key aspects of naturally occurring age-dependent human infection can be reproduced in neonatal rats. Here, we employ transposon-directed insertion site sequencing to identify genes essential for the in vitro growth of E. coli K1 and genes that contribute to the colonization of susceptible rats. The presence of bottlenecks to invasion of the blood and cerebrospinal compartments precluded insertion site sequencing analysis, but we identified genes for survival in serum.

Bacteria under stress by complement and coagulation

The complement and coagulation systems are two related protein cascades in plasma that serve important roles in host defense and hemostasis, respectively. Complement activation on bacteria supports cellular immune responses and leads to direct killing of bacteria via assembly of the Membrane Attack Complex (MAC). Recent studies have indicated that the coagulation system also contributes to mammalian innate defense since coagulation factors can entrap bacteria inside clots and generate small antibacterial peptides. In this review, we will provide detailed insights into the molecular interplay between these protein cascades and bacteria. We take a closer look at how these pathways are activated on bacterial surfaces and discuss the mechanisms by which they directly cause stress to bacterial cells. The poorly understood mechanism for bacterial killing by the MAC will be reevaluated in light of recent structural insights. Finally, we highlight the strategies used by pathogenic bacteria to modulate these protein networks. Overall, these insights will contribute to a better understanding of the host defense roles of complement and coagulation against bacteria.

Killing of Gram-negative bacteria with normal human serum and normal bovine serum: use of lysozyme and complement proteins in the death of Salmonella strains O48

Serum is an environment in which bacterial cells should not exist. The serum complement system provides innate defense against microbial infections. It consists of at least 35 proteins, mostly in pre-activated enzymatic forms. The activation of complement is achieved through three major pathways: the classical, alternative, and lectin. Lysozyme, widely present in body fluids, catalyzes the hydrolysis of beta 1,4 linkage between N-acetyloglucosamine and N-acetylmuramic acid in the bacterial cell wall and cooperates with the complement system in the bactericidal action of serum. In this study, ten strains of serotype O48 Salmonella, mainly associated with warm-blooded vertebrates and clinically important causing diarrhea in infants and children, were tested. The results demonstrated that the most efficient killing of Salmonella O48 occurred when all the components of normal bovine serum (NBS) and normal human serum (NHS) cooperated. To prove the role of lysozyme in the bactericidal activity of bovine and human serum, the method of serum adsorption onto bentonite (montmorillonite, MMT) was used. In order to investigate structural transitions accompanying the adsorption of serum components, we applied X-ray diffraction methods. The results of this investigation suggested that apart from lysozyme, other proteins (as, e.g., C3 protein or IgG immunoglobulin) were adsorbed on MMT particles. It was also shown that Ca(2+) cations can be adsorbed on bentonite. This may explain the different sensitivities of the serovars belonging to the same O48 Salmonella serotype to NBS and NHS devoid of lysozyme.

Inherited complement deficiencies and bacterial infections

A wide variety of bacteria are recognized by the complement system through the early components that trigger the three pathways of complement activation, leading to the release of biologically active products involved in opsonization, recruitment of phagocytes and bacterial killing. Deficiencies of complement components and regulators provide a model to understand the in vivo role of complement as a defense system against bacterial infections. An increased susceptibility to these types of infections is frequently seen in individuals with C2, C3, late component, properdin and factor I deficiencies. The identification of these deficiencies is essential for the adoption of preventive measures aimed to reduce the risk of bacterial infections. Vaccination represents the treatment of choice to protect these subjects, although further studies on a large number of C-deficient individuals are needed to prove the protective effect of vaccines.

The Vibrio cholerae ToxR-regulated porin OmpU confers resistance to antimicrobial peptides

BPI (bactericidal/permeability-increasing) is a potent antimicrobial protein that was recently reported to be expressed as a surface protein on human gastrointestinal tract epithelial cells. In this study, we investigated the resistance of Vibrio cholerae, a small-bowel pathogen that causes cholera, to a BPI-derived peptide, P2. Unlike in Escherichia coli and Salmonella enterica serovar Typhimurium, resistance to P2 in V. cholerae was not dependent on the BipA GTPase. Instead, we found that ToxR, the master regulator of V. cholerae pathogenicity, controlled resistance to P2 by regulating the production of the outer membrane protein OmpU. Both toxR and ompU mutants were at least 100-fold more sensitive to P2 than were wild-type cells. OmpU also conferred resistance to polymyxin B sulfate, suggesting that this porin may impart resistance to cationic antibacterial proteins via a common mechanism. Studies of stationary-phase cells revealed that the ToxR-repressed porin OmpT may also contribute to P2 resistance. Finally, although the mechanism of porin-mediated resistance to antimicrobial peptides remains elusive, our data suggest that the BPI peptide sensitivity of OmpU-deficient V. cholerae is not attributable to a generally defective outer membrane.

Two-component systems in Haemophilus influenzae: a regulatory role for ArcA in serum resistance

Knockout mutations were constructed in the arcA gene of a virulent type b strain of Haemophilus influenzae, and the behavior of the resulting mutants was investigated in a number of conditions that mimicked distinct steps in the natural infection pathway. In arcA mutants, synthesis of capsule and lipooligosaccharide (LOS) and growth in synthetic media were unaltered compared to synthesis of capsule and LOS and growth in synthetic media in the wild-type H. influenzae type b parent strain. However, the virulence of the arcA mutants for BALB/c mice was significantly reduced. Upon exposure to human blood or serum, the arcA mutants showed markedly reduced survival compared with the survival of its wild-type parent. Serum resistance could be fully restored by complementation in cis with the H. influenzae arcA gene but not by complementation in cis with the homologous gene from Escherichia coli. The proteomes of wild-type and mutant bacteria were markedly different, especially under anaerobic conditions, underscoring the global regulatory role of ArcAB in H. influenzae. Evaluation of antibody titers and classical complement activities in various serum samples pointed to complement-mediated bactericidal activity as the factor that distinguishes between the arcA mutant and wild-type phenotypes. Comparative analysis of the membrane fractions of the arcA mutants and the wild-type strain revealed several ArcA-regulated proteins, some of which may be implicated in the serum hypersensitivity phenotype.

Deletion of wboA enhances activation of the lectin pathway of complement in Brucella abortus and Brucella melitensis

Brucella spp. are gram-negative intracellular pathogens that survive and multiply within phagocytic cells of their hosts. Smooth organisms present O polysaccharides (OPS) on their surface. These OPS help the bacteria avoid the bactericidal action of serum. The wboA gene, coding for the enzyme glycosyltransferase, is essential for the synthesis of O chain in Brucella. In this study, the sensitivity to serum of smooth, virulent Brucella melitensis 16M and B. abortus 2308, rough wboA mutants VTRM1, RA1, and WRR51 derived from these two Brucella species, and the B. abortus vaccine strain RB51 was assayed using normal nonimmune human serum (NHS). The deposition of complement components and mannose-binding lectin (MBL) on the bacterial surface was detected by flow cytometry. Rough B. abortus mutants were more sensitive to the bactericidal action of NHS than were rough B. melitensis mutants. Complement components were deposited on smooth strains at a slower rate compared to rough strains. Deposition of iC3b and C5b-9 and bacterial killing occurred when bacteria were treated with C1q-depleted, but not with C2-depleted serum or NHS in the presence of Mg-EGTA. These results indicate that (i) OPS-deficient strains derived from B. melitensis 16M are more resistant to the bactericidal action of NHS than OPS-deficient strains derived from B. abortus 2308, (ii) both the classical and the MBL-mediated pathways are involved in complement deposition and complement-mediated killing of Brucella, and (iii) the alternative pathway is not activated by smooth or rough brucellae.

Complement evasion by the Lyme disease spirochete Borrelia burgdorferi grown in host-derived tissue co-cultures: role of fibronectin in complement-resistance

The effectiveness of complement-mediated killing of Borrelia burgdorferi, the causative agent of Lyme disease, in the presence of host-derived tissues was studied. Second and high passage forms of B. burgdorferi 297 isolate were grown in a LEW/N rat joint tissue co-culture system and in artificial BSK medium. Guinea pig complement and third week immune serum from hamsters with experimental Lyme disease were added to the cultures. Both high and low passage borrelia grown in BSK medium died and did not revive after 3 weeks incubation in BSK medium. However, 5-12% of tissue co-cultured borrelia survived the first complement-mediated lysis. Repeated re-growth and lysis cycles in tissue co-culture resulted in isolation of an 85% complement-resistant population of B. burgdorferi. Joint tissue culture supernatant collected on the third day of tissue culture, and fibronectin (25 micrograms/ml), also protected spirochetes from complement-mediated lysis in contrast to BSK or fresh co-culture medium. Complement-mediated lysis may not be an effective mechanism in eradication of borrelia, and the chronicity of Lyme disease may be due to resistance of B. burgdorferi variants to host immune defense mechanisms in the presence of host-derived tissues.

C9-mediated killing of bacterial cells by transferred C5b-8 complexes: transferred C5b-9 complexes are nonbactericidal

The formation of the C5b-9 complex on the outer membrane of complement-sensitive cells of Escherichia coli results in inhibition of inner membrane function and the death of the cell. Cells bearing a precursor of the C5b-9 site, the C5b-8 complex, suffer no loss in viability. Antibiotic-sensitive, complement-sensitive donor cells bearing precursor C5b-8 complexes were incubated with equal numbers of antibiotic-resistant, complement-sensitive acceptor cells that had not been exposed to a complement source. This cell mixture was incubated with 5 mM EDTA for 5 min and then with calcium chloride (20 mM) for various times. The excess calcium ion concentration was effectively reduced with additional EDTA, and the cell mixture was washed and resuspended in buffer. The viability of the acceptor cells was assayed by plating on antibiotic-containing media. C9 was added to the mixture, and the mixture was incubated for 10 min at 37 degrees C and then plated as described above. It was found that the acceptor cells were killed by the addition of purified C9 only after incubation with donor cells bearing C5b-8 sites during the transfer procedure. This indicates that precursor C5b-8 sites that support C9-mediated killing could be transferred between cells. No loss in viability was detected for acceptor cells subjected to the procedure described above in the presence of donor cells bearing complete C5b-9 complexes, formed prior to mixing with acceptor cells for the transfer procedure.

Sub-minimal inhibitory concentrations of cefmetazole enhance serum bactericidal activity in vitro by amplifying poly-C9 deposition

Serum-resistant organisms grown in sub-minimal inhibitory concentrations (subMICs) of antibiotics in vitro may be rendered sensitive to complement-mediated, serum bactericidal activity. We measured 125I-C3 and 125I-C9 deposition on genetically serum resistant Salmonella montevideo SH5770 (SH5770) that was rendered serum sensitive by growth in sub-MICs of cefmetazole (CMZ), a parenteral, second generation, cephamycin-group antibiotic. Three times as much C3 and over six times as much C9 bound to SH5770 grown in one-fourth the MIC of CMZ compared to broth-grown bacteria. SDS-PAGE analysis and autoradiography showed that neither the ratio of C3b:iC3b (approximately 1:2.5) nor the nature of the C3-bacterial bond was changed by growing the organisms in CMZ. Large amounts of complement membrane attack complexes containing poly-C9 were seen only on CMZ-grown SH5770 by SDS-PAGE and autoradiography. Poly-C9 was also detected only on CMZ-grown bacteria by indirect immunofluorescence and ELISA using a murine monoclonal antibody directed against a neoantigen of poly-C9. Bacterial hydrophobicity increased after growth in CMZ, and transmission electron micrographs of CMZ-grown SH5770 showed cell wall disruption and blebbing. These results indicate that growth in subMICs of CMZ increases bacterial hydrophobic domains available for interacting with the membrane attack complex, C5b-9, allowing formation and stable insertion of bactericidal complexes containing poly-C9.

N-linked oligosaccharides of human transferrin are not required for binding to bacterial transferrin receptors

Derivatives of human transferrin (hTf) with removed or modified N-linked oligosaccharides were compared with native hTf with respect to their binding to bacterial hTf receptors from Neisseria meningitidis, N. gonorrhoeae, and Haemophilus influenzae. Partially and fully deglycosylated hTf were prepared by enzymatic deglycosylation with glycopeptidase F and isolated by concanavalin A-Sepharose affinity chromatography. Oligosaccharide-modified hTf was prepared via mild periodate oxidation. Competition and direct binding experiments with the hTf derivatives demonstrated that the hTf oligosaccharides are not essential for binding to the bacterial hTf receptors.

Transfer of preformed terminal C5b-9 complement complexes into the outer membrane of viable gram-negative bacteria: effect on viability and integrity

An efficient fusion system between Gram-negative bacteria and liposomes incorporating detergent-extracted C5b-9 complexes has been developed that allows delivery of preformed terminal complexes to the cell envelope (Tomlinson et al., 1989b). Fusion of Salmonella minnesota Re595 and Escherichia coli 17 with C5b-9-incorporated liposomes resulted in the transfer of 1900 C5b-9 complexes to each target bacterial cell. No loss in viability of bacteria was observed following fusion, even though the deposotion of 900 complexes onto the envelope following exposure to lysozyme-free serum effected a greater than 99% loss of viability. Increased sensitivity to antibiotics normally excluded from the cell by an integral outer membrane (OM), as well as the ability of the chromogenic substrate PADAC to gain access to periplasmically located beta-lactamase, indicated that transferred C5b-9 complexes functioned as water-filled channels through the OM. A similar conclusion was drawn from measurements demonstrating the uptake by cells of the lipophilic cation tetraphenylphosphonium (bromide), a result further indicating that the membrane potential across the cytoplasmic membrane was maintained following C5b-9 transfer to the OM. Examination of S. minnesota Re595 by electron microscopy revealed no obvious difference between cells exposed to lethal concentrations of lysozyme-free serum and cells following fusion with C5b-9-incorporated liposomes. These data suggest either that there are critical sites in the OM to which liposome-delivered C5b-9 complexes are unable to gain access or that bacterial cell death is related to events occurring during polymerization of C9 on the cell surface.

Transfer of phospholipid and protein into the envelope of gram-negative bacteria by liposome fusion

A liposome-bacterial fusion system was developed in order to introduce preformed terminal complement complexes, C5b-9, into the outer membrane of Gram-negative bacteria. Liposomes were prepared from a total phospholipid extract of Salmonella minnesota Re595. Fusion between liposomes and Salmonella sp. or Escherichia coli 17 was dependent on time, temperature, pH, and Ca2+ and PO4- concentration. Only Salmonella sp. with attenuated LPS core regions were able to fuse efficiently with liposomes. It was demonstrated that fusion of liposomes with S. minnesota Re595 or E. coli 17 under optimum conditions resulted in (i) quantitative transfer of the self-quenching fluorescent membrane probe octadecyl rhodamine B chloride from the liposomal bilayer to the bacterial envelope, (ii) transfer of radiolabeled liposomal phospholipid to the bacterial outer membrane and its subsequent translocation to the cytoplasmic membrane, demonstrated by isolation of the bacterial membranes following fusion, and (iii) delivery of liposome-entrapped horseradish peroxidase (HRP) to the periplasmic space, confirmed by a chemiluminescent assay. Following fusion of liposomes incorporating C5b-9 complexes with S. minnesota Re595 or E. coli 17, immunological analysis of the isolated membranes revealed C5b-9 complexes located exclusively in the outer membrane.

Killing of gram-negative bacteria by complement. Fractionation of cell membranes after complement C5b-9 deposition on to the surface of Salmonella minnesota Re595

The effect of C5b-9 deposition on the envelope of target Gram-negative bacteria was studied. In order to understand the changes occurring after complement deposition on the bacterial surface, the preparation of Gram-negative bacterial membranes by different methods involving the osmotic lysis of spheroplasts was investigated. Subsequent fractionation of the outer membrane (OM) and cytoplasmic membrane (CM) by sucrose-density-gradient centrifugation showed differences in the membrane profiles obtained. The results indicate that optimum separation of OM and CM components requires effective digestion of DNA in the total membrane preparation before density-gradient fractionation. Salmonella minnesota Re595 carrying the intermediate complement complex C5b-7 (BC1-7) or C5b-8 (BC1-8) were efficiently killed upon incubation with purified C8 + C9 or C9 respectively. Human-alpha-thrombin-cleaved C9 (C9n), which is unable to form tubular poly(C9), was shown to be more effective at killing than native C9. By using an optimized system for the separation of OM and CM, it was found that, subsequent to lethal complement attack, the CM could not be recovered when C9 was used as the terminal complement component, but was recovered with reduced yield when C9n replaced C9. The results show that inability to recover the CM on sucrose density gradients after complement attack may not be a consequence of an essential membrane damage event required for complement-mediated killing of Gram-negative bacteria.

The cytolytic C5b-9 complement complex: feedback inhibition of complement activation

We describe a regulatory function of the terminal cytolytic C5b-9 complex [C5b-9(m)] of human complement. Purified C5b-9(m) complexes isolated from target membranes, whether in solution or bound to liposomes, inhibited lysis of sensitized sheep erythrocytes by whole human serum in a dose-dependent manner. C9 was not required for the inhibitory function since C5b-7 and C5b-8 complexes isolated from membranes were also effective. No effect was found with the cytolytically inactive, fluid-phase SC5b-9 complex. However, tryptic modification of SC5b-9 conferred an inhibitory capacity to the complex, due probably to partial removal of the S protein. Experiments using purified components demonstrated that C5b-9(m) exerts a regulatory effect on the formation of the classical- and alternative-pathway C3 convertases and on the utilization of C5 by cell-bound C5 convertases. C5b-9(m) complexes were unable to inhibit the lysis of cells bearing C5b-7(m) by C8 and C9. Addition of C5b-9(m) to whole human serum abolished its bactericidal effect on the serum-sensitive Escherichia coli K-12 strain W 3110 and suppressed its hemolytic function on antibody-sensitized, autologous erythrocytes. Feedback inhibition by C5b-9(m) represents a biologically relevant mechanism through which complement may autoregulate its effector functions.

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.

Interaction of Escherichia coli and macrophages: alteration by treatment of bacteria with beta-lactam antibiotics

Antibiotics are known to exert an influence on the host-parasite relationship either by impairment of immunocompetent cells or by alteration of the bacterium, such as changes of surface properties or the production of toxins. The main problem in investigating the effect of antibiotics on the surface properties of bacteria consists in morphological changes of bacteria (round cell or filament formation) after treatment e.g. with beta-lactam antibiotics. These changes of morphology lead to problems in the comparison of such bacterial forms with untreated organisms. Therefore, in this study outer membrane vesicles from bacteria were used as a model to investigate the effect of antibiotics on the surface properties of Escherichia coli with regard to the interaction with mouse peritoneal macrophages tested by chemiluminescence reaction. It could be shown that these membrane vesicles induce a luminol dependent chemiluminescence response. Treatment of E. coli with different beta-lactams lead to an increase of the stimulating properties. The relative effectiveness of certain antibiotics depended on the particular E. coli strain. Analysis of the different adhesions involved in the stimulation of macrophages revealed that only mannose-sensitive adhesins were increased after treatment with beta-lactam antibiotics. No stimulation of the membrane-bound NAD(P)H-oxidase could be found following the reaction with outer membrane vesicles. Even the treatment of bacteria with antibiotics did not evoke such a reaction.

Influence of beta-lactam antibiotics and ciprofloxacin on cell envelope of Escherichia coli

The effects of subinhibitory concentrations of different beta-lactam antibiotics and one quinolone on the quantitative composition of the outer membrane (OM) of two strains of Escherichia coli, on lipid translocation into the OM, and on the production of capsular K1 polysaccharide were studied. The phospholipid/amino acid ratio was reduced in almost all OM preparations from antibiotic-treated bacteria. In one strain, antibiotic treatment increased the lipopolysaccharide/amino acid ratio. The amount of peptidoglycan fragments bound to the OM was increased by all the antibiotics. In pulse-chase experiments with a radioactive lipid precursor, ciprofloxacin, imipenem, and aztreonam inhibited phospholipid translocation into the OM. Furthermore, imipenem, cephaloridine, and ciprofloxacin induced a pronounced reduction of the production of capsular K1 polysaccharide. Thus, antibiotics seem to induce marked changes of the quantitative composition of the cell envelope of E. coli. Possible connections of these data with findings on the influence of antibiotics on functional parameters of the host-parasite relationship such as OM immunogenicity and serum resistance are discussed.

Sensitive ELISA for quantitating the terminal membrane C5b-9 and fluid-phase SC5b-9 complex of human complement

Activation of the complement system to completion results either in the generation of a pore-forming, cytolytic C5b-9(m) complex, or of a cytolytically inactive, fluid-phase SC5b-9 complex. In this paper, we describe a sensitive and reliable, sandwich ELISA for C5b-9(m) and SC5b-9, which is based on the use of a monoclonal antibody to a neoantigen of C5b-9 in combination with affinity-purified, polyclonal rabbit antibodies. The ELISA has been calibrated with purified C5b-9(m) and SC5b-9, and can detect 3 ng/ml C5b-9(m) and 20 ng/ml SC5b-9. We show that maximal conversion of C5-C9 in pooled human serum by insulin or zymosan activation generates 220 +/- 40 micrograms/ml SC5b-9. 65 of 100 normal human EDTA plasma samples analyzed in this study contained 100-600 ng/ml SC5b-9, corresponding to 0.04-0.24% of maximal conversion. Levels of circulating SC5b-9 in other donors were below the limit of detection. Incubation of serum at 37 degrees C always led to spontaneous generation of SC5b-9; concentrations ranged from 490-4725 ng/ml after 60 min, 37 degrees C, with a mean of 1848 +/- 1031 (SD) ng/ml amongst 25 donors studied. The terminal complement complex present in EDTA plasma was partially purified by PEG precipitation, DEAE-ion exchange chromatography and sucrose density gradient centrifugation, and was found to contain C8, C9 and S-protein as demonstrable by SDS-PAGE immunoblotting. Thus, the material most probably represented genuine SC5b-9. No significant age- or sex-dependent variations in SC5b-9 levels were noted. The present data call for a critical re-appraisal of several previously published methods for the determination of SC5b-9 levels in human plasma and serum.

Formation of transmural complement pores in serum-sensitive Escherichia coli

The binding of C9 at 0 and 37 degrees C to viable Escherichia coli K-12 cells carrying C5b-8 complexes was quantified. At low temperature, limited average binding of only 1 to 1.4 molecules of C9 per C8 molecule occurred, whereas 6 to 8 C9 molecules were bound per C8 molecule at 37 degrees C. Despite incorporation of C9 into C5b-9 complexes at 0 degrees C, these terminal complexes caused no loss of bacterial viability even when present in very large numbers (1,000 to 1,500 per CFU) on the bacterial cells. In contrast, generation of 50 to 100 C5b-9 complexes carrying multiple C9 molecules per CFU caused loss of viability. The failure of C5b-81C91 complexes to generate transmural pores was confirmed by measurements of o-nitrophenyl-beta-D-galactoside influx into the cells. Whereas treatment of C5b-8-laden cells with C9 at 32 degrees C caused virtually instantaneous influx of the marker, almost no influx was registered in cells receiving C9 at 0 degrees C. When cells carrying C5b-7 were brought into the stationary phase and given C8 and C9 at 32 degrees C, a C9-dependent disruption of the outer membrane permeability barrier immediately occurred as demonstrated by cleavage of a chromogenic substrate by periplasmic beta-lactamase. In sharp contrast, o-nitrophenyl-beta-D-galactoside influx was markedly retarded over a prolonged period, with abrupt permeability increases of the inner membrane toward this molecule being noted just before bacterial cell division occurred. We conclude that killing of E. coli requires binding of C5b-9 complexes containing C9 oligomers to the outer membrane and suggest that formation of pores in the inner membrane occurs when these complexes are "hit" by transiently forming zones of bioadhesion. Formation of the latter may be a dynamic process that is accentuated during cell division and quiescent during the stationary phase.

Influence of beta-lactam antibiotics and ciprofloxacin on composition and immunogenicity of Escherichia coli outer membrane

The effects of subinhibitory concentrations of different beta-lactam antibiotics and one quinolone on the sedimentation of outer membranes (OMs) of Escherichia coli and on the qualitative properties and immunogenicity of OM components were studied. Membranes were prepared by osmotic lysis of plasmolyzed bacteria. OM and cytoplasmic membrane vesicles were separated by sucrose density ultracentrifugation. Two peaks of OM vesicles with different buoyant densities could be isolated; the quantitative contribution of these to the total OM varied, depending upon the growth phase. In early log phase, the OM consisted mainly of lighter material; in late log and stationary phases, the OM consisted mainly of heavier material. Moxalactam, imipenem, and ciprofloxacin inhibited the formation of heavier material in all growth phases. The immunogenicity of OM vesicles was tested in mice by the hemolytic plaque test. The lighter OM material was markedly less immunogenic than the heavier OM material. The vesicles from antibiotic-treated bacteria and those from early-log-phase cells were less immunogenic than vesicles from untreated late-log-phase and stationary-phase bacteria. These changes were found for the immune response against lipopolysaccharides, as well as against OM proteins. Thus, the immunogenicity of OM components seems to be dependent upon the quantitative composition of lighter and heavier compounds, which is strongly influenced by growth phase and treatment with certain antibiotics.

Does complement kill E. coli by producing transmural pores?

Three lines of evidence are presented to indicate that C5b-9 kills serum-sensitive E. coli K 12 cells by generating functional pores across the outer and inner bacterial membrane. First, viable cells carrying C5b-8 complexes are impermeable to o-nitrophenyl-beta-D-galactoside (ONPG), but lose viability and become permeable to this marker upon post-treatment with purified C9 in the absence of lysozyme. Cells killed with colicin E1 or gentamicin are also impermeable to ONPG but take up the marker if they are post-treated with lysozyme-free serum. Second, killing by C5b-9 is highly effective, deposition of only a small number of complexes being lethal. This has been demonstrated in experiments where viable cells carrying 2000-4000 C5b-7 complexes per CFU were permitted to multiply in broth culture, and the daughter generations subsequently treated with purified C8 and C9. Fifty percent killing was observed in the fifth to sixth generation, corresponding to a dilution of C5b-7 complexes to 50-100 molecules/CFU. In the presence of 2 mM EDTA, further dilution of C5b-7 down to 8-30 complexes/CFU still caused 50% killing of daughter cells. Third, treatment of C5b-7 cells with purified CC8 and C9 results in the release of intracellular K+, which commences immediately after addition of C8/C9. This was shown in experiments where C5b-7 cells were packed to high density in saline, post-treated with C8 + C9, and K+ directly measured in the cell supernatants. Based on these results, we propose that C5b-9 pores deposited in the outer bacterial membrane periodically fuse with the inner membrane, the transmural pores thus generated permitting rapid K+ efflux, with cell death ensuing through the collapse of membrane potential.

Multimeric complement component C9 is necessary for killing of Escherichia coli J5 by terminal attack complex C5b-9

We studied the molecular composition of the complement C5b-9 complex required for optimal killing of Escherichia coli strain J5. J5 cells were incubated in 3.3%, 6.6%, or 10.0% C8-deficient serum previously absorbed to remove specific antibody and lysozyme. This resulted in the stable deposition after washing of 310, 560, and 890 C5b67 molecules per colony-forming unit, respectively, as determined by binding of 125I-labeled C7. Organisms were then incubated with excess C8 and various amounts of 131I-labeled C9. Plots of the logarithm (base 10) of E. coli J5 cells killed (log kill) vs. C9 input were sigmoidal, confirming the multihit nature of the lethal process. When C9 was supplied in excess, 3300, 5700, and 9600 molecules of C9 were bound per organism for cells bearing 310, 560, and 890 C5b-8 complexes, respectively, leading to C9-to-C7 ratios of 11.0:1, 10.8:1, and 11.4:1 and to log kill values of 1.3, 2.1, and 3.9. However, at low inputs of C9 that lead to C9-to-C7 ratios of less than 3.3:1, no killing occurred, and this was independent of the number of C5b-9 complexes bound. Formation of multimeric C9 at C9-to-C7 ratios permissive for killing was confirmed by electron microscopy and by binding of 125I-labeled antibody with specificity for multimeric but not monomeric C9. These experiments are the first to demonstrate a biological function for C9 polymerization and suggest that multimeric C9 is necessary for optimal killing of E. coli J5 cells by C5b-9.

Stable insertion of C5b-9 complement complexes into the outer membrane of serum treated, susceptible Escherichia coli cells as a prerequisite for killing

Escherichia coli 17, a K12 derivative, was rapidly killed by human serum following a short lag period of 10 min. Stable binding of terminal C5b-9 complement complexes was investigated in time course experiments. Serum treated E. coli cells were lysed osmotically and the resulting outer and cytoplasmic membrane vesicles separated by sucrose gradient centrifugation. Exposure of E. coli 17 to serum rapidly reduced the degree of recoverability of cytoplasmic membrane vesicles. Electron microscopy revealed no interaction of C5b-9 complexes with CM vesicles. In contrast there was a clear time-dependent deposition of terminal complement complexes onto OM-vesicles. Very few complexes were detected during the prekilling phase of the reaction; initiation of the active killing phase was accompanied by a large increase in complement lesions. In contrast, no C5b-9 complexes could be visualised on outer or cytoplasmic membrane vesicles of a smooth, serum-resistant E. coli strain. We conclude that complement-mediated killing is a consequence of stable binding of C5b-9 complexes to the outer membrane of susceptible strains.

Phenotypic modification of the reactivity in human serum of Escherichia coli K1 isolates

Three Escherichia coli strains producing the K1 antigen and shown to be resistant to the complement-mediated bactericidal action of human serum when grown in batch culture, were cultivated in the chemostat under conditions of carbon-, nitrogen-, magnesium- and phosphate-limitation. All strains were fully serum resistant when grown under carbon-limiting conditions but became phenotypically serum sensitive when limited by magnesium. One strain, belonging to serogroup O7:K1, also displayed serum sensitivity when nitrogen limited and showed an intermediate serum response when phosphate was used as the limiting nutrient.