Protective role of smooth lipopolysaccharide in the serum bactericidal reaction

More than a Pore: Nonlytic Antimicrobial Functions of Complement and Bacterial Strategies for Evasion

The complement system is an evolutionarily ancient defense mechanism against foreign substances. Consisting of three proteolytic activation pathways, complement converges on a common effector cascade terminating in the formation of a lytic pore on the target surface. The classical and lectin pathways are initiated by pattern recognition molecules binding to specific ligands, while the alternative pathway is constitutively active at low levels in circulation. Complement-mediated killing is essential for defense against many Gram-negative bacterial pathogens, and genetic deficiencies in complement can render individuals highly susceptible to infection, for example, invasive meningococcal disease. In contrast, Gram-positive bacteria are inherently resistant to the direct bactericidal activity of complement due to their thick layer of cell wall peptidoglycan. However, complement also serves diverse roles in immune defense against all bacteria by flagging them for opsonization and killing by professional phagocytes, synergizing with neutrophils, modulating inflammatory responses, regulating T cell development, and cross talk with coagulation cascades. In this review, we discuss newly appreciated roles for complement beyond direct membrane lysis, incorporate nonlytic roles of complement into immunological paradigms of host-pathogen interactions, and identify bacterial strategies for complement evasion.

Studies on serum resistance in Escherichia coli

Serum-sensitive mutants and their serum-resistant smooth parental E. coli strains (Wf8, Wf26, and WF 52) have been investigated in respect to their binding of different complement components. These pairs consisting of a wild-type and its mutants represent a better model for the investigation of the mechanism of serum resistance than the comparison of unrelated strains. Both strains of a pair bind equivalent amounts of C3. In binding assays using radiolabeled terminal components C6, C7, C8, and C9, the serum-sensitive strains do bind more late acting components than their resistant parental strains. An active membrane attack complex stably bound to the cell surface was found on the mutants, whereas with wild-type bacteria a complex could be isolated from the supernatant which is composed of the late acting complement components and S-protein. This complex is released from the surface of the wild-type bacteria without participation of C9.

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.

Serum sensitivity of strains isolated and antibodies against O antigen in gram-negative bacteraemia

The sensitivity of blood culture isolates to the bactericidal activity of normal human serum (NHS) has been studied in 101 patients with gram-negative sepsis. These results were compared with clinical status and outcome, and to the presence of specific IgG or IgM antibodies to O antigens of bacteraemic strains in autologous serum. 23% of the strains were markedly resistant, 27% markedly sensitive and 50% intermediately sensitive to the bactericidal activity of NHS. Shock or death occurred more frequently in immunocompromised patients and those infected with serum-resistant strains. IgG antibody titres to O antigens were significantly lower in patients with serum-resistant organisms regardless of their immune status. Resistance to natural bactericidal antibodies and low immunogenicity of the infecting organism, plus immunodeficiency in the host, may account for apparent increased virulence of some gram-negative bacilli.

Studies on the mechanism of bacterial resistance to complement-mediated killing. I. Terminal complement components are deposited and released from Salmonella minnesota S218 without causing bacterial death

The mechanism of resistance of gram-negative bacteria to killing by complement was investigated. Complement consumption and uptake of purified, radiolabeled complement components on bacteria was studied using a serum- sensitive and a serum-resistant strain of Salmonella minnesota. Twice as many molecules of (125)I C3 were bound per colony-forming unit (CFU) of the smooth, serum-resistant S. minnesota S218 as were bound per CFU of the rough, serum-sensitive S. minnesota Re595 in 10 percent pooled normal human serum (PNHS), although 75-80 percent of C3 was consumed by both organisms. Hemolytic titrations documented total consumption of C9 by 5 min and more than 95 percent consumption of C5 and C7 by 15 min in the reaction with S218 with 10 percent PNHS. In contrast, negligible C5 depletion, 10 percent C7 consumption, and only a 26 percent decrease in C9 titer occurred with the serum-sensitive Re595. Binding of (125)I C5, (125)I C7, and (125)I C9 to S218 and Re595 was measured in 10 percent PNHS. A total of 6,600 molecules C5/CFU, 5,200 molecules C7/CFU, and 3,100 molecules C9/CFU bound to S218 after 5-10 min of incubation at 37 degrees C, but 50-70 percent of the C5, C7, and C9 bound to S218 was released from the organism during incubation at 37 degrees C for 60 min. Binding of 2,000 molecules C5/CFU, 1,900 molecules C7/CFU, and 9,000 molecules C9/CFU to Re595 was achieved by 20 min and was stable. The ratio of bound C9 molecules to bound C7 molecules, measured using (131)I C9 and (125)I C7, was constant for both organisms after 15 min and was 4.3:1 on Re595 and 0.65:1 on S218 in 10 percent PNHS. With addition of increasing amounts of purified, unlabeled (29 to 10 percent PNHS, there was no change in the C9:C7 ratio on Re595. However, with S218 there was a linear increase of the C9:C7 ratio, which approached the ratio on Re595. There was no (14)C release from S218 incubated in PNHS, nor was there evidence by electron microscopy of outer membrane damage to S218. Therefore, S. minnesota S218 is resistant to killing by PNHS, despite the fact that the organism consumes terminal complement components efficiently and that terminal components are deposited on the surface in significant amounts. The C5b-9 complex is released from the surface of S218 without causing lethal outer membrane damage.

Use of chemotaxis chambers for studying in vitro bacterial colonization of biomaterials

Blind-well chemotaxis chambers were used to study the in vitro bacterial adhesion and colonization of biomaterials. Staphylococcus aureus and Pseudomonas aeruginosa were selected for the bacterial inocula. Abundant growth on the surfaces of methyl methacrylate, polyethylene, stainless steel, and Vitallium was detected by using scanning electron microscopy after 24 h of incubation. The culture technique employed proved to be of value for the study of surface bacterial colonization of inert materials.

The role of bacterial surface structures in pathogenesis

Modern research has revealed that the true surfaces of animal cells consist of polysaccharide chains that are linked to proteins hydrophobically anchored in the membrane and protrude to form a dense glycocalyx. It has become increasingly clear that most pathogenic bacteria must position themselves at the surface of their "target" cell in order to exert their toxic or otherwise deleterious effects. The true surface of most pathogenic bacteria has also been recently shown to consist of a protruding mass of polysaccharide chains--the bacterial glycocalyx--that is composed of teichoic acids in many gram-positive species and of acid polysaccharides in many gram-negative organisms. Through this bacterial glycocalyx certain cell surface proteins and organized protein structures (e.g., pili) are known to project, so that the bacterial surface is a mosaic of polysaccharides and proteins; both of these types of molecules have been implicated in instances of specific pathogenic adhesion. Besides their role in specific adhesion to target cells, these surface components interpose a highly charged, and often very extensive, barrier that can prevent the penetration of antibodies and antibiotics to their target sites in the bacterial cell. They may also frustrate mucociliary clearance, phagocytosis, and other clearance mechanisms of the host. We will discuss the chemical and physical nature of these bacterial surface components that mediate pathogenic adhesion and counteract host defense mechanisms sufficiently to allow infections to become established.

Resistance of gram-negative bacteria to purified bactericidal leukocyte proteins: relation to binding and bacterial lipopolysaccharide structure

The sensitivity or resistance of gram-negative bacteria to antibacterial systems appears to be related to the length of the saccharide chain of the bacterial envelope lipopolysaccharides (LPS). To explore this relationship further, we made use of two bactericidal, membrane-active cationic proteins, recently purified to near homogeneity, one from human and one from rabbit polymorphonuclear leukocytes (PMN). We have studied the effects of these two closely similar proteins on strains of Salmonella typhimurium and Escherichia coli, each separate strain differing in the saccharide chain length of its outer membrane LPS. Binding of these proteins to the bacterial outer membrane is required for killing, and is accompanied by an almost immediate increase in outer membrane permeability to normally impermeant actinomycin D. Sensitivity to the bactericidal and permeability-increasing activities of the human and rabbit proteins increases with decreasing LPS-saccharide chain length (chemotype: [S < Ra < Rb(3) < Rc < Rd(1)]). S. typhimurium G-30 and E. coli J5, mutant strains lacking UDP-galactose-4-epimerase, synthesize incomplete LPS (chemotype Rc) when grown without galactose, and are then as sensitive to both PMN proteins as the S. typhimurium strains 395 R10 (Rd(1)) and R5 (Rb(3)). However, when these mutants are grown with galactose, they synthesize complete LPS (chemotype S) and exhibit nearly the same relative insensitivity as the smooth strains S. typhimurium 395 MS and E. coli 0111:B4. The differences among strains in sensitivity to the effects of the proteins on bacterial viability and permeability correspond to differences in bacterial binding of these PMN proteins. Thus, at protein concentrations that produce maximal antibacterial activity toward the rough bacteria, but little or no activity toward the smooth strains, rough bacteria bind from 3- to 10-fold more protein (S. typhimurium 395 R10; S. typhimurium G-30, and E. coli J5 [grown without galactose]) than do the smooth bacteria (S. typhimurium 395 MS; E. coli 0111:B4; S. typhimurium G-30 and E. coli J5 [grown with galactose]). These findings suggest that bacterial sensitivity or resistance to these purified bactericidal PMN proteins is determined by the binding properties of the outer membrane, which in turn depends upon the LPS-saccharide chain length.

Enterobacterial common antigen

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Reversible envelope effects during and after killing of Escherichia coli w by a highly-purified rabbit polymorpho-nuclear leukocyte fraction

The effects of a highly-purified, potently bactericidal fraction from rabbit polymorphonuclear leukocytes on the envelope of Escherichia coli (W) have been examined. This leukocyte fraction has equally enriched bactericidal, permeability-increasing and phospholipase A2 activities, and is essentially devoid of lysozyme, myeloperoxidase and protease activities (Weiss, J., Franson, R.C., Beckerdite, S., Schmeidler, K. and Elsbach, P. (1975) J. Clin. Invest. 55, 33-42). Rapid killing of E. coli by this fraction is accompanied by two almost immediate alterations in the bacterial envelope: (1) a discrete increase in envelope permeability (measured by inhibition of bacterial leucine incorporation by normally impermeant actinomycin D), and, (2) hydrolysis of 14C-labeled fatty acid-prelabeled E. coli phospholipids. Both envelope effects are promptly reversed during further incubation at 37 degrees C, But not at 0 degrees C, with 40 mM Mg2+. Reversal is also produced by Ca2+ (40 mM) and trypsin (200 mug/ml), but 200 mM K+ causes only partial recovery and Na+ and hyperosmolar sucrose are ineffective. Upon addition of Mg2+, phospholipid degradation ceases abruptly and the labeled products of hydrolysis (free fatty acids and lysocompounds) disappear with a corresponding reaccumulation of radioactive diacylphosphatides. The time course of resynthesis of phospholipids coincides with that of restoration of the permeability barrier. Higher concentrations of the leukocyte fraction and prolonged incubation increase both the extent of phospholipid degradation and the time required for reversal of both envelope effects. These findings suggest that both the initiation of the increased permeability and its reversal are linked to respectively the breakdown and resynthesis of major E. coli membrane phospholipids, and thus depend on the fact that the biochemical apparatus of E. coli remains capable of biosynthesis despite loss of viability. Treatment of E. coli, exposed to the leukocyte fraction, with albumin results in extracellular sequestration of the products of hydrolysis and also restores the permeability barrier to actinomycin D, suggesting that the accumulation of lytic products of lipid hydrolysis within the bacterial envelope, rather than the loss of phospholipids per se, causes increased permeability Whereas the effects on the envelope are reversible as long as 2 h after nearly complete loss of ability to multiply by E. coli, the effect on bacterial multiplication is irreversible within 5 min.

Comparison of the cell envelope structure of a lipopolysaccharide-defective (heptose-deficient) strain and a smooth strain of Salmonella typhimurium

The cell envelope structure of Salmonella typhimurium LT2, which has a heptose-deficient lipopolysaccharide (LPS), is significantly different from that of an isogenic strain with a normal LPS. The rough strain, when examined by freeze-etching, lacks most surface structures that are routinely present in the smooth strain (surface particles and flagella) and has few transmemberane studs in the cytoplasmic membrane (those present are generally found in aggregates), and the outer membrane cleavage is substantially stronger than that of the smooth strain. These envelope differences were independent of both growth temperature and culture age. Examination of ultrathin sections indicated that the rough strain has an outer membrane which forms a much more defined double-track artifact than the smooth strain. The addition of MgCl2 to the growth medium of the rough strain decreased the extent of outer membrane cleavage, and flagella became evident in freeze-etched preparations. The presence of supplemental MgCl2 in the growth medium, which resulted in these morphological changes in the rough strain, also produced growth at a previously restrictive temperature and a decrease in the leakage of periplasmic enzymes. The smooth strain was unaltered morphologically or physiologically by MgCl2 under identical conditions. It is suggested that the outer membrane of the rough strain is more planar.

Ultrastructure of cell envelopes of bacteria of the bovine rumen

Most of the bacteria found in rumen fluid samples taken from cows fed hay, or a concentrate diet, had cell walls of the gram-negative type. Most were intact, with only a small proportion of lysed cells, and many of the cells contained electron-translucent cytoplasmic deposits similar to the carbohydrate reserve material described in pure cultures of rumen organisms. All of the bacteria observed in these samples had an external "coat" layer outside the outer membrane when fixed in glutaraldehyde and osmium, stained with uranyl acetate and lead citrate, and examined as sectioned material. These coat layers varied from thin (ca. 8 nm) structures to very extensive fibrous systems, sometimes including concentric arrangements and radial fibers extending up to 1,200 nm from the cell. The thin-coat layers sometimes exhibited a rough periodicity. In all, 10 different types of coat layers were distinguishable on a morphological basis. It is proposed that these external coat layers have protective and adherence functions for the rumen bacteria in the environment.

Interaction of complement components with a serum-resistant strain of Salmonella typhimurium

Salmonella typhimurium C5 is under normal conditions (physiological saline containing 0.002 M Mg2+) resistant to the action of antibody and complement (C). It becomes sensitive, however, when suspended in tris(hydroxymethyl)-aminomethane buffer (Reynolds and Pruul, 1971; Reynolds and Rowley, 1969). The interaction of complement components with this strain sensitized with specific antibody has been studied to identify the intermediate step at which inhibition occurs. The components C1 yields C2 react normally, as has been shown by lysis of complement-treated cells incubated with complement in ethylenediaminetetraacetic acid. Also, the reaction of C3 can be demonstrated by positive immune adherence and agglutination with anti-C3. The complement-treated cells do not, however, react with rabbit C6 to 9 or rabbit serum lacking C6 in tris(hydroxymethyl)aminomethan buffer. We conclude from these date that C5 can not react effectively under normal conditions. In contrast, if bacteria-antibody complexes are pretreated with rabbit serum lacking C6 in tris(hydroxymethyl)aminomethane buffer, they are readily lysed by incubation with C6 to 9. Thus, C5 can react with the bacterial surface in tris(hydroxymethyl)-aminomethane buffer.

Ultrastructure and adhesion properties of Ruminococcus albus

Morphological studies have shown that cells of the anaerobic rumen bacterium Ruminococcus albus have electron-translucent granules of reserve carbohydrate in their cytoplasm, and that they have a polysaccharide "coat" layer external to their gram-negative cell wall. This coat layer, which stains specifically with ruthenium red, forms a compact mat of fibers adjacent to the cell, and fibrous elements also project as much as 0.6 mum from the cells. These radial fibers are clearly visualized by freeze-etching, and can be seen to extend throughout the extensive intercullular space in centrifuged pellets of these bacteria. Cells of R. albus adhere to cellulose fibers added to the culture medium, and the coat material is seen to mediate this adhesion in addition to its function in the general protection of these cells.

Bactericidal activity of blood of rabbits vaccinated with homologous antigens of Campylobacter fetus (Vibrio fetus)

Rabbits were vaccinated with the following Campylobacter fetus var. venerealis (Vibrio fetus) antigens: whole-cell (WC), autoclaved (A), boiled (B), and purified postgrowth broth (PGB). Bactericidal activity of freshly drawn heparinized blood against the organism was determined after each vaccination. In all cases bactericidal activity of the blood of vaccinated rabbits was higher than for nonvaccinated rabbits. The in vitro bactericidal activity of the blood was determined in two separate experiments. In experiment I the bactericidal activity of the blood of rabbits vaccinated with PGB antigen was the same as that of rabbits vaccinated with WC antigen and higher than that of rabbits vaccinated with A antigen after the third vaccination. In experiment II the bactericidal activity of blood of rabbits vaccinated with PGB antigen was the same as that of those vaccinated with WC antigen after the second and third vaccinations and higher than for rabbits vaccinated with A antigen after the third vaccination. Blood of rabbits vaccinated with A antigen was less bactericidal than blood of rabbits vaccinated with B antigen after the third vaccination, indicating the presence of a surface antigen destroyed by autoclaving but not by boiling. The in vivo and in vitro whole blood bactericidal tests are more sensitive for measuring the response of rabbits vaccinated with WC, B, A, or PGB antigens than is the plate agglutination test.

Structure and function of the cell envelope of gram-negative bacteria

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Interaction of complement and polymyxin with gram-negative bacteria

The interaction of complement and polymyxin with gram-negative bacteria has been investigated by using three strains of Salmonella typhimurium in an attempt to determine the loci of complement action. It has been shown that the bactericidal activities of complement and polymyxin towards a smooth gram-negative organism are similarly affected by various components of the bactericidal test system. Further, complement and polymyxin have been shown to act synergistically in the bactericidal event. Evidence is presented which suggests that each agent produces lesions in the outer membrane of gram-negative bacteria, allowing lysozyme to interact with its glycopeptide substrate. An attack on the inner cytoplasmic membrane follows, since cell respiration is rapidly inhibited and this membrane becomes sufficiently disorganized to permit massive leakage of beta-galactosidase from the cytoplasm of the target cells.