Identification of a new genetic site (sac-3+) in Neisseria gonorrhoeae that affects sensitivity to normal human serum

Phase variable changes in genes lgtA and lgtC within the lgtABCDE operon of Neisseria gonorrhoeae can modulate gonococcal susceptibility to normal human serum

The α-chain of the core oligosaccharide of the lipo-oligosaccharide (LOS) produced by Neisseria gonorrhoeae can undergo reversible and rapid changes in structure due to phase-variable production of certain enzymes employed in the biosynthesis of the lacto- N-neotetraose structure. Five of these enzymes are encoded by the lgtABCDE operon, and polynucleotide tracts within three of these genes ( lgtA, lgtC and lgtD) can be substrates for slipped-strand mispairing events that lead to nucleotide insertions or deletion events which result in variable production of their respective gene products. We now report that phase-variable synthesis of the lgtA and lgtC gene products in strain FA19 results in the production of elongated LOS α-chains and that the presence of these LOS species can result in gonococci being sensitive to the bacteriolytic action of serum-antibody and complement. Hence, phase variation within the lgtABCDE operon can significantly impact the ability of gonococci to subvert this important host defense system.

α-2,3-sialyltransferase expression level impacts the kinetics of lipooligosaccharide sialylation, complement resistance, and the ability of Neisseria gonorrhoeae to colonize the murine genital tract

Neisseria meningitidis and Neisseria gonorrhoeae modify the terminal lacto-N-neotetraose moiety of their lipooligosaccharide (LOS) with sialic acid. N. gonorrhoeae LOS sialylation blocks killing by complement, which is mediated at least in part by enhanced binding of the complement inhibitor factor H (FH). The role of LOS sialylation in resistance of N. meningitidis to serum killing is less well defined. Sialylation in each species is catalyzed by the enzyme LOS α-2,3-sialyltransferase (Lst). Previous studies have shown increased Lst activity in N. gonorrhoeae compared to N. meningitidis due to an ~5-fold increase in lst transcription. Using isogenic N. gonorrhoeae strains engineered to express gonococcal lst from either the N. gonorrhoeae or N. meningitidislst promoter, we show that decreased expression of lst (driven by the N. meningitidis promoter) reduced LOS sialylation as determined by less incorporation of tritium-labeled cytidine monophospho-N-acetylneuraminic acid (CMP-NANA; the donor molecule for sialic acid). Diminished LOS sialylation resulted in reduced rates of FH binding and increased pathway activation compared to N. gonorrhoeae promoter-driven lst expression. The N. meningitidislst promoter generated sufficient Lst to sialylate N. gonorrhoeae LOS in vivo, and the level of sialylation after 24 h in the mouse genital tract was sufficient to mediate resistance to human serum ex vivo. Despite demonstrable LOS sialylation in vivo, gonococci harboring the N. meningitidislst promoter were outcompeted by those with the N. gonorrhoeaelst promoter during coinfection of the vaginal tract of estradiol-treated mice. These data highlight the importance of high lst expression levels for gonococcal pathogenesis.

Neisseria gonorrhoeae has become resistant to nearly every therapeutic antibiotic used and is listed as an “urgent threat” by the Centers for Disease Control and Prevention. Novel therapies are needed to combat drug-resistant N. gonorrhoeae. Gonococci express an α-2,3-sialyltransferase (Lst) that can scavenge sialic acid from the host and use it to modify lipooligosaccharide (LOS). Sialylation of gonococcal LOS converts serum-sensitive strains to serum resistance, decreases antibody binding, and combats killing by neutrophils and antimicrobial peptides. Mutant N. gonorrhoeae that lack Lst (cannot sialylate LOS) are attenuated in a mouse model. Lst expression levels differ among N. gonorrhoeae strains, and N. gonorrhoeae typically expresses more Lst than Neisseria meningitidis. Here we examined the significance of differential lst expression levels and determined that the level of LOS sialylation is critical to the ability of N. gonorrhoeae to combat the immune system and survive in an animal model. LOS sialylation may be an ideal target for novel therapies.

Phosphoethanolamine residues on the lipid A moiety of Neisseria gonorrhoeae lipooligosaccharide modulate binding of complement inhibitors and resistance to complement killing

Loss of phosphoethanolamine (PEA) from the lipid A of gonococcal strain FA19 results in increased sensitivity to killing by the classical pathway of complement. Here we demonstrate that loss of PEA from lipid A diminishes binding of the complement regulatory protein C4b binding protein (C4BP) to the FA19 porin B (PorB), providing a molecular basis to explain the susceptibility of an lptA null strain of FA19 to killing by normal human serum (NHS). Loss of PEA from lipid A in three additional gonococcal strains that expressed diverse PorB molecules also resulted in decreased C4BP binding, increased deposition of C4b, and increased susceptibility to killing by NHS. Complementation of lptA null strains with lptA restored C4BP binding, decreased C4b deposition, and increased resistance to killing by NHS. These effects of lipid A PEA on C4BP binding to gonococcal PorB and serum resistance were simulated when gonococcal PorB was expressed in a meningococcal background. Loss of PEA from lipid A also affected binding of the alternative pathway regulator factor H (fH) to PorB of some strains. For instance, PorB molecules of lptA null mutants of strains 252 and 1291 bound less fH than those of their parent strains when lipooligosaccharide (LOS) was sialylated, whereas PorB molecules of lptA null mutants of strains FA1090 and 273 retained the ability to bind fH when LOS was sialylated. These data indicate that replacement of lipid A with PEA alters binding of C4BP and fH to PorB and contributes to the ability of gonococci to resist complement-mediated killing.

Lipooligosaccharide Structure is an Important Determinant in the Resistance of Neisseria Gonorrhoeae to Antimicrobial Agents of Innate Host Defense

The strict human pathogen Neisseria gonorrhoeae has caused the sexually transmitted infection termed gonorrhea for thousands of years. Over the millennia, the gonococcus has likely evolved mechanisms to evade host defense systems that operate on the genital mucosal surfaces in both males and females. Past research has shown that the presence or modification of certain cell envelope structures can significantly impact levels of gonococcal susceptibility to host-derived antimicrobial compounds that bathe genital mucosal surfaces and participate in innate host defense against invading pathogens. In order to facilitate the identification of gonococcal genes that are important in determining levels of bacterial susceptibility to mediators of innate host defense, we used the Himar I mariner in vitro mutagenesis system to construct a transposon insertion library in strain F62. As proof of principle that this strategy would be suitable for this purpose, we screened the library for mutants expressing decreased susceptibility to the bacteriolytic action of normal human serum (NHS). We found that a transposon insertion in the lgtD gene, which encodes an N-acetylgalactosamine transferase involved in the extension of the α-chain of lipooligosaccharide (LOS), could confer decreased susceptibility of strain F62 to complement-mediated killing by NHS. By complementation and chemical analyses, we demonstrated both linkage of the transposon insertion to the NHS-resistance phenotype and chemical changes in LOS structure that resulted from loss of LgtD production. Further truncation of the LOS α-chain or loss of phosphoethanolamine (PEA) from the lipid A region of LOS also impacted levels of NHS-resistance. PEA decoration of lipid A also increased gonococcal resistance to the model cationic antimicrobial polymyxin B. Taken together, we conclude that the Himar I mariner in vitro mutagenesis procedure can facilitate studies on structures involved in gonococcal pathogenesis.

Phosphoethanolamine substitution of lipid A and resistance of Neisseria gonorrhoeae to cationic antimicrobial peptides and complement-mediated killing by normal human serum

The capacity of Neisseria gonorrhoeae to cause disseminated gonococcal infection requires that such strains resist the bactericidal action of normal human serum. The bactericidal action of normal human serum against N. gonorrhoeae is mediated by the classical complement pathway through an antibody-dependent mechanism. The mechanism(s) by which certain strains of gonococci resist normal human serum is not fully understood, but alterations in lipooligosaccharide structure can affect such resistance. During an investigation of the biological significance of phosphoethanolamine extensions from lipooligosaccharide, we found that phosphoethanolamine substitutions from the heptose II group of the lipooligosaccharide beta-chain did not impact levels of gonococcal (strain FA19) resistance to normal human serum or polymyxin B. However, loss of phosphoethanolamine substitution from the lipid A component of lipooligosaccharide, due to insertional inactivation of lptA, resulted in increased gonococcal susceptibility to polymyxin B, as reported previously for Neisseria meningitidis. In contrast to previous reports with N. meningitidis, loss of phosphoethanolamine attached to lipid A rendered strain FA19 susceptible to complement killing. Serum killing of the lptA mutant occurred through the classical complement pathway. Both serum and polymyxin B resistance as well as phosphoethanolamine decoration of lipid A were restored in the lptA-null mutant by complementation with wild-type lptA. Our results support a role for lipid A phosphoethanolamine substitutions in resistance of this strict human pathogen to innate host defenses.

A variable genetic island specific for Neisseria gonorrhoeae is involved in providing DNA for natural transformation and is found more often in disseminated infection isolates

Neisseria gonorrhoeae (the gonococcus) is the causative agent of the sexually transmitted disease gonorrhoea. Most gonococcal infections remain localized to the genital tract but, in a small proportion of untreated cases, the bacterium becomes systemic to produce the serious complication of disseminated gonococcal infection (DGI). We have identified a large region of chromosomal DNA in N. gonorrhoeae that is not found in a subset of gonococcal isolates (a genetic island), in the closely related pathogen, Neisseria meningitidis or in commensal Neisseria that do not usually cause disease. Certain versions of the island carry a serum resistance locus and a gene for the production of a cytotoxin; these versions of the island are found preferentially in DGI isolates. All versions of the genetic island encode homologues of F factor conjugation proteins, suggesting that, like some other pathogenicity islands, this region encodes a conjugation-like secretion system. Consistent with this hypothesis, a wild-type strain released large amounts of DNA into the medium during exponential growth without cell lysis, whereas an isogenic strain mutated in a peptidoglycan hydrolase gene (atlA) was drastically reduced in its ability to donate DNA for transformation during growth. This genetic island constitutes the first major discriminating factor between the gonococcus and the other Neisseria and carries genes for providing DNA for genetic transformation.

The elusive determinants of bacterial interference with non-specific host defences

The identification of the determinants of bacterial interference with non-specific host defences during the early stages of infection is approached rather than attained. Recognizing a relevant biological property (e.g. resistance to phagocytosis) by an in vitro test and associating bacterial surface components with it are relatively easy. Proving causation, however, is usually not completed because the biological test is complex and the surface component(s) act only in situ . Nevertheless, evidence in addition to mere association can be sought to show that a putative determinant is strongly implicated in biological activity. Even then, proving that the biological activity concerned is relevant to infection in vivo , and that the putative determinant is produced there, is often not accomplished. Again, however, distinction can be made between those cases probably relevant in vivo and those only possibly so. Finally, bacteria grown in vitro can be deficient in some of the determinants of pathogenicity expressed during infection, and this situation requires the study of organisms grown in vivo . These points are discussed and then illustrated in a brief survey of the activities of many pathogenic bacteria and a description of recent work on the resistance of gonococci to killing by human serum and phagocytes.

A pplications of genetics to studies of bacterial virulence

Classical genetic techniques have made it possible in many instances to discern which bacterial factors are directly involved in causation of infection, as opposed to those that are associated with but do not directly contribute to virulence. By use of these methods as well as monoclonal antibodies, recombinant DNA, and other new techniques it has been shown that bacterial virulence is complicated, with involvement of many different bacterial factors at each step of infection; bacterial factors that facilitate one step of infection may actually impede a subsequent step. Interestingly, a large number of genes involved in toxin production or bacterial cell-surface structure are carried on unstable elements (phage, plasmids). In addition, many chromosomal genes affecting surface antigens or appendages involved in pathogenesis are subject to high-frequency variation, enabling the bacterium to adapt rapidly to different ecological niches or to evade host immunological defences. Genetic approaches have greatly increased our appreciation for the sophistication of successful bacterial pathogens, and are rapidly being used to create exciting new vaccines.

Construction of isogenic gonococci with variable porin structure: effects on susceptibility to human serum and antibiotics

Protein I (PI) is the most abundant protein on the gonococcal cell surface and besides its porin function it may have important properties contributing to pathogenicity. By allelic exchange using cloned PI genes from FA19 (PIA) and MS11 (PIB) and a selectable marker introduced closely downstream of these genes, we constructed sets of isogenic gonococcal strains that differ only in their PI gene. Analysis revealed that PI has a major effect on stable resistance to normal human serum, and a slight effect on low-level resistance to antibiotics. All PIA/B hybrids were hypersusceptible to serum, suggesting a possible explanation for why such hybrids do not occur in nature.

Comparison of two serum bactericidal assays for Neisseria gonorrhoeae

Pooled normal human serum killing of 14 strains of Neisseria gonorrhoeae was assessed by dilution plate and microtiter methods. In both assays, the strains presented a spectrum of sensitivity to the serum. In the dilution plate assay, results with two different concentrations of human serum were similar for most, but not all of the strains tested. When data for all of the strains were compared, no correlation was found between the dilution plate and microtiter bactericidal assays. Finally, we found that the bactericidal capacities of intact and complement-depleted human sera were very similar when assessed by microtiter methods, suggesting a non-complement-mediated serum killing mechanism.

Complement binding on serum-sensitive and serum-resistant transformants of Neisseria gonorrhoeae: effect of presensitization with a non-bactericidal monoclonal antibody

The binding of C3 and C9 on serum sensitive (FA635) and serum resistant (FA638) transformants of serum sensitive Neisseria gonorrhoeae strain F62 was examined. Previous studies showed that these transformants have Protein IAs which are minimally different by proteinase K cleavage and primary structural and peptide mapping and bear LPS which vary slightly on SDS-PAGE. Binding of C3 and C9 on FA635 exceeded binding on FA638 in NHS and in adsorbed NHS. Monoclonal antibody 4G5, which binds to PI on FA638 but not FA635, increases C9 binding on FA638 to levels 3-3.5 fold greater than on FA635 but does not result in killing. The majority of additional 125IC9 deposited on FA638 following presensitization with 4G5 is released from the bacterial surface by trypsin. These results extend our earlier results with N. gonorrhoeae by showing that, although PI monoclonals can lead to substantial deposition of non-bactericidal C5b-9, this C5b-9 is not fully inserted into the gonococcal outer membrane.

Topographical alterations in proteins I of Neisseria gonorrhoeae correlated with lipooligosaccharide variation

Four transformant strains of Neisseria gonorrhoeae were generated, two of which (WS3 and WS5) had protein I subclass A (P.IA) and two which (WS2 and WS4) had protein I subclass B (P.IB). Analysis of the strains demonstrated that the two P.IA-bearing strains differed in lipooligosaccharide (LOS) and H.8 antigen, as assessed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting. The WS5 strain had slow-migrating LOS and H.8 antigen, and the WS3 strain had fast-migrating LOS and H.8 antigen. The P.IB-bearing strains also had either slow-migrating LOS and H.8 antigen (WS4) or fast-migrating LOS and H.8 antigen (WS2). Structural and exposure analysis revealed that although the P.IAs were identical in the WS3 and WS5 strains, there was a slight alteration of the exposure of the proteins which correlated with altered LOS and/or H.8 antigen. The P.IBs were also shown to be structurally identical, but the LOS and/or H.8 antigen variation in these strains correlated with a more pronounced alteration in the exposure of the P.IB molecules. The differences in protein I (P.I) exposure were generally found in highly negatively charged regions of the molecule, suggesting that the immunogenicity and/or antigenicity of the P.I molecules may vary as a result of LOS and/or H.8 antigen alterations.

A recombinant molecule from a disseminating strain of Neisseria gonorrhoeae that confers serum bactericidal resistance

A cosmid gene library was prepared from Neisseria gonorrhoeae JC1, a serum-resistant clinical isolate from a patient with disseminated gonococcal infection. From this library a recombinant molecule, pWM3, was isolated which had the ability to transform F62, a serum-sensitive strain of N. gonorrhoeae, to serum resistance. This plasmid contained 2.2 kilobases of insert gonococcal DNA that coded for two peptides, one of 29 kilodaltons (kDa) and one of 17.5 kDa. Deletion of the region coding for the 29-kDa peptide resulted in the loss of the ability of the plasmid to transform F62 to serum resistance. N. gonorrhoeae F62 acquired the ability to bind blocking antibody when transformed with pWM3 or subclones that code for only the 29-kDa protein. Although similar in size, the cloned 29-kDa protein and protein III are antigenically distinct.

Serum susceptibility of bovine pasteurellas

In this study, the serum sensitivity of 23 P. haemolytica isolates and 18 P. multocida isolates was determined by incubating dilutions of bacteria with equal volumes of fresh or heat-inactivated bovine serum for one, two, or three hours. Clinical isolates of both Pasteurella species were resistant to serum, whereas isolates from asymptomatic cattle varied in serum susceptibility. The classical pathway of complement appeared to be the principal means of complement mediated killing as detected by incubation in the presence or absence of EGTA-MgCl2. Lyzozyme and iron saturation of serum did not greatly affect serum susceptibility with either of the Pasteurella species.

Characterization of protein I from serum-sensitive and serum-resistant transformants of Neisseria gonorrhoeae

The protein IAs of serum-sensitive (FA635) and serum-resistant (FA638) transformants of Neisseria gonorrhoeae, which have identical pedigrees, have been shown to be different by the use of a monoclonal antibody and were also shown to be different by proteinase K cleavage and primary structural and surface peptide mapping. The difference in structure is within the surface-exposed region of the molecule. The only other difference observed between the two strains was a very slight difference in lipooligosaccharide silver staining in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. These data suggest that protein I alone or in combination with lipooligosaccharide may significantly contribute to serum resistance.

Recombination near the antibiotic resistance locus penB results in antigenic variation of gonococcal outer membrane protein I

In gonococci, the nonspecific antimicrobial resistance locus penB is known to be closely linked to loci designated nmp that alter the Mr and antigenicity of the outer membrane porin protein I (P.I). We report that after selection for the linked donor penB locus, occasional recombinants expressed P.I with some epitopes from each parent. These hybrid P.I antigens were stable on subculture and were transformed at a locus closely linked to penB. The hybrid P.I antigens were detected with monoclonal antibodies in both coagglutination and Western blot assays. The alterations of P.I antigenicity may have resulted from recombination between structural genes for P.I that are closely linked to penB.

Serum susceptibility of Haemophilus somnus from bovine clinical cases and carriers

The serum susceptibility of 64 isolates of Haemophilus somnus from cattle was determined in a bactericidal assay with undiluted fresh or inactivated bovine serum with serial dilutions of bacterial suspension in RPMI 1640 medium. A total of 27 strains isolated from cattle with clinical disease (4 with thromboembolic meningoencephalitis, 13 with pneumonia, and 10 with reproductive failure) were compared with 35 strains from asymptomatic carriers (11 from the vagina and 24 from the prepuce). Essentially, all clinical isolates were serum resistant, whereas approximately 25% of preputial isolates were serum susceptible, as judged after 1 h of incubation in serum; a majority of vaginal isolates showed delayed serum susceptibility. Lysozyme played no role in serum killing, and the alternative complement pathway played only a minor role. Iron saturation, however, appeared to impart greater serum resistance to serum-susceptible strains from the vagina and prepuce. Perhaps the serum-susceptible strains from carriers would be useful vaccine candidates, but resistant strains from carriers may be pathogenic.

Identification of an envelope mutation (env-10) resulting in increased antibiotic susceptibility and pyocin resistance in a clinical isolate of Neisseria gonorrhoeae

A mutation (env-10) conferring increased susceptibility to drugs, dyes, and detergents was detected in a clinical isolate of Neisseria gonorrhoeae. In certain strains, env-10 also affected susceptibility to pyocins. This mutation was phenotypically similar to but genotypically distinct from previously described env mutations.

Effect of dilution rate on lipopolysaccharide and serum resistance of Neisseria gonorrhoeae grown in continuous culture

Growth of Neisseria gonorrhoeae strain FA171 in continuous culture under glucose-limiting conditions resulted in a growth-rate-dependent change in the lipopolysaccharide (LPS). The evidence for this change is an alteration in the mobility of purified alkali-treated LPS on sodium dodecyl sulfate-polyacrylamide gels and a quantitative difference in the amount of the LPS serotype antigen. The LPS from cells grown at a low dilution rate (0.12 h-1) contained ca. eightfold less serotype antigen than the LPS from cells grown at a high dilution rate (0.56 h-1). The decrease in LPS serotype antigen was associated with an increase in sensitivity to the bactericidal activity of normal human serum and an increase in cell surface hydrophobicity. An increase in the amount of serotype antigen was associated with a reduction in the accessibility of a monoclonal antibody to a core LPS determinant, an increase in resistance to normal human serum, and a decrease in cell surface hydrophobicity. The microheterogeneity of gonococcal LPS with respect to the content of serotype antigen may result from an alteration in the metabolism of glucose.

Confirmation of association of protein I serotype of Neisseria gonorrhoeae with ability to cause disseminated infection

Previous work indicates that strains of Neisseria gonorrhoeae isolated in Seattle, Wash., and Atlanta, Ga., show an association between serotypes 1 and 2 of protein I of the outer membrane and the ability to cause disseminated infection (T.M. Buchanan and J.F. Hildebrandt, Infect. Immun. 32:985-994, 1981). By using the same serotyping system, we confirmed the association between those serotypes and both disseminated infection and serum resistance in strains from North Carolina. Some strains of the same serotype had protein I species with different apparent molecular weights.

Pyocin-resistant lipopolysaccharide mutans of Neisseria gonorrhoeae: alterations in sensitivity to normal human serum and polymyxin B

Pyocins from Pseudomonas aeruginosa were used to select several lipopolysaccharide (LPS) mutants of Neisseria gonorrhoeae strain FA19. Three classes of LPS mutans were found in the initial group selected for study. The LPS of one class lacked galactose. That of a second group lacked the typical heptose found in the parental LPS, was reduced in glucose, galactose, and N-acetylglucosamine content, appeared to contain a new unidentified sugar component, and consisted of two species of LPS separable on sodium dodecyl sulfate-polyacrylamide gels. The LPS of a third strain lacked the heptose, glucose, galactose, and N-acetylglucosamine found in the oligosaccharide portion of parental FA19 LPS. The minimal inhibitory concentration for polymyxin B of the mutant strains was 3 to 4 times that of the parental strain. The strains lacking only galactose were as resistant as the parent to the bactericidal action of normal human serum, but cells of the other two classes were quickly killed by serum. Gonococcal LPS thus appears to be important in determining phenotypic properties of the cells.