The structural basis for pyocin resistance in Neisseria gonorrhoeae lipooligosaccharides

Relationship between UDP-glucose 4-epimerase activity and oligoglucose glycoforms in two strains of Neisseria meningitidis

Sodium dodecyl sulfate-polyacrylamide gel analysis of lipooligosaccharide (LOS) from Neisseria meningitidis has demonstrated considerable microheterogeneity in the variable region of LOS due to the presence of novel glycoforms. As a step toward understanding the basis for the expression of these novel glycoforms, we have examined the LOS structures and UDP-glucose 4-epimerase (epimerase) activity levels in two strains (NMB and MA-1) and their respective galE mutants. Strain NMB was found to have low epimerase activity and to contain multiple glycoforms, some of which appear to contain only glucose sugars. The galE mutant had only the oligoglucose glycoforms. Strain MA-1 had higher epimerase activity at both log and stationary phases (2- and 12.5-fold, respectively) and one glycoform with a putative lactosyl structure. Strain MA-1 galE had two glycoforms that contained one or two glucose residues. To understand the molecular basis for the different epimerase activities, we examined the predicted amino acid sequences of the respective galE open reading frames and determined the relative amounts of GalE protein. We found no significant differences between the predicted amino acid sequence of the GalE protein in NMB and that in MA-1. We observed no significant differences in the level of GalE protein between MA-1 and NMB at exponential or stationary phase. We also observed an 8.2-fold drop in epimerase activity in NMB between the log and stationary phases that was not due to the GalE protein level or low glucose levels.

An essential saccharide binding domain for the mAb 2C7 established for Neisseria gonorrhoeae LOS by ES-MS and MSn

A study of bacterial surface oligosaccharides were investigated among different strains of Neisseria gonorrhoeae to correlate structural features essential for binding to the MAb 2C7. This epitope is widely expressed and conserved in gonococcal isolates, characteristics essential to an effective candidate vaccine antigen. Sample lipooligosaccharides (LOS), was prepared by a modification of the hot phenol-water method from which de-O-acetylated LOS and oligosaccharide (OS) components were analyzed by ES-MS-CID-MS and ES-MSnin a triple quadrupole and an ion trap mass spectrometer, respectively. Previously documented natural heterogeneity was apparent from both LOS and OS preparations which was admixed with fragments induced by hydrazine and mild acid treatment. Natural heterogeneity was limited to phosphorylation and antenni extensions to the alpha-chain. Mild acid hydrolysis to release OS also hydrolyzed the beta(1-->6) glycosidic linkage of lipid A. OS structures were determined by collisional and resonance excitation combined with MS and multistep MSn which provided sequence information from both neutral loss, and nonreducing terminal fragments. A comparison of OS structures, with earlier knowledge of MAb binding, enzyme treatment, and partial acid hydrolysis indicates a generic overlapping domain for 2C7 binding. Reoccurring structural features include a Hepalpha(1-->3)Hepbeta(1-->5)KDO trisaccharide core branched on the nonreducing terminus (Hep-2) with an alpha(1-->2) linked GlcNAc (gamma-chain), and an alpha-linked lactose (beta-chain) residue. From the central heptose (Hep-1), a beta(1-->4) linked lactose (alpha-chain), moiety is required although extensions to this residue appear unnecessary.

The R-type pyocin of Pseudomonas aeruginosa C is a bacteriophage tail-like particle that contains single-stranded DNA

Pseudomonas aeruginosa R-type pyocin particles have been described as bacteriocins that resemble bacteriophage tail-like structures. Because of their unusual structure, we reexamined whether they contained nucleic acids. Our data indicated that pyocin particles isolated from P. aeruginosa C (pyocin C) contain DNA. Probes generated from this DNA by the random-primer extension method hybridized to distinct bands in restriction endonuclease-digested P. aeruginosa C genomic DNA. These probes also hybridized to genomic DNA from 6 of 18 P. aeruginosa strains that produced R-type pyocins. Asymmetric PCR, complementary oligonucleotide hybridization, and electron microscopy indicated that pyocin C particles contained closed circular single-stranded DNA, approximately 4.0 kb in length. Examination of total intracellular DNA from mitomycin C-induced cultures revealed the presence of two extrachromosomal DNA molecules, a double-stranded molecule and a single-stranded molecule, which hybridized to pyocin DNA. Sequence analysis of 7,480 nucleotides of P. aeruginosa C chromosomal DNA containing the pyocin DNA indicated the presence of pyocin open reading frames with similarities to open reading frames from filamentous phages and cryptic phage elements. We did not observe any similarities to known phage structural proteins or previously characterized pseudomonal prt genes expressing R-type pyocin structural proteins. These studies demonstrate that pyocin particles from P. aeruginosa C are defective phages that contain a novel closed circular single-stranded DNA and that this DNA was derived from the chromosome of P. aeruginosa C.

Neisseria gonorrhoeae that infect men have lipooligosaccharides with terminal N-acetyllactosamine repeats

Infectious Neisseria gonorrhoeae make relatively large lipooligosaccharides (LOS) that structurally resemble human glycosphingolipids. MS11mkC is an LOS variant of N. gonorrhoeae strain MS11 which was isolated from men at the onset of dysuria (Schneider, H., Griffiss, J. M., Boslego, J. W., Hitchcock, P. J., Zahos, K. M., and Apicella, M. A. (1991) J. Exp. Med. 174, 1601-1605). Delayed extraction matrix-assisted laser desorption and ionization and electrospray ionization mass spectrometry of O-deacylated MS11mkC LOS produced ions consistent with known LOS which have lacto-N-neotetraose (Galbeta1-->4GlcNAcbeta1-->3Galbeta1-->4Glc; paraglobosyl; monoclonal antibodies (mAbs) 1B2(+) and 06B4(+)) and GalNAc-->lacto-N-neotetraose (gangliosyl; mAb 1-1-M+) oligosaccharides. Ion peaks for a larger LOS which also bound mAb 1B2 indicated the addition of a hexose (+162 Da) to gangliosyl LOS or the addition of a hexose and a N-acetylhexosamine (+365 Da) to paraglobosyl LOS. Analysis of HF-treated and O-deacylated LOS revealed three major components present in a phosphoethanolamine (PEA)0 and a PEA1 series. Digestion of MS11mkC LOS by beta-N-acetylhexosaminidase and beta-galactosidase, alone and sequentially, combined with mAb binding patterns, confirmed the presence of a nonreducing terminal repeating LacNAc ((Galbeta1-->4GlcNAc)2) on the largest LOS, rather than a parallel oligosaccharide structure.

Identification of the gene (lgtG) encoding the lipooligosaccharide beta chain synthesizing glucosyl transferase from Neisseria gonorrhoeae

The lipooligosaccharide from Neisseria gonorrhoeae (GC), consists of lipid A, an oligosaccharide core and three branches, alpha, beta, and gamma. We report the cloning of the gene (lgtG, lipooligosaccharide glycosyl transferase G) encoding the glucosyl transferase of GC that initiates the beta chain which consists of a lactosyl moiety. This gene contains a homopolymeric tract of cytidine [poly(C)] and we demonstrate that changes in the number of Cs in poly(C) account for the variation of beta chain expression in different GC strains. Biochemical analyses and mass spectrometry clearly attribute the reactivity of mAb 2C7 to the presence of the lactosyl beta chain. In addition, we demonstrate that in the absence of the lactosyl group, a phosphoethanolamine is added to generate a new antigenic epitope as evidenced by the gain of reactivity to mAb 2-L1-8. These results show that, like the alpha chain, the beta chain of lipooligosaccharide is subject to antigenic variation.

Characterization of terminal NeuNAcalpha2-3Galbeta1-4GlcNAc sequence in lipooligosaccharides of Neisseria meningitidis

Group B and C Neisseria meningitidis are the major cause of meningococcal disease in the United States and in Europe. N . meningitidis lipooligosaccharide (LOS), a major surface antigen, can be divided into 12 immunotypes of which L1 through L8 were found among Group B and C organisms. Groups B and C but not Group A may sialylate their LOSs with N-acetylneuraminic acid (NeuNAc) at the nonreducing end because they synthesize CMP-NeuNAc. Using sialic acid-galactose binding lectins as probes in an ELISA format, six of the eight LOS immunotypes (L2, L3, L4, L5, L7, and L8) in Groups B and C bound specifically to Maackia amurensis leukoagglutinin (MAL), which recognizes NeuNAcalpha2-3Galbeta1-4GlcNAc/Glc sequence, but not to Sambucus nigra agglutinin, which binds NeuNAcalpha2-6Gal sequence. The combination of SDS-PAGE and MAL-blot analyses revealed that these six LOSs contained only the NeuNAcalpha2-3Galbeta1-4GlcNAc trisaccharide sequence in their 4.1 kDa LOS components, which have a common terminal lacto-N-neotetraose (LNnT, Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc) structure when nonsialylated as shown by previous studies. The LOS-lectin binding was abolished when the LOSs were treated with Newcastle disease viral neuraminidase which cleaves alpha2-->3 linked sialic acid. Methylation analysis of a representative LOS (L2) confirmed that NeuNAc is 2-->3 linked to Gal. Thus, these LOSs structurally mimic certain glycolipids, i.e., paragloboside (LNnT-ceramide) and sialylparagloboside and some glycoproteins in having LNnT and N-acetyllactosamine sequences, respectively, with or without alpha2-->3 linked NeuNAc. The molecular mimicry of the LOSs may play a role in the pathogenesis of N.meningitidis by assisting the organism to evade host immune defenses in man.

Antigenic variation in Neisseria gonorrhoeae: production of multiple lipooligosaccharides

Individual cells of Neisseria gonorrhoeae may express a single lipooligosaccharide (LOS) component on their cell surfaces, or they may simultaneously express multiple LOS structures. Strain FA19 expresses LOS components that react with monoclonal antibodies (MAbs) 2-1-L8 and 1B2. The genetic locus responsible for this phenotype in FA19 was identified by isolating a clone that is able to impart the ability to simultaneously express both LOS molecules to strain 1291, a strain expressing only the MAb 1B2-reactive LOS. This clone, pCLB1, was characterized, and the gene responsible for the expression of both LOS components was determined to be lsi2. DNA sequence analysis of lsi2(Fa19) indicates that there are several differences between the DNA sequences of lsi2(FA19) and lsi2(1291). The region responsible for the LOS-specific phenotype change in lsi2(FA19) was identified by deletion and transformation analysis, mapping to a polyguanine tract within lsi2 where lsi2(FA19) possesses a +2 frameshift relative to lsi2(1291). The polyguanine tract in lsi2(FA19) was modified by site-directed mutagenesis to change the sequence to GGGAGGTGGCGGA to prevent frameshifting during DNA replication, transcription, and/or translation. Transformants of strain 1291 containing this DNA sequence express a single MAb 2-1-L8-reactive LOS component, the same phenotype exhibited by lsi2-defective strains. These data indicate that FA19 is able to generate a small amount of functional Lsi2 protein via transcriptional and/or translational frameshifting, and this limited amount of protein allows for the expression of MAb 1B2-reactive LOS molecules.

Molecular mimicry of host structures by bacterial lipopolysaccharides and its contribution to disease

The core oligosaccharides of low-molecular-weight lipopolysaccharide (LPS), also termed lipooligosaccharide (LOS), of pathogenic Neisseria spp. mimic the carbohydrate moieties of glycosphingolipids present on human cells. Such mimicry may serve to camouflage the bacterial surface from the host. The LOS component is antigenically and/or chemically identical to lactoneoseries glycosphingolipids and can become sialylated in Neisseria gonorrhoeae when the bacterium is grown in the presence of cytidine 5'-monophospho-N-acetylneuraminic acid, the nucleotide sugar of sialic acid. Strains of Neisseria meningitidis and Haemophilus influenzae also express similarly sialylated LPS. Sialylation of the LOS influences susceptibility to bactericidal antibody, may decrease or prevent phagocytosis, cause down-regulation of complement activation, and decrease adherence to neutrophils and the subsequent oxidative burst response. The core oligosaccharides of LPS of Campylobacter jejuni serotypes which are associated with the development of the neurological disorder, Guillain-Barré syndrome (GBS), exhibit mimicry of gangliosides. Cross-reactive antibodies between C. jejuni LPS and gangliosides are considered to play an important role in GBS pathogenesis. In contrast, the O-chain of a number of Helicobacter pylori strains exhibit mimicry of Lewis(x) and Lewis(y) blood group antigens. The role of this mimicry remains to be investigated, but may play a role in bacterial camouflage, the induction of autoimmunity and immune suppression in H. pylori-associated disease.

Cloning, complementation, and characterization of an rfaE homolog from Neisseria gonorrhoeae

Neisseria gonorrhoeae WS1 is a spontaneous pyocin (a bacteriocin produced by Pseudomonas aeruginosa)-resistant mutant of N. gonorrhoeae FA19 that produces a truncated lipooligosaccharide (LOS) and is non-transformable. The LOS-specific mutation in WS1 was moved into a transformable background by transforming FA19 with chromosomal DNA from WS1 (generating strain JWS-1). A clone (pJCL2) capable of restoring JWS-1 to wild-type LOS expression, as detected by its acquisition of reactivity with monoclonal antibodies and by its complemented sodium dodecyl sulfate-polyacrylamide gel electrophoresis profile, was isolated. Sequential unidirectional deletion and DNA sequence analysis of pJCL2 identified an open reading frame, designated lsi-7, that could complement the defect in JWS-1. Homology searches against various databases indicated that lsi-7 bad homology with several Escherichia coli genes involved in the phosphorylation of sugars. lsi-7 is adjacent to the lsi-6 gene, another gene involved in LOS biosynthesis. Complementation studies using Salmonella typhimurium lipopolysaccharide mutants showed lsi-6 and lsi-7 to be gonococcal homologs of S. typhimurium rfaD and rfaE, respectively. Reverse transcriptase PCR analysis demonstrated that lsi-6 and lsi-7 are part of the same transcriptional unit.

The lipooligosaccharides of pathogenic gram-negative bacteria

Lipooligosaccharides (LOSs) are the major glycolipids expressed on mucosal Gram-negative bacteria, including members of the genera Neisseria, Haemophilus, Bordetella, and Branhamella. They can also be expressed on some enteric bacteria such as Campylobacter jejuni and Campylobacter coli strains. LOS is analogous to the lipopolysaccharide (LPS) found in other Gram-negative families. LOSs share similar lipid A structures with an identical array of functional activities as LPSs. LOSs lack O-antigen units with the LOS oligosaccharide structures limited to 10 saccharide units. The LOS species of pathogenic Neisseria can play a major role in pathogenesis through enhancing the resistance of the organism to killing by normal human serum. Other distinguishing characteristics of LOS are the structural and antigenic similarity of some LOS species to human glycolipids and the potential for certain LOSs to be modified in vivo by host substances or secretions. These modifications of LOS in different environments of the host result in synthesis of new LOS structures that probably benefit the survival of the pathogen. The LOS of N. gonorrhoeae can act as a ligand of human receptors, promoting invasion of host cells. It is becoming clearer that LOSs are crucial factors in the pathogenesis of bacteria that express them.

The lipooligosaccharides of Haemophilus ducreyi are highly sialylated

The major lipooligosaccharides of the sexually transmitted pathogen Haemophilus ducreyi 35000 have been previously found to terminate in N-acetyllactosamine and sialyl-N-acetyllactosamine, Neu5Ac alpha 2-->3Gal beta 1-->4GlcNAc (W. Melaugh, N. J. Phillips, A. A. Campagnari, M. V. Tullius, and B. W. Gibson, Biochemistry 33: 13070-13078, 1994). In this study, mass spectrometry and composition analyses have shown that the lipooligosaccharides from three other H. ducreyi strains also contain N-acetyllactosamine and are highly sialylated (approximately 30 to 50%), although one African strain was found to contain neither of these structural features.

Genetic basis of Neisseria gonorrhoeae lipooligosaccharide antigenic variation

Neisseria gonorrhoeae lipooligosaccharide (LOS) undergoes antigenic variation at a high rate, and this variation can be monitored by changes in a strain's ability to bind LOS-specific monoclonal antibodies. We report here the cloning and identification of a gene, lsi-2, that can mediate this variation. The DNA sequence of lsi-2 has been determined for N. gonorrhoeae 1291, a strain that expresses a high-molecular-mass LOS, and a derivative of this strain, RS132L, that produces a truncated LOS. In the parental strain, lsi-2 contains a string of 12 guanines in the middle of its coding sequence. In cells that had antigenically varied to produce a truncated LOS, the number of guanines in lsi-2 was altered. Site-specific deletions were constructed to verify that expression of a 3.6-kDa LOS is due to alterations in lsi-2.

A lipooligosaccharide-binding site on HepG2 cells similar to the gonococcal opacity-associated surface protein Opa

The lacto-N-neotetraose-containing lipooligosaccharide (LOS) present on the surface of most Neisseria gonorrhoeae organisms may serve many important functions in gonococcal pathogenesis. This surface glycolipid contains the cross-reactive epitope to human paragloboside and can be sialylated by gonococci grown in the presence of CMP-N-acetylneuraminic acid. Another possible role for this glycolipid could be to mimic human asialocarbohydrates and act as a ligand for asialoglycoprotein receptors contained on numerous human cells. The most noted of this large family of receptors is that expressed on the surface of hepatic cells. In a model cell system, using the hepatoma tissue culture cell line HepG2, we wanted to investigate if the presence of this asialoglycoprotein receptor influenced the adherence and/or invasion of gonococci expressing the lacto-N-neotetraose structure. Piliated variants of the gonococcal wild-type strain 1291 and its isogeneic LOS mutant 1291E were used in adherence-invasion assays. This gonococcal strain is somewhat unusual in that it expresses large amounts of predominantly one species of LOS, thus reducing the complexity of interpreting the data. The data from these assays suggested that the Gal(beta 1-4)GlcNAc(beta 1-3)Gal(beta 1-4)Glc carbohydrate structure on the wild-type LOS affected the adherence-invasion of gonococci into the HepG2 cells. In studies to determine whether the major hepatic asialoglycoprotein receptor was involved in these interactions, we found that the HepG2 cells contained two receptors which bound gonococcal LOS. One of these was the asialoglycoprotein receptor, and the data concerning this receptor will be reported elsewhere. The data on the second receptor are reported here. Purified, 125I-labeled gonococcal LOS was used to identify specific high-affinity LOS-binding sites. These binding experiments revealed one major binding site corresponding to a protein with a molecular mass of 70 kDa (p70). Several lines of evidence in this study suggested that the oligosaccharide region of LOS played an important role in LOS binding to the p70 of HepG2 cells. In addition, we show that this human LOS receptor has some similarities to the gonococcal Opa proteins.

A mutation in the Neisseria gonorrhoeae rfaD homolog results in altered lipooligosaccharide expression

The gonococcal lsi-6 locus was cloned and shown by DNA sequence analysis to have homology with the E. coli rfaD gene, which encodes ADP-L-glycero-D-mannoheptose epimerase. This enzyme is involved in the biosynthesis of the lipopolysaccharide precursor ADP-L-glycero-D-mannoheptose. A site-directed frameshift mutation in lsi-6 was constructed by PCR amplification and introduced into the chromosome of Neisseria gonorrhoeae MS11 P+ by transformation. The lipooligosaccharides (LOS) of mutant and parental strains were characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The lsi-6 mutant produced LOS components with apparent molecular masses of 2.6 and 3.6 kDa as compared with a 3.6-kDa band of the MS11 P+ strain. The parental LOS phenotype was expressed when a revertant was constructed by transformation of the cloned wild-type gene into the lsi-6 mutant. The immunoreactivity of LOS from parental and constructed strains was examined by SDS-PAGE and Western blotting. Only the parental and reconstructed wild-type strains produced a 3.6-kDa LOS component that reacted with monoclonal antibody 2-1-L8. These results suggest that the lsi-6 locus is involved in gonococcal LOS biosynthesis and that the nonreactive mutant 3.6-kDa LOS component contains a conformational change or altered saccharide composition that interferes with immunoreactivity.

Neisseria gonorrhoeae utilizes and enhances the biosynthesis of the asialoglycoprotein receptor expressed on the surface of the hepatic HepG2 cell line

One of the lipooligosaccharide (LOS) structures of Neisseria gonorrhoeae contains a terminal Gal(beta 1-4)GlcNAc residue which is a good candidate to serve as a ligand for human asialoglycoprotein receptors (ASGP-R). These receptors have been shown to be present on macrophages, sperm cells, and hepatocytes. The human tissue culture cell line used most often to study this receptor, HepG2, was used in our investigations only as a model. We also chose N. gonorrhoeae 1291 for these studies because, unlike many other gonococcal strains, this strain expresses one main species of LOS. The LOS structure expressed by this strain has also been fully characterized. Using well-established assays for the utilization of the ASGP-R, we found that incubation of HepG2 cells with gonococci expressing the terminal Gal(beta 1-4)GlcNAc asialo-LOS carbohydrate structure competitively inhibited the ASGP-R from binding to one of its well-known ligands, asialo-alpha-acid-1-glycoprotein. The inhibition was specific to the ASGP-R, since binding of two other ligands to their specific receptors in the same model cell system was not affected. Immunoblot analysis for the ASGP-R suggested that gonococci seemed to stimulate the HepG2 cells to increase the expression of the major (46-kDa) receptor species. This observation was confirmed both by functional analysis, which showed that the concentration of total receptor molecules, as well as surface receptors, was about 60% higher after incubation with gonococci than in control cells and by Northern (RNA) blot analysis using a cDNA probe of the major human H1 subunit. Poly(A) RNA purified from control and HepG2 cells exposed to gonococci indicated the presence of increased amounts of mRNA coding for the ASGP-R after incubation with gonococci. This result supports the idea that the molecular mechanism controlling the receptor level after gonococcal exposure is under transcriptional regulation.

Lipooligosaccharide biosynthesis in Neisseria gonorrhoeae: cloning, identification and characterization of the alpha 1,5 heptosyltransferase I gene (rfaC)

The identical partial deep-core structure of Hep alpha 1-3Hep alpha 1-5KDO in Salmonella typhimurium LT2 LPS and Neisseria gonorrhoeae LOS enabled us to isolate a DNA fragment from N. gonorrhoeae that was able to complement the alpha 1,5 LOS heptosyltransferase defect in the S. typhimurium rfaC630 (SA1377) mutant. SDS-PAGE analysis confirmed the production of wild-type LPS in the transformant. Subcloning revealed that complementation was due to a 1.2 kb fragment. Sequence analysis revealed a complete open reading frame capable of encoding a 36-37 kDa peptide. In vitro transcription-translation analysis of the 1.2 kb clone confirmed that a 37 kDa protein was encoded by this DNA fragment. The DNA sequence-deduced protein had 36% identity and 58% similarity to S. typhimurium heptosyltransferase I (RfaC). Primer extension analysis indicated that transcription of the cloned gene in N. gonorrhoeae strain 1291 begins 144 bp upstream of the start codon at a G nucleotide. An isogenic mutant of N. gonorrhoeae strain 1291 with an m-Tn3 insertion inside the coding sequence expressed a single truncated LOS with a similar molecular mass to S. typhimurium rfaC LPS. We conclude that the 1.2 kb fragment encodes the alpha 1,5 LOS heptosyltransferase I (RfaC) in N. gonorrhoeae. Our studies also provide further evidence that the third KDO residue in S. typhimurium LPS is added after the core synthesis is completed.

Genetic basis of pyocin resistance in Neisseria gonorrhoeae

The genetic basis for pyocin resistance in Neisseria gonorrhoeae 1291d, 1291e, and FA5100 was determined by Southern blot and DNA sequence analyses. The genes defective in these strains are present as single copies in the gonococcal chromosome. The mutant regions of 1291d, 1291e, and FA5100 were amplified by the PCR. Sequence analysis of the mutant regions demonstrated that strain 1291d contains a 12-bp deletion that results in the loss of four amino acids in phosphoglucomutase, while strain 1291e contains a point mutation that results in the change of an uncharged glycine residue to a charged glutamic acid residue in the same protein. FA5100 contains a nonsense mutation in the gene encoding heptosyltransferase II. The gene previously described as lsi-1 was shown to complement an rfaF mutation in Salmonella typhimurium and has been renamed rfaF.

Tn916-generated, lipooligosaccharide mutants of Neisseria meningitidis and Neisseria gonorrhoeae

A library of Tn916-generated, tetracycline-resistant (Tc) mutants of the group B Neisseri meningitidis strain NMB was screened by using monoclonal antibodies (MAbs) that recognize structural differences in neisserial lipooligosaccharide (LOS). The LOS of parental strain NMB had a relative molecular mass of 4.5 kDa, reacted with MAbs 3F11 and 6B4 but not with MAb 4C4 or 6E4, and contained a lacto-N-neotetrose unit. Two phenotypically stable mutants, SS3 and R6, altered in LOS, were identified by colony immunoblots, electrophoresis, and Western immunoblots. The LOS of mutant SS3 was 3.4 kDa and reacted with MAbs 4C4 and 6E4 but not MAb 3E11 or 6B4. The LOS of mutant R6 was 3.1 to 3.2 kDa and reacted with MAb 6E4 but not MAb 3F11, 6B4, or 4C4. Thus, the LOSs of the R6 and SS3 mutants were predicted to contain different truncations of the core oligosaccharide. The LOS phenotype of each mutant was linked to Tc(r), as determined by transformation of the parent strain with DNA from the mutant. Southern hybridizations and single-specific-primer PCR revealed in each mutant a single truncated tn916 insertion which had lost genes required for mobilization. Tn916 mutagenesis was used to identify two distinct genetic sites in the meningococcal chromosome involved in biosynthesis of the oligosaccharide chain of LOS and to create genetically defined LOS mutants of N. meningitidis and Neisseria gonorrhoeae.

Use of pyocin to select a Haemophilus ducreyi variant defective in lipooligosaccharide biosynthesis

Haemophilus ducreyi, a cause of genital ulcer disease in developing countries, appears to facilitate the heterosexual transmission of the human immunodeficiency virus in Africa. Despite an increase in studies of this gram-negative human pathogen, little is known about the pathogenesis of chancroid. Our studies have shown that the lipooligosaccharides (LOS) of H. ducreyi may play an important role in ulcer formation. Monoclonal antibody and mass spectrometric analyses identified a terminal trisaccharide present on H. ducreyi LOS that is immunochemically similar to human paragloboside. This epitope is present on the LOS of Neisseria gonorrhoeae, and it may be the site of attachment for pyocin lysis. We have used pyocin, produced by Pseudomonas aeruginosa, to select LOS variants with sequential saccharide deletions from N. gonorrhoeae. On the basis of the similarities between N. gonorrhoeae and H. ducreyi LOS, we employed the same technique to determine if H. ducreyi strains were susceptible to pyocin lysis. In this study, we report the generation of a pyocin N-resistant H. ducreyi strain which synthesizes a truncated version of the parental LOS. Further studies have shown that this H. ducreyi variant has lost the terminal LOS epitope defined by monoclonal antibody 3F11. This report demonstrates that H. ducreyi is sensitive to pyocins and that this technique can be used to generate H. ducreyi LOS variants. Such variants could be used in comparative studies to relate LOS structure to biologic function in the pathogenesis of chancroid.

Role of phosphoglucomutase in lipooligosaccharide biosynthesis in Neisseria gonorrhoeae

A region of pSG30 that complements the pyocin-derived gonococcal lipooligosaccharide (LOS) mutants 1291d and 1291e was characterized by DNA sequence analysis and an open reading frame of 1,380 bases was identified that is 89% similar and 56% identical over 452 amino acids to the algC gene product from Pseudomonas aeruginosa that encodes phosphomannomutase. Enzymatic analysis of gonococcal crude protein extracts demonstrated that pSG30 encodes phosphoglucomutase (PGM) and phosphomannomutase activity. This activity is absent in 1291d and 1291e but is restored upon introduction of pSG30. PGM encoded by pSG34, a subclone of pSG30, was able to complement Escherichia coli PGM1, a strain deficient in PGM, as determined by bacteriophage C21 plaque formation. A revertant of 1291d that binds monoclonal antibody 2-1-L8 (specific for a 3.6-kDa LOS component) was isolated. The construction of a site-specific deletion of this region in the chromosome of 1291 confirms the role of this open reading frame in LOS biosynthesis.

Lipopolysaccharides of Campylobacter jejuni serotype O:19: structures of core oligosaccharide regions from the serostrain and two bacterial isolates from patients with the Guillain-Barré syndrome

Lipopolysaccharides from phenol-water extraction of cells of Campylobacter jejuni serotype O:19 were separated into a water-soluble gel of low M(r) and a water-soluble component of high M(r). Acetic acid hydrolysis of the ketosidic linkages to lipid A furnished respectively a core oligosaccharide, the structure of which is reported herein, and an O antigenic polysaccharide. Structural investigations were performed on the O-deacetylated lipopolysaccharide of low M(r), the liberated core oligosaccharide and the various products from removal of neuraminic acid and phosphate residues, and from the Smith degradation. It is concluded that the lipopolysaccharide from the serostrain has a core region with two types of closely related oligosaccharide chains showing striking homologies with gangliosides, the first with a single N-acetylneuraminic acid residue in an outer chain resembling GM1 and the second with two N-acetyl-neuraminic acid residues with a terminal region resembling GD1a. Similar experiments were carried out on lipopolysaccharides of low M(r) from bacterial isolates OH 4384 and OH 4382 serotyped as O:19 that had been obtained from two patients who subsequently developed the Guillain-Barré syndrome. The core oligosaccharide region of lipopolysaccharide from the former isolate differed only slightly from that of the serostrain, whereas that from the latter isolate was distinctly shorter.

Cloning of a gonococcal DNA sequence that complements the lipooligosaccharide defects of Neisseria gonorrhoeae 1291d and 1291e

An isogenic set of gonococcal lipooligosaccharide (LOS) mutants derived from pyocin treatment of Neisseria gonorrhoeae 1291 was used to identify cloned gonococcal DNA fragments. A gene bank from N. gonorrhoeae 1291c chromosomal DNA was made in pLEE10, a shuttle vector that replicates in the gonococcus and Escherichia coli. A plasmid (pSG30) that could transform the LOS mutants 1291d and 1291e to reactivity with monoclonal antibody 3F11 and to production of an LOS component with migration identical to that of the parent, 1291, was identified. pSG30 contains a 9-kb EcoRI fragment. Curing studies indicate that pSG30 encodes gene products that affect LOS biosynthesis in trans. Subcloning identified a 2.6-kb HincII fragment (pSG38) that retained the ability to modify the LOS of 1291d and 1291e. The DNA regions involved in modification of 1291d and 1291e were named lsi-4 and lsi-5, respectively. The region of pSG38 that was involved in LOS modification was further localized by the construction of exonuclease III deletion plasmids. Transformation of these constructs identified a 750-bp fragment that retains the ability to modify 1291e and a 540-bp fragment which retains the ability to modify 1291d.

Chemical structures of the core regions of Campylobacter jejuni serotypes O:1, O:4, O:23, and O:36 lipopolysaccharides

Complete structures, including the location of N-acetylneuraminic acid (Neu5Ac) residues, were assigned for the core regions of Campylobacter jejuni serotypes O:1, O:4, and O:23 and O:36 lipopolysaccharides (LPS). In continuation of earlier studies, structure determinations of liberated oligosaccharides and, where necessary, of intact LPS, were by 1H-NMR spectroscopy, Smith degradation, chromium trioxide and enzymic degradations, in conjunction with methylation studies supported by fast-atom-bombardment mass spectrometry and linkage analyses by gas chromatography/mass spectrometry. It was concluded on the basis of the following structures, in which each was linked 1-->5 to a terminal 3-deoxy-D-manno-octulosonic acid residue, that the core regions with qualitatively similar sugar compositions showed serotypic differences in one or more of their sequences, linkage types, and anomeric configurations: [formula: see text] [corrected]. The outer regions of each structure carry Neu5Ac residues linked 2-->3 to available beta-D-Galp residues and show striking similarities with various glycosphingolipids of the ganglioside family. However, Neu5Ac epitopes are not apparently involved in determining serospecificity.

Lipo-oligosaccharides (LOS) of mucosal pathogens: molecular mimicry and host-modification of LOS

Immunochemical studies of the lipo-oligosaccharides (LOS) of the Gram-negative bacteria Neisseria gonorrhoeae and Neisseria meningitidis have revealed some interesting structural characteristics of these LOS that might relate to their roles during pathogenesis. The carbohydrate moieties of the LOS of pathogenic Neisseria mimic carbohydrates present in glycosphingolipids of human cells. Firstly, an LOS component present among a number of Neisseria species is antigenically and/or chemically identical to lactoneoseries glycosphingolipids present in human cells. The lactoneoseries LOS becomes sialylated on Neisseria gonorrhoeae when they are grown in the presence of cytidine 5'-monophospho-N-acetyl-neuraminic acid (CMP-NANA), the nucleotide sugar for sialic acid. Examination of gonococci present in exudates from males with natural infection indicates that sialylation also occurs in vivo. The mechanism for this process apparently involves a bacterial sialyltransferase scavenging available host CMP-NANA ("host-modification" of LOS) and transferring the sialic acid to the lactoneoserieslike LOS. Strains of N. meningitidis and Haemophilus influenzae also express similarly sialylated LOS suggesting that this is a common mechanism of pathogenesis among these bacteria. Additional examples of LOS that mimic other glycosphingolipid series have been identified also and the fact that multiple series can be expressed in a single population of gonococci suggests that a diverse set of LOS can be presented to the host during infection. It is possible that this diverse set of LOS serve different functions for the bacteria in various hosts and/or environments during infection.

Structural studies of the lipooligosaccharides from Haemophilus influenzae type b strain A2

The outer membrane lipooligosaccharides (LOS) from Haemophilus influenzae type b strain A2 are a heterogeneous mixture of glycolipids containing a conserved Lipid A structure and a variable oligosaccharide moiety. After O-deacylation by treatment with anhydrous hydrazine, the O-deacylated LOS mixture was analyzed by electrospray mass spectrometry and shown to contain 11 components, ranging in M(r) from 2277.8 to 3416.4. The majority of these structures contained a variable number of hexoses, three L-glycero-D-manno-heptoses, and one 3-deoxy-D-manno-octulosonic acid (KDO) residue attached to a diphosphorylated O-deacylated Lipid A moiety. Additional phosphate and phosphoethanolamine (PEA) groups were also present on the oligosaccharide structures. Two minor high molecular weight components were also observed that contained N-acetylhexosamine and sialic acid. Neuraminidase treatment of the O-deacylated LOS mixture resulted in the loss of sialic acid from these latter two species. After mild acid hydrolysis and separation by size-exclusion chromatography, liquid secondary ion mass spectrometry identified six major and four minor oligosaccharides, ranging in M(r) from 1243.4 to 2215.8. These released oligosaccharides contained a common heptose trisaccharide core structure with anhydro-KDO at the reducing terminus, which arises as an artifact of the hydrolysis procedure by beta-elimination of a phosphate group from the 4-position of KDO. Selected oligosaccharide fractions were subjected to composition and methylation analyses and sequenced by tandem mass spectrometry. Taken together, these data defined the major O-deacylated LOS as follows: [formula: see text] Higher molecular weight structures in the mixture contained galactose, N-acetylglucosamine, and sialic acid as additional branch sugars, suggesting that H. influenzae A2 is capable of forming a sialylated lactosamine structure.

Structural analysis of lipooligosaccharide produced by Neisseria gonorrhoeae, strain MS11mk (variant A): a precursor for a gonococcal lipooligosaccharide associated with virulence

We studied the structure of the lipooligosaccharide (LOS) that is produced by a variant A of strain MS11mk. This variant produces a single LOS that is recognized by monoclonal antibody (MAb) 2-1-L8. In a recent study of the pathogenesis of Neisseria gonorrhoeae in male volunteers, variant A gave rise to other phase variants that produce higher molecular weight LOSs, and these LOS were associated with virulence. Definition of the structure of the variant A LOS is important to understand the biosynthesis of LOS and its expression in vivo. The dephosphorylated oligosaccharide (OS) structure derived from the variant A LOS was analyzed by two-dimensional NMR and methylation analysis. The OS structure was found to be a truncated form of the LOS produced by strain F62 [Yamasaki et al. (1991) Biochemistry 30, 10566-10575]; the variant A OS is a hexamer, a beta-lactosyl residue linked to a tetrasaccharide: Gal beta 1-->4Glc beta 1-->4[GlcNAc alpha 1-->2Hep alpha 1-->3]Hep alpha 1-->KDO. We determined that the variant A LOS is a precursor for the synthesis of higher MW LOS. We also studied expression of the MAb 2-1-L8-defined epitope present on the variant A LOS. Our data indicate that the MAb-defined epitope is not a linear beta-lactosyl residue but its specificity is directed toward the phosphorylated GlcNAc-Hep-Hep residue. Since this MAb binds to gonococci, at least part of the phosphorylated diheptose area is exposed on the gonococcal surface.

Structural characterization of the cell surface lipooligosaccharides from a nontypable strain of Haemophilus influenzae

Oligosaccharides released from the lipooligosaccharides (LOS) of Haemophilus influenzae nontypable strain 2019 by mild acid hydrolysis were fractionated by size exclusion chromatography and analyzed by liquid secondary ion mass spectrometry. The major component of the heterogeneous mixture was found to be a hexasaccharide of Mr 1366, which lost two phosphoethanolamine groups upon treatment with 48% aqueous HF. The dephosphorylated hexasaccharide was purified and shown by tandem mass spectrometry, composition analysis, methylation analysis, and two-dimensional nuclear magnetic resonance studies to be Gal beta 1----4Glc beta 1----(Hep alpha 1----2Hep alpha 1----3) 4Hep alpha 1----5anhydro-KDO, where Hep is L-glycero-D-manno-heptose and KDO is 3-deoxy-D-manno-octulosonic acid. An analogous structure containing authentic KDO was generated from LOS that had been HF-treated prior to acetic acid hydrolysis, suggesting that the reducing terminal anhydro-KDO moiety is produced as an artifact of the hydrolysis procedure by beta-elimination of a phosphate substituent from C-4 of KDO. Mass spectral analyses of O-deacylated LOS and free lipid A confirmed that, in addition to the two phosphoethanolamines on the oligosaccharide and two phosphates on the lipid A, another phosphate group exists on the KDO. This KDO does not appear to be further substituted with additional KDO residues in intact H. influenzae 2019 LOS. The terminal disaccharide epitope, Gal beta 1----4Glc beta 1----, of the hexasaccharide is also present on lactosylceramide, a precursor to human blood group antigens. It is postulated that the presence of this structure on H. influenzae LOS may represent a form of host mimicry by the pathogen.

Lipooligosaccharides (LOS) of some Haemophilus species mimic human glycosphingolipids, and some LOS are sialylated

The lipooligosaccharides (LOS) of strains of Haemophilus ducreyi, Neisseria gonorrhoeae, Neisseria meningitidis, and Neisseria lactamica contain epitopes that are antigenically and structurally similar to carbohydrates present in human glycosphingolipids. LOS from strains of Haemophilus influenzae and H. influenzae biogroup aegyptius were tested for the binding of monoclonal antibodies (MAbs) that bind to human glycosphingolipids possessing Gal beta 1-4GlcNAc (MAb 3F11) and Gal alpha 1-4Gal beta 1-4Glc (MAb anti-Pk). In solid-phase radioimmunoassays, the LOS of 18 of 19 H. influenzae type b (Hib), 8 of 19 nontypeable H. influenzae, and 10 of 20 H. influenzae biogroup aegyptius strains bound MAb anti-Pk. The LOS of 13 of 19 Hib, 10 of 16 nontypeable H. influenzae, and 2 of 18 H. influenzae biogroup aegyptius strains bound MAb 3F11. Neuraminidase treatment of the strains increased the binding of MAb 3F11 by more than twofold in 47% of the H. influenzae strains, suggesting that sialic acid occluded the LOS structure recognized by MAb 3F11. The material released from neuraminidase-treated Hib LOS was confirmed to be sialic acid by high-performance anion-exchange chromatography. A recombinant plasmid containing genes involved in Hib LOS biosynthesis directed the expression (assembly) of the 3F11 epitope in Escherichia coli. These studies demonstrate that H. influenzae and H. influenzae biogroup aegyptius express at least two LOS epitopes that are similar to those present in human glycosphingolipids. Sialic acid was present on the LOS of some H. influenzae strains and prevented the binding of MAb 3F11 to its epitope. The oligosaccharide portion of sialylated LOS may also resemble sialylated oligosaccharides present in human glycosphingolipids (gangliosides).

Meningococcal molecular mimicry and the search for an ideal vaccine

The carbohydrates expressed on the surface of meningococcal strains of groups B and C mimic those commonly found on human cells and thus are not functionally antigenic in infancy. In order to develop an effective vaccine, it will be necessary to find ways of circumventing this molecular mimicry. Three possible ways of achieving this are discussed. (i) The surface polysaccharides can theoretically present conformationally different epitopes, some of which might be recognized as antigenic by the host. Experimental evidence is presented that such differences do indeed exist; what is needed is to determine which of these conformations are unique to the organism and hence potentially antigenic. (ii) Precursors of the surface lipooligosaccharides may be unable to mimic human antigens, and so may be potential candidates for vaccine development. (iii) Natural immunity to some strains of meningococci develops in young children who are colonized with strains of Neisseria lactamica, and it is possible that its development could be enhanced by widespread intentional colonization by N. lactamica strains that are particularly efficient inducers of broad immunity.