Phase variation in meningococcal lipooligosaccharide biosynthesis genes.

Neisseria meningitidis expresses a range of lipooligosaccharide (LOS) structures, comprising of at least 13 immunotypes (ITs). Meningococcal LOS is subject to phase variation of its terminal structures allowing switching between ITs, which is proposed to have functional significance in disease. The objectives of this study were to investigate the repertoire of structures that can be expressed in clinical isolates, and to examine the role of phase-variable expression of LOS genes during invasive disease. Southern blotting was used to detect the presence of LOS biosynthetic genes in two collections of meningococci, a global set of strains previously assigned to lineages of greater or lesser virulence, and a collection of local clinical isolates which included paired throat and blood isolates from individual patients. Where the phase-variable genes lgtA, lgtC or lgtG were identified, they were amplified by PCR and the homopolymeric tracts, responsible for their phase-variable expression, were sequenced. The results revealed great potential for variation between alternate LOS structures in the isolates studied, with most strains capable of expressing several alternative terminal structures. The structures predicted to be currently expressed by the genotype of the strains agreed well with conventional immunotyping. No correlation was observed between the structural repertoire and virulence of the isolate. Based on the potential for LOS phase variation in the clinical collection and observations with the paired patient isolates, our data suggest that phase variation of LOS structures is not required for translocation between distinct compartments in the host.

Identification and characterisation of a novel conserved outer membrane protein from Neisseria meningitidis.

We have identified a homologue of the adhesin AIDA-I of Escherichia coli in Neisseria meningitidis. This gene was designated nhhA (Neisseria hia homologue), as analysis of the complete coding sequence revealed that it is more closely related to the adhesins Hia and Hsf of Haemophilus influenzae. The sequence of nhhA was determined from 10 strains, and found to be highly conserved. Studies of the localisation by Western immunoblot analysis of total cell proteins and outer membrane complex preparations and by immunogold electron microscopy revealed that NhhA is located in the outer membrane. A strain survey showed that nhhA is present in 85/85 strains of N. meningitidis representative of all the major disease-associated serogroups, based on Southern blot analysis. It is expressed in the majority of strains tested by Western immunoblot.

Tetranucleotide repeats identify novel virulence determinant homologues in Neisseria meningitidis.

Numerous outer membrane components of Neisseria meningitidis and N. gonorrhoeae exhibit phase variable expression (the rapid, reversible on/off switching of phenotypic expression). Many of the genes encoding these outer membrane components contain simple repetitive DNA motifs (mononucleotides, dinucleotides, tetranucleotides and other repeats) which mediate this variation. One such repeat motif, the tetranucleotide 5;-(GCAA)n-3;, is associated with phase-variable LPS biosynthetic genes in the pathogen Haemophilus influenzae. We have previously shown that N. meningitidis strain MC58 contains this repeat motif in at least three distinct genetic loci. In this study all three of these loci were investigated: two were cloned and identified as novel loci and designated nmrep1 and nmrep2. The third locus was assigned to a previously cloned gene and here is designated nmrep3. The distribution of these loci, and the number of repeat units at each locus was investigated in a range of strains. This analysis revealed that the nmrep1 and nmrep2 loci are present in all 45 strains examined, with 41/45 containing nmrep3. Sequences associated with nmrep1 showed no homology with reported proteins, but amino acid sequences of open reading frames of nmrep2 and nmrep3 exhibited sequence homology to the adhesins Aida of Escherichia coli and Prn of Bordetella sppand IcsA of Shigella flexneri which is involved in intracellular spread.

Tetrameric repeat units associated with virulence factor phase variation in Haemophilus also occur in Neisseria spp. and Moraxella catarrhalis.

The tetrameric repeat units 5'-CAAT-3' and 5'-GCAA-3' are associated with phase variable expression of lipopolysaccharide biosynthetic genes in Haemophilus influenzae. Four other tetrameric repeat units have also been reported from H. influenzae strain Rd, 5'-CAAC-3', 5'-GACA-3', 5'-AGCT-3', and 5'-TTTA-3', which are also associated with putative virulence factors. Using oligonucleotide probes corresponding to five tandem copies of each of these tetramers, we have screened three strains of Neisseria meningitidis and one each of Neisseria gonorrhoeae, Neisseria lactamica, Haemophilus parainfluenzae, Bordetella pertussis, Bordetella parapertussis, Bordetella bronchiceptica and Moraxella catarrhalis for the presence of these motifs. We have demonstrated the presence of multiple copies of the 5'-GCAA-3' motif in all the Neisseria strains tested, and also the repeated motif 5'-CAAC-3' in M. catarrhalis. We have further demonstrated by Southern blot analysis that the 5'-CAAC-3' repeats detected in M. catarrhalis are probably associated with the same genes as in H. influenzae, but that the 5'-GCAA-3' motifs in N. meningitidis are not. The use of characterised tetrameric DNA sequences as hybridisation probes may prove useful in the identification of novel phase variable virulence determinants in organisms other than H. influenzae.

Molecular analysis of a locus for the biosynthesis and phase-variable expression of the lacto-N-neotetraose terminal lipopolysaccharide structure in Neisseria meningitidis.

Lipopolysaccharide (LPS) is a major determinant of Neisseria, meningitidis virulence. A key feature of meningococcal LPS is the phase-variable expression of terminal structures which are proposed to have disparate roles in pathogenesis. In order to identify the biosynthetic genes for terminal LPS structures and the control mechanisms for their phase-variable expression, the lic2A gene, which is involved in LPS biosynthesis in Haemophilus influenzae, was used as a hybridization probe to identify a homologous gene in N. meningitidis strain MC58. The homologous region of DNA was cloned and nucleotide sequence analysis revealed three open reading frames (ORFs), two of which were homologous to the H. influenzae lic2A gene. All three ORFs were mutagenized by the insertion of antibiotic-resistance cassettes and the LPS from these mutant strains was analysed to determine if the genes had a role in LPS biosynthesis. Immunological and tricine-SDS-PAGE analysis of LPS from the mutant strains indicated that all three genes were probably transferases in the biosynthesis of the terminal lacto-N-neotetraose structure of meningococcal LPS. The first ORF of the locus contains a homopolymeric tract of 14 guanosine residues within the 5'-end of the coding sequence. As the lacto-N-neotetraose structure in meningococcal LPS is subject to phase-variable expression, colonies that no longer expressed the terminal structure, as determined by monoclonal antibody binding, were isolated. Analysis of an 'off' phase variant revealed a change in the number of guanosine residues resulting in a frameshift mutation, indicating that a slipped-strand mispairing mechanism, operating in the first ORF, controls the phase-variable expression of lacto-N-neotetraose.

Opc- and pilus-dependent interactions of meningococci with human endothelial cells: molecular mechanisms and modulation by surface polysaccharides.

The interplay between four surface-expressed virulence factors of Neisseria meningitidis (pili, Opc, capsule and lipopolysaccharide (LPS)) in host cell adhesion and invasion was examined using derivatives of a serogroup B strain, MC58, created by mutation (capsule, Opc) and selection of variants. To examine the role of Opc and of additional expression of pili, bacteria lacking the expression of Opa proteins were used. The effects of different LPS structures were examined in variants expressing either sialylated (L3 immunotype) or truncated non-sialylated (L8 immunotype) LPS. Studies showed that (i) pili were essential for meningococcal interactions with host cells in both capsulate and acapsulate bacteria with the sialylated L3 LPS immunotype, (ii) the Opc-mediated invasion of host cells by piliated and non-piliated bacteria was observed only in acapsulate organisms with L8 LPS immunotype, and (iii) expression of pili in Opc-expressing bacteria resulted in increased invasion. Investigations on the mechanisms of cellular invasion indicated that the Opc-mediated invasion was dependent on the presence of serum in the incubation medium and was mediated by serum proteins with arginine-glycine-aspartic acid (RGD) sequence. Cellular invasion in piliated Opc+ phenotype also required bridging molecules containing the RGD recognition sequence and appeared to involve the integrin alpha v beta 3 as a target receptor on endothelial cells. These studies extend the previous observations on variants of a serogroup A strain (C751) and show that Opc mediates cellular invasion in distinct meningococcal strains and provide confirmation of its mechanism of action. This is the first investigation that evaluates, using derivatives of a single strain, the interplay between four meningococcal surface virulence factors in host cell invasion.