Transformational exchanges in the dihydropteroate synthase gene of Neisseria meningitidis: a novel mechanism for acquisition of sulfonamide resistance.

The nucleotide sequences of the chromosomal dihydropteroate synthase (dhps) genes in sulfonamide-susceptible and sulfonamide-resistant strains of Neisseria meningitidis of serogroups A, B and C were determined. The molecular weights and the amino acid sequences showed similarity to those of all other known dihydropteroate synthase polypeptides. Sequence comparison of the N. meningitidis dhps genes indicated horizontal transfer of DNA segments rather than point mutations as the cause for resistance in meningococci. The dhps genes in three of four sulfonamide-resistant meningococci contained identical central regions of 424 bp. Compared with the corresponding genes in susceptible strains, each central region included an insert of 6 bp. In one of the sulfonamide-resistant strains, the dhps gene was similar to the corresponding genes in the sensitive strains in its NH2-terminal and C-terminal parts. Its central region, however, was identical to the corresponding regions of two of the other resistant genes, and thus it could be seen as a hybrid dhps gene. Transformation experiments and mapping of transformed dhps genes indicated the existence of a novel mechanism for the dissemination of sulfonamide resistance in N. meningitidis. The origin of the resistance-mediating segment of the gene is unknown, but hybridization results showed the presence of homologous dhps genes in Neisseria gonorrhoeae and N. lactamica but not in N. subflava or Branhamella catarrhalis.     

PilC of Neisseria meningitidis is involved in class II pilus formation and restores pilus assembly, natural transformation competence and adherence to epithelial cells in PilC-deficient gonococci

Type 4 pili produced by the pathogenic Neisseria species constitute primary determinants for the adherence to host tissues. In addition to the major pilin subunit (PilE), neisserial pili contain the variable PilC proteins represented by two variant gene copies in most pathogenic Neisseria isolates. Based upon structural differences in the conserved regions of PilE, two pilus classes can be distinguished in Neisseria meningitidis . For class I pili found in both Neisseria gonorrhoeae and N. meningitidis , PilC proteins have been implicated in pilus assembly, natural transformation competence and adherence to epithelial cells. In this study, we used primers specific for the pilC2 gene of N. gonorrhoeae strain MS11 to amplify, by the polymerase chain reaction, and clone a homologous pilC gene from N. meningitidis strain A1493 which produces class II pili. This gene was sequenced and the deduced amino acid sequence showed 75.4% and 73.8% identity with the gonococcal PilC1 and PilC2, respectively. These values match the identity value of 74.1% calculated for the two N. gonorrhoeae MS11 PilC proteins, indicating a horizontal relationship between the N. gonorrhoeae and N. meningitidis pilC genes. We provide evidence that PilC functions in meningococcal class II pilus assembly and adherence. Furthermore, expression of the cloned N. meningitidis pilC gene in a gonococcal pilC1,2 mutant restores pilus assembly, adherence to ME-180 epithelial cells, and transformation competence to the wild-type level. Thus, PilC proteins exhibit indistinguishable functions in the context of class I and class II pili.     

Sequence diversity within the argF, fbp and recA genes of natural isolates of Neisseria meningitidis: interspecies recombination within the argF gene

Studies of natural populations of Neisseria meningitidis using multilocus enzyme electrophoresis have shown extensive genetic variation within this species, which, it has been proposed, implies a level of sequence diversity within meningococci that is greater than that normally considered as the criterion for species limits in bacteria. To obtain a direct measure of the sequence diversity among meningococci, we obtained the nucleotide sequences of most of the argF, recA and fbp genes of eight meningococci of widely differing electrophoretic type (from the reference collection of Caugant). Sequence variation between the meningococcal strains ranged from 0-0.6% for fbp, 0-1.3% for argF, and 0-3.3% for recA. These levels of diversity are no greater than those found within Escherichia coli 'housekeeping' genes and suggest that multilocus enzyme electrophoresis may overestimate the extent of nucleotide sequence diversity within meningococci. The average sequence divergence between the Neisseria meningitidis strains and N. gonorrhoeae strain FA19 was 1.0% for fbp and 1.6% for recA. The argF gene, although very uniform among the eight meningococcal isolates, had a striking mosaic structure when compared with the gonococcal argF gene: two regions of the gene differed by greater than 13% in nucleotide sequence between meningococci and gonococci, whereas the rest of the gene differed by less than 1.7%. One of the diverged regions was shown to have been introduced from the argF gene of a commensal Neisseria species that is closely related to Neisseria cinerea. The source of the other region was unclear.     

Networks and groups within the genus Neisseria: analysis of argF, recA, rho, and 16S rRNA sequences from human Neisseria species.

To understand the pattern of nucleotide sequence variation among bacteria that frequently exchange chromosomal genes, we analyzed sequences of the recA, argF, and rho genes, as well as part of the small-subunit (16S) rRNA gene, from about 50 isolates of human commensal Neisseria species and the pathogenic N. meningitidis and N. gonorrhoeae. Almost all isolates of these species could be assigned to five phylogenetic groups that are found for all genes examined and generally are supported by high bootstrap values. In contrast, the phylogenetic relationships among groups varied according to the gene analyzed with notable incongruences involving N. cinerea and N. lactamica. Further analysis using split decomposition showed that for each gene, including 16S rRNA, the patterns of sequence divergence within N. meningitidis and closely related species were inconsistent with a bifurcating treelike phylogeny and better represented by an interconnected network. These data indicate that the human commensal Neisseria species can be separated into discrete groups of related species but that the relationships both within and among these groups, including those reconstructed using 16S rRNA, have been distorted by interspecies recombination events.     

Interspecies recombination between the penA genes of Neisseria meningitidis and commensal Neisseria species during the emergence of penicillin resistance in N. meningitidis: natural events and laboratory simulation.

The penicillin-binding protein 2 genes (penA) of penicillin-resistant Neisseria meningitidis have a mosaic structure that has arisen by the introduction of regions from the penA genes of Neisseria flavescens or Neisseria cinerea. Chromosomal DNA from both N. cinerea and N. flavescens could transform a penicillin-susceptible isolate of N. meningitidis to increased resistance to penicillin. With N. flavescens DNA, transformation to resistance was accompanied by the introduction of the N. flavescens penA gene, providing a laboratory demonstration of the interspecies recombinational events that we believe underlie the development of penicillin resistance in many meningococci in nature. Surprisingly, with N. cinerea DNA, the penicillin-resistant transformants did not obtain the N. cinerea penA gene. However, the region of the penA gene derived from N. cinerea in N. meningitidis K196 contained an extra codon (Asp-345A) which was not found in any of the four N. cinerea isolates that we examined and which is known to result in a decrease in the affinity of PBP 2 in gonococci.     

Detection of the lst gene in different serogroups and LOS immunotypes of Neisseria meningitidis

The sialylation of the lipooligosaccharide (LOS) of Neisseria meningitidis is mediated by the LOS sialyltransferase enzyme encoded by the lst gene. PCR using four sets of primers that targeted to different regions of the lst gene was used to survey the distribution of lst in different serogroups and LOS immunotypes of N. meningitidis as well as other Neisseria species. While the lst gene was found in N. meningitidis strains regardless of their capsular serogroup and LOS structures, the gene is also found in N. gonorrhoeae, N. lactamica, N. polysaccharea, and N. subflava biovar subflava. The presence of the lst gene in these organisms and the sialylation of their LOS antigens were discussed.     

Adaptation to sulfonamide resistance in Neisseria meningitidis may have required compensatory changes to retain enzyme function: kinetic analysis of dihydropteroate synthases from N. meningitidis expressed in a knockout mutant of Escherichia coli.

Previously, the effects of three point mutations (at amino acid positions 31, 84, and 194) in the gene coding for a sulfonamide-resistant dihydropteroate synthase of Neisseria meningitidis were analyzed by site-directed mutagenesis. Changes at positions 31 and 194 abolished the phenotypic expression of sulfonamide resistance, while a change at position 84 appeared to be neutral. These studies are here extended to correlate the alterations in phenotype with effects on enzyme kinetics by expressing the cloned meningococcal genes in an Escherichia coli strain that had its dhps gene partially deleted and replaced by a resistance determinant. The most dramatic effects were produced by mutations at position 31. A change from the Leu found in the resistant isolate to a Phe (the residue found in sensitive isolates) led to a 10-fold decrease in the Km and a concomitant drop in the Ki. Changes at position 194 also affected both the Km and Ki but not to the same extent as mutations at position 31. Changing position 84 altered the Km only slightly but significantly. This latter change was interpreted as a compensatory change modulating the function of the enzyme. In another type of resistance gene, 2 amino acid residues, proposed to be an insertion, were deleted, resulting in a sensitive enzyme. However, the resulting Km was raised 10-fold, suggesting that compensatory changes have accumulated in this type of resistance determinant as well. This resistance gene differs by as much as 10% from the sensitive isolates, which makes identification of important mutations difficult.     

HmbR outer membrane receptors of pathogenic Neisseria spp.: iron-regulated, hemoglobin-binding proteins with a high level of primary structure conservation.

We have recently cloned and characterized the hemoglobin receptor gene from Neisseria meningitidis serogroup C. N. meningitidis cells expressing HmbR protein were able to bind biotinylated hemoglobin, and the binding was specifically inhibited by unlabeled hemoglobin and not heme. The HmbR-mediated hemoglobin binding activity of N. meningitidis cells was shown to be iron regulated. The presence of hemoglobin but not heme in the growth medium stimulated HmbR-mediated hemoglobin binding activity. The efficiency of utilization of different hemoglobins by the HmbR-expressing N. meningitidis cells was shown to be species specific; human hemoglobin was the best source of iron, followed by horse, rat, turkey, dog, mouse, and sheep hemoglobins, The phenotypic characterization of HmbR mutants of some clinical strains of N. meningitidis suggested the existence of two unrelated hemoglobin receptors. The HmbR-unrelated hemoglobin receptor was shown to be identical to Hpu, the hemoglobin-haptoglobin receptor of N. meningitidis. The Hpu-dependent hemoglobin utilization system was not able to distinguish between different sources of hemoglobin; all animal hemoglobins were utilized equally well. HmbR-like genes are also present in N. meningitidis serogroups A and B, Neisseria gonorrhoeae MS11 and FA19, Neisseria perflava, and Neisseria polysaccharea. The hemoglobin receptor genes from N. meningitidis serogroups A and B and N. gonorrhoeae MS11 were cloned, and their nucleotide sequences were determined. The nucleotide sequence identity ranged between 86.5% (for N. meningitidis serogroup B hmbR and MS11 hmbR) and 93.4% (for N. meningitidis serogroup B hmbR and N. meningitidis serogroup C hmbR). The deduced amino acid sequences of these neisserial hemoglobin receptors were also highly related, with overall 84.7% conserved amino acid residues. A stop codon was found in the hmbR gene of N. gonorrhoeae MS11. This strain was still able to use hemoglobin and hemoglobin-haptoglobin complexes as iron sources, indicating that some gonococci may express only the HmbR-independent hemoglobin utilization system.     

Inventory of the proteins in Neisseria meningitidis serogroup B strain MC58

A protein inventory of Neisseria meningitidis strain MC58, a meningococcal strain belonging to the serogroup B, was performed by proteomics. A differential extraction procedure was employed and 238 protein species were identified by 2D mini-maps and MALDI-ToF analyses. In this catalog, we detected protein products from 33 genes, which were not yet annotated in previous N. meningitidis proteomic studies. This approach is suitable for high-throughput studies on differential expression of N. meningitidis genomes.     

A Sco homologue plays a role in defence against oxidative stress in pathogenic Neisseria

Sco proteins are found in mitochondria and in a variety of oxidase positive bacteria. Although Sco is required for the formation of the Cu(A) centre in a cytochrome oxidase of the aa(3) type, it was observed that oxidases with a Cu(A) centre are not present in many bacteria that contain a Sco homologue. Two bacteria of this type are the pathogens Neisseria meningitidis and Neisseria gonorrhoeae. The sco genes of N. gonorrhoeae strain 1291 and N. meningitidis strain MC58 were cloned, inactivated by inserting a kanamycin resistance cassette and used to make knockout mutants by allelic exchange. Both N. gonorrhoeae and N. meningitidis sco mutants were highly sensitive to oxidative killing by paraquat, indicating that Sco is involved in protection against oxidative stress in these bacteria.     

Deoxyribonucleic Acid Homologies Among Species of the Genus Neisseria

Eleven aerobic species of Neisseria, a Mima sp., and a Herellea sp. were tested for deoxyribonucleic acid (DNA) homology in direct hybridization experiments. DNA labeled with either (14)C or (32)P was prepared from five species of Neisseria. Unlabeled DNA from the various microorganisms was immobilized on membrane filters, which, after pretreatment, were incubated with labeled DNA (4,000 counts per min per filter) for 14 hr at 67 C. The measure of relatedness was expressed as the relative percentage of direct binding compared to that obtained with homologous DNA. All serological types of N. meningitidis, including the newly proposed types, were homologous to the standard strain of N. meningitidis with one possible exception, type Z. The genus Neisseria is heterogeneous in nature, forming at least three distinct groups: first, N. meningitidis and N. gonorrhoeae; second, N. perflava, N. subflava, N. sicca, N. flavescens, and N. flava; third, N. catarrhalis and N. caviae. Mima and Herellea species show no significant homology with the Neisseria.     

Transposition of Tn1545-delta 3 in the pathogenic Neisseriae: a genetic tool for mutagenesis.

The ability to study the virulence of pathogenic Neisseria spp. has been greatly limited by the absence of genetic tools which allow the construction of defined mutants. We have engineered a transposon system which allows random mutagenesis of the Neisseria genome at relatively high frequency. Tn1545-delta 3 is a 3.4-kb derivative of the gram-positive transposon Tn1545 encoding resistance to kanamycin. Tn1545-delta 3 was subcloned into an erythromycin-resistant derivative of the mobilizable shuttle vector pLES2 to yield the plasmid pMGC20. This latter plasmid was introduced by conjugation from Escherichia coli S17-1 into Neisseria meningitidis 8013N and Neisseria gonorrhoeae 15063G. Kanamycin-resistant 8013N and 15063G transconjugants appeared at frequencies of 10(-5) and 10(-6), respectively. Restriction enzyme analysis and Southern blot hybridization of these transconjugants showed that, in Neisseria spp., the transposon excised spontaneously from pMGC20 and integrated into chromosomal DNA. Our studies revealed that (i) transposition of Tn1545-delta 3 was in numerous, apparently distinct sites, (ii) in most cases, for each transconjugant a single copy of Tn1545-delta 3 was integrated into the chromosome, and (iii) several passages on selective media did not induce secondary transposition. The kanamycin resistance marker expressed by the transconjugants was subsequently transformed into a parental background without change in the chromosomal location of the transposon. To assess the role of the general recombination system in the transposition of Tn1545-delta 3, the recA gene of N. meningitidis has been cloned and a rec derivative of 8013N has been engineered. Similar results were obtained when this latter strain was used as recipient, suggesting that recA function were not required for Tn1545-delta 3 transposition in N. meningitidis. Transposition with Tn1545-delta 3 may be an important technique for mutagenesis of the pathogenic neisseriae.     

Identification and characterization of specific sequences encoding pathogenicity associated proteins in the genome of commensal Neisseria species

Abstract The distribution of distinct sequences in pathogenic and commensal Neisseria species was investigated systematically by dot blot analysis. Probes representing the genes of Rmp, pilin and IgA1 protease were found to hybridize exclusively to the chromosomal DNA of the pathogenic species, Neisseria gonorrhoeae and/or Neisseria meningitidis . In contrast, specific sequences for the genes of the porin protein Por and the opacity protein (Opa) were also detected in a panel of commensal Neisseria species such as N. lactamica, N. subflava, N, flava, N. mucosa and N. sicca . Using opa -specific oligonucleotides as probes in chromosomal blots, the genomes of the commensal Neisseria species show a totally reduced repertoire of cross-hybridizing loci compared to the complex opa gene family of N. gonorrhoeae . DNA sequence analysis of one opa -related gene derived from N. flava and N. sicca , respectively, revealed a large degree of homology with previously described gonococcal and meningococcal genes e.g., a typical repetitive sequence in the leader peptide and the distribution of the hypervariable and conserved regions. This observation, together with the finding, that the gene is constitutively transcribed, leads to the assumption that some of the commensal Neisseria species may have the potential for the expression of a protein harboring similar functions as the Opa proteins in pathogenic Neisseriae .     

Ecological separation and genetic isolation of Neisseria gonorrhoeae and Neisseria meningitidis

BACKGROUND: Classifying bacteria into species is problematic. Most microbiologists consider species to be groups of isolates that share some arbitrary degree of relatedness of biochemical or molecular (such as DNA sequence) features and that, ideally, are clearly delineated from all other groups of isolates. The main problem in applying to bacteria a biological concept of species based on the ability or inability of their genes to recombine, is that recombination appears to be rare in bacteria in nature, as indicated by the strong linkage disequilibrium between alleles found in most bacterial populations. However, there are some naturally transformable bacteria in which assortative recombination appears to be so frequent that alleles are in, or close to, linkage equilibrium. For these recombining populations a biological concept of species might be applicable. RESULTS: Populations of Neisseria gonorrhoeae and Neisseria meningitidis from Spain were analysed by multilocus enzyme electrophoresis. The data indicate that assortative recombination occurs frequently within populations, but not between populations. Similarly, the sequences of two house-keeping genes show no evidence of intragenic recombination between N. gonorrhoeae and N. meningitidis. CONCLUSIONS: N. gonorrhoeae and N. meningitidis represent extremely closely related 'sexual' populations that appear to be genetically isolated in nature, and thus conform to the biological concept of species. The extreme uniformity of N. gonorrhoeae house-keeping genes suggests that this species may have arisen recently as a clone of N. meningitidis that could colonize the genital tract. Ecological isolation - of populations that can colonize the genital tract from those that can colonize the nasopharynx - may have been an important component in speciation, leading to a lower frequency of recombination between species than within species.     

Comparative characterization of the iga gene encoding IgA1 protease in Neisseria meningitidis, Neisseria gonorrhoeae and Haemophilus influenzae

Cloning and sequencing of the IgA1 protease gene (iga) from Neisseria meningitidis strain HF13 showed an overall structure equivalent to iga genes from Neisseria gonorrhoeae and Haemophilus influenzae, although no region corresponding to the gonococcal α-peptide was evident. An additional 18 N. meningitidis and 3 H. influenzae iga genes were amplified by the polymerase chain reaction technique and sequenced corresponding approximately to the N-terminal half of the mature enzyme. Comparative analyses of a total of 29 iga genes showed that pathogenic Neisseria have iga genes with a significantly lower degree of heterogeneity than H. influenzae iga genes. Recombinational events indicated by mosaic-like structures corresponding to those found among N. gonorrhoeae protease genes were detected among N. meningitidis iga genes. One region showed characteristic differences in sequence and length which correlated with each of the different cleavage specificities. Meningococci were extremely conserved in this region with no evidence of recombination between isolates of different cleavage specificities. Sequences further downstream showed no obvious relationship with enzyme cleavage type. This region consisted of conserved areas interspersed with highly variable areas. Amino acid sequence homologies in the variable regions of meningococci reflected the antigenic types defined by using polyclonal neutralizing antibodies.     

Genome analysis and strain comparison of correia repeats and correia repeat-enclosed elements in pathogenic Neisseria

Whole genome sequences of Neisseria meningitidis strains Z2491 and MC58 and Neisseria gonorrhoeae FA1090 were analyzed for Correia repeats (CR) and CR-enclosed elements (CREE). A total of 533, 516, and 256 copies of CR and 270, 261, and 102 copies of CREE were found in these three genomes, respectively. The lengths of CREE range from 28 to 348 bp, and the lengths of multicopy CREE appear mainly in the ranges of 154 to 156 bp and 105 to 107 bp. The distribution of CREE lengths is similar between the two N. meningitidis genomes, with a greater number of 154- to 156-bp CREE (163 and 152 copies in N. meningitidis strain Z2491 and N. meningitidis strain MC58, respectively) than 105- to 107-bp CREE (72 and 77 copies). In the N. gonorrhoeae strain FA1090 genome there are relatively more 105- to 107-bp CREE (51 copies) than 154- to 156-bp CREE (36 copies). The genomic distribution of 107-bp CREE also shows similarity between the two N. meningitidis strains (15 copies share the same loci) and differences between N. meningitidis strains and N. gonorrhoeae FA1090 (only one copy is located in the same locus). Detailed sequence analysis showed that both the terminal inverted repeats and the core regions of CREE are composed of distinct basic sequence blocks. Direct TA dinucleotide repeats exist at the termini of all CREE. A survey of DNA sequence upstream of the sialyltransferase gene, lst, in several Neisseria isolates showed that 5 N. meningitidis strains contain a 107-bp CREE in this region but 25 N. gonorrhoeae strains show an exact absence of a 105-bp sequence block (i.e., the 107-bp CREE without a 5' TA dinucleotide) in the same region. Whole-genome sequence analysis confirmed that this 105-bp indel exists in many homologous 107-bp CREE loci. Thus, we postulate that all CREE are made of target TA with indels of various lengths. Analysis of 107-bp CREE revealed that they exist predominantly in intergenic regions and are often near virulence, metabolic, and transporter genes. The abundance of CREE in Neisseria genomes suggests that they may have played a role in genome organization, function, and evolution. Their differential distribution in different pathogenic Neisseria strains may contribute to the distinct behaviors of each Neisseria species.