Studies on gonococcus infection. II. Freeze-fracture, freeze-etch studies on gonocci

Low Oxygen Concentration Reduces Neisseria gonorrhoeae Susceptibility to Resazurin

Neisseria gonorrhoeae has developed resistance to every antibiotic currently approved for the treatment of gonorrhea, prompting the development of new therapies. The phenoxazine dye resazurin exhibits robust antimicrobial activity against N. gonorrhoeae in vitro but fails to limit vaginal colonization by N. gonorrhoeae in a mouse model. The lack of in vivo efficacy may be due to oxygen limitation as in vitro susceptibility assays with resazurin are conducted under atmospheric oxygen while a microaerophilic environment is present in the vagina. Here, we utilized broth microdilution assays to determine the susceptibility of N. gonorrhoeae to resazurin under low and atmospheric oxygen conditions. The minimal inhibitory concentration of resazurin for multiple N. gonorrhoeae clinical isolates was significantly higher under low oxygen. This effect was specific to resazurin as N. gonorrhoeae was equally susceptible to other antibiotics under low and atmospheric oxygen conditions. The reduced susceptibility of N. gonorrhoeae to resazurin under low oxygen was largely attributed to reduced oxidative stress, as the addition of antioxidants under atmospheric oxygen mimicked the reduced susceptibility to resazurin observed under low oxygen. Together, these data suggest oxygen concentration is an important factor to consider when evaluating the efficacy of new antibiotics against N. gonorrhoeae in vitro.

Expression, Localization, and Protein Interactions of the Partitioning Proteins in the Gonococcal Type IV Secretion System

Partitioning proteins are well studied as molecular organizers of chromosome and plasmid segregation during division, however little is known about the roles partitioning proteins can play within type IV secretion systems. The single-stranded DNA (ssDNA)-secreting gonococcal T4SS has two partitioning proteins, ParA and ParB. These proteins work in collaboration with the relaxase TraI as essential facilitators of type IV secretion. Bacterial two-hybrid experiments identified interactions between each partitioning protein and the relaxase. Subcellular fractionation demonstrated that ParA is found in the cellular membrane, whereas ParB is primarily in the membrane, but some of the protein is in the soluble fraction. Since TraI is known to be membrane-associated, these data suggest that the gonococcal relaxosome is a membrane-associated complex. In addition, we found that translation of ParA and ParB is controlled by an RNA switch. Different mutations within the stem-loop sequence predicted to alter folding of this RNA structure greatly increased or decreased levels of the partitioning proteins.

Complete Genome Sequences of Seven Neisseria gonorrhoeae Clinical Isolates from Mucosal and Disseminated Gonococcal Infections

Neisseria gonorrhoeae is a Gram-negative bacterium that causes the sexually transmitted infection gonorrhea. N. gonorrhoeae has progressively developed resistance to all currently prescribed antibiotics, and no vaccine is available. Here, we report the closed, completed, annotated genome sequences for seven N. gonorrhoeae strains obtained by single-molecule real-time (SMRT) long-read genome sequencing.

The serogroup B meningococcal outer membrane vesicle-based vaccine 4CMenB induces cross-species protection against Neisseria gonorrhoeae

There is a pressing need for a gonorrhea vaccine due to the high disease burden associated with gonococcal infections globally and the rapid evolution of antibiotic resistance in Neisseria gonorrhoeae (Ng). Current gonorrhea vaccine research is in the stages of antigen discovery and the identification of protective immune responses, and no vaccine has been tested in clinical trials in over 30 years. Recently, however, it was reported in a retrospective case-control study that vaccination of humans with a serogroup B Neisseria meningitidis (Nm) outer membrane vesicle (OMV) vaccine (MeNZB) was associated with reduced rates of gonorrhea. Here we directly tested the hypothesis that Nm OMVs induce cross-protection against gonorrhea in a well-characterized female mouse model of Ng genital tract infection. We found that immunization with the licensed Nm OMV-based vaccine 4CMenB (Bexsero) significantly accelerated clearance and reduced the Ng bacterial burden compared to administration of alum or PBS. Serum IgG and vaginal IgA and IgG that cross-reacted with Ng OMVs were induced by 4CMenB vaccination by either the subcutaneous or intraperitoneal routes. Antibodies from vaccinated mice recognized several Ng surface proteins, including PilQ, BamA, MtrE, NHBA (known to be recognized by humans), PorB, and Opa. Immune sera from both mice and humans recognized Ng PilQ and several proteins of similar apparent molecular weight, but MtrE was only recognized by mouse serum. Pooled sera from 4CMenB-immunized mice showed a 4-fold increase in serum bactericidal50 titers against the challenge strain; in contrast, no significant difference in bactericidal activity was detected when sera from 4CMenB-immunized and unimmunized subjects were compared. Our findings directly support epidemiological evidence that Nm OMVs confer cross-species protection against gonorrhea, and implicate several Ng surface antigens as potentially protective targets. Additionally, this study further defines the usefulness of murine infection model as a relevant experimental system for gonorrhea vaccine development.

Neisseria gonorrhoeae PBP3 and PBP4 Facilitate NOD1 Agonist Peptidoglycan Fragment Release and Survival in Stationary Phase

Neisseria gonorrhoeae releases peptidoglycan fragments during growth, and these molecules induce an inflammatory response in the human host. The proinflammatory molecules include peptidoglycan monomers, peptidoglycan dimers, and free peptides. These molecules can be released by the actions of lytic transglycosylases or an amidase. However, >40% of the gonococcal cell wall is cross-linked, where the peptide stem on one peptidoglycan strand is linked to the peptide stem on a neighboring strand, suggesting that endopeptidases may be required for the release of many peptidoglycan fragments. Therefore, we characterized mutants with individual or combined mutations in genes for the low-molecular-mass penicillin-binding proteins PBP3 and PBP4. Mutations in either dacB, encoding PBP3, or pbpG, encoding PBP4, did not significantly reduce the release of peptidoglycan monomers or free peptides. A mutation in dacB caused the appearance of a larger-sized peptidoglycan monomer, the pentapeptide monomer, and an increased release of peptidoglycan dimers, suggesting the involvement of this enzyme in both the removal of C-terminal d-Ala residues from stem peptides and the cleavage of cross-linked peptidoglycan. Mutation of both dacB and pbpG eliminated the release of tripeptide-containing peptidoglycan fragments concomitantly with the appearance of pentapeptide and dipeptide peptidoglycan fragments and higher-molecular-weight peptidoglycan dimers. In accord with the loss of tripeptide peptidoglycan fragments, the level of human NOD1 activation by the dacB pbpG mutants was significantly lower than that by the wild type. We conclude that PBP3 and PBP4 overlap in function for cross-link cleavage and that these endopeptidases act in the normal release of peptidoglycan fragments during growth.

Selective Inhibition of Neisseria gonorrhoeae by a Dithiazoline in Mixed Infections with Lactobacillus gasseri

The Gram-negative human pathogen Neisseria gonorrhoeae has progressively developed resistance to antibiotic monotherapies, and recent failures of dual-drug therapy have heightened concerns that strains resistant to all available antibiotics will begin circulating globally. Targeting bacterial cell wall assembly has historically been effective at treating infections with N. gonorrhoeae, but as the effectiveness of β-lactams (including cephalosporins) is challenged by increasing resistance, research has expanded into compounds that target the numerous other enzymes with roles in peptidoglycan metabolism. One example is the dithiazoline compound JNJ-853346 (DTZ), which inhibits the activity of an Escherichia coli serine protease l,d-carboxypeptidase (LdcA). Recently, the characterization of an LdcA homolog in N. gonorrhoeae revealed localization and activity differences from the characterized E. coli LdcA, prompting us to explore the effectiveness of DTZ against N. gonorrhoeae We found that DTZ is effective at inhibiting N. gonorrhoeae in all growth phases, unlike the specific stationary-phase inhibition seen in E. coli Surprisingly, DTZ does not inhibit gonococcal LdcA enzyme activity, and DTZ sensitivity is not significantly decreased in ldcA mutants. While effective against numerous N. gonorrhoeae strains, including recent multidrug-resistant isolates, DTZ is much less effective at inhibiting growth of the commensal species Lactobacillus gasseri DTZ treatment during coinfections of epithelial cells resulted in significant lowering of gonococcal burden and interleukin-8 secretion without significantly impacting recovery of viable L. gasseri This selective toxicity presents a possible pathway for the use of DTZ as an effective antigonococcal agent at concentrations that do not impact vaginal commensals.

Neisseria gonorrhoeae Crippled Its Peptidoglycan Fragment Permease To Facilitate Toxic Peptidoglycan Monomer Release

Neisseria gonorrhoeae (gonococci) and Neisseria meningitidis (meningococci) are human pathogens that cause gonorrhea and meningococcal meningitis, respectively. Both N. gonorrhoeae and N. meningitidis release a number of small peptidoglycan (PG) fragments, including proinflammatory PG monomers, although N. meningitidis releases fewer PG monomers. The PG fragments released by N. gonorrhoeae and N. meningitidis are generated in the periplasm during cell wall remodeling, and a majority of these fragments are transported into the cytoplasm by an inner membrane permease, AmpG; however, a portion of the PG fragments are released into the extracellular environment through unknown mechanisms. We previously reported that the expression of meningococcal ampG in N. gonorrhoeae reduced PG monomer release by gonococci. This finding suggested that the efficiency of AmpG-mediated PG fragment recycling regulates the amount of PG fragments released into the extracellular milieu. We determined that three AmpG residues near the C-terminal end of the protein modulate AmpG's efficiency. We also investigated the association between PG fragment recycling and release in two species of human-associated nonpathogenic Neisseria: N. sicca and N. mucosa Both N. sicca and N. mucosa release lower levels of PG fragments and are more efficient at recycling PG fragments than N. gonorrhoeae Our results suggest that N. gonorrhoeae has evolved to increase the amounts of toxic PG fragments released by reducing its PG recycling efficiency.

IMPORTANCE

Neisseria gonorrhoeae and Neisseria meningitidis are human pathogens that cause highly inflammatory diseases, although N. meningitidis is also frequently found as a normal member of the nasopharyngeal microbiota. Nonpathogenic Neisseria, such as N. sicca and N. mucosa, also colonize the nasopharynx without causing disease. Although all four species release peptidoglycan fragments, N. gonorrhoeae is the least efficient at recycling and releases the largest amount of proinflammatory peptidoglycan monomers, partly due to differences in the recycling permease AmpG. Studying the interplay between bacterial physiology (peptidoglycan metabolism) and pathogenesis (release of toxic monomers) leads to an increased understanding of how different bacterial species maintain asymptomatic colonization or cause disease and may contribute to efforts to mitigate disease.

TraK and TraB are conserved outer membrane proteins of the Neisseria gonorrhoeae Type IV secretion system and are expressed at low levels in wild-type cells

Neisseria gonorrhoeae uses a type IV secretion system (T4SS) to secrete chromosomal DNA into the medium, and this DNA is effective in transforming other gonococci via natural transformation. In addition, the T4SS is important in the initial stages of biofilm development and mediates intracellular iron uptake in the absence of TonB. To better understand the mechanism of type IV secretion in N. gonorrhoeae, we examined the expression levels and localization of two predicted T4SS outer membrane proteins, TraK and TraB, in the wild-type strain as well as in overexpression strains and in a strain lacking all of the T4SS proteins. Despite very low sequence similarity to known homologues, TraB (VirB10 homolog) and TraK (VirB9 homolog) localized similarly to related proteins in other systems. Additionally, we found that TraV (a VirB7 homolog) interacts with TraK, as in other T4SSs. However, unlike in other systems, neither TraK nor TraB required the presence of other T4SS components for proper localization. Unlike other gonococcal T4SS proteins we have investigated, protein levels of the outer membrane proteins TraK and TraB were extremely low in wild-type cells and were undetectable by Western blotting unless overexpressed or tagged with a FLAG3 triple-epitope tag. Localization of TraK-FLAG3 in otherwise wild-type cells using immunogold electron microscopy of thin sections revealed a single gold particle on some cells. These results suggest that the gonococcal T4SS may be present in single copy per cell and that small amounts of T4SS proteins TraK and TraB are sufficient for DNA secretion.

Peptidoglycan fragment release from Neisseria meningitidis

Neisseria meningitidis (meningococcus) is a symbiont of the human nasopharynx. On occasion, meningococci disseminate from the nasopharynx to cause invasive disease. Previous work showed that purified meningococcal peptidoglycan (PG) stimulates human Nod1, which leads to activation of NF-κB and production of inflammatory cytokines. No studies have determined if meningococci release PG or activate Nod1 during infection. The closely related pathogen Neisseria gonorrhoeae releases PG fragments during normal growth. These fragments induce inflammatory cytokine production and ciliated cell death in human fallopian tubes. We determined that meningococci also release PG fragments during growth, including fragments known to induce inflammation. We found that N. meningitidis recycles PG fragments via the selective permease AmpG and that meningococcal PG recycling is more efficient than gonococcal PG recycling. Comparison of PG fragment release from N. meningitidis and N. gonorrhoeae showed that meningococci release less of the proinflammatory PG monomers than gonococci and degrade PG to smaller fragments. The decreased release of PG monomers by N. meningitidis relative to N. gonorrhoeae is partly due to ampG, since replacement of gonococcal ampG with the meningococcal allele reduced PG monomer release. Released PG fragments in meningococcal supernatants induced significantly less Nod1-dependent NF-κB activity than released fragments in gonococcal supernatants and tended to induce less interleukin-8 (IL-8) secretion in primary human fallopian tube explants. These results support a model in which efficient PG recycling and extensive degradation of PG fragments lessen inflammatory responses and may be advantageous for maintaining meningococcal carriage in the nasopharynx.

Mating pair formation homologue TraG is a variable membrane protein essential for contact-independent type IV secretion of chromosomal DNA by Neisseria gonorrhoeae

Neisseria gonorrhoeae uses a type IV secretion system (T4SS) to secrete chromosomal DNA into the surrounding milieu. The DNA is effective in transforming gonococci in the population, and this mechanism of DNA donation may contribute to the high degree of genetic diversity in this species. Similar to other F-like T4SSs, the gonococcal T4SS requires a putative membrane protein, TraG, for DNA transfer. In F-plasmid and related systems, the homologous protein acts in pilus production, mating pair stabilization, and entry exclusion. We characterized the localization, membrane topology, and variation of TraG in N. gonorrhoeae. TraG was found to be an inner-membrane protein with one large periplasmic region and one large cytoplasmic region. Each gonococcal strain carried one of three different alleles of traG. Strains that carried the smallest allele of traG were found to lack the peptidoglycanase gene atlA but carried a peptidoglycan endopeptidase gene in place of atlA. The purified endopeptidase degraded gonococcal peptidoglycan in vitro, cutting the peptide cross-links. Although the other two traG alleles functioned for DNA secretion in strain MS11, the smallest traG did not support DNA secretion. Despite the requirement for a mating pair stabilization homologue, static coculture transformation experiments demonstrated that DNA transfer was nuclease sensitive and required active uptake by the recipient, thus demonstrating that transfer occurred by transformation and not conjugation. Together, these results demonstrate the TraG acts in a process of DNA export not specific to conjugation and that different forms of TraG affect what substrates can be transported.

The physical basis of type 4 pilus-mediated microcolony formation by Vibrio cholerae O1

The Vibrio cholerae toxin co-regulated pilus (TCP) is a type 4b pilus that mediates bacterial microcolony formation, which is essential for intestinal colonization. Structural analyses have defined a surface domain of the TcpA pilin subunit that is displayed repeatedly around the pilus filament surface and forms the molecular basis for pilus-pilus interactions required for microcolony formation. The physical attributes of this domain that lead to pilus-pilus association between bacteria are not known. Mutational analysis has revealed alterations within this domain that allow pilus-pilus interactions among pili expressed by individual bacteria, but do not allow pilus-pilus mediated association between bacteria. We characterized these altered strains using conventional microscopy, as well as three-dimensional high-resolution field emission scanning electron microscopy (FESEM), to reveal the physical difference between nonproductive and productive pilus associations that lead to interactions among multiple bacteria and result in microcolony formation. These findings pave the way towards investigation of the biophysical parameters involved in this basic bacterial property that promotes colonization of intestinal and other biological surfaces.

XerCD-mediated site-specific recombination leads to loss of the 57-kilobase gonococcal genetic island

Most strains of Neisseria gonorrhoeae carry the 57-kb gonococcal genetic island (GGI), as do a few strains of Neisseria meningitidis. The GGI is inserted into the chromosome at the dif site (difA) and is flanked by a partial repeat of the dif site (difB). Since dif is a sequence recognized by the site-specific recombinases XerC and XerD and the GGI shows evidence of horizontal acquisition, we hypothesized that the GGI may be acquired or lost by XerCD-mediated site-specific recombination. We show that while the GGI flanked by wild-type dif sites, difA and difB, is not readily lost from the gonococcal chromosome, the substitution of difB with another copy of difA allows the frequent excision and loss of the GGI. In mutants carrying two difA sites (difA(+) difA(+)), the GGI can be detected as an extrachromosomal circle that exists transiently. A mutation of xerD diminished GGI excision from the chromosome of a difA(+) difA(+) strain, while mutations in recA or type IV secretion genes had no effect on the loss of the GGI. These data indicate that the GGI is maintained by the replication of the chromosome and that GGI excision and loss are dependent upon the dif sequence and xerD. The detection of a circular form of the GGI in a wild-type strain suggests that GGI excision may occur naturally and could function to facilitate GGI transfer. These data suggest a model of GGI excision and loss explaining the absence of the GGI from some gonococcal strains and the maintenance of variant GGIs in some gonococcal and meningococcal isolates.

Type IV secretion machinery promotes ton-independent intracellular survival of Neisseria gonorrhoeae within cervical epithelial cells

Survival of Neisseria gonorrhoeae within host epithelial cells is expected to be important in the pathogenesis of gonococcal disease. We previously demonstrated that strain FA1090 derives iron from a host cell in a process that requires the Ton complex and a putative TonB-dependent transporter, TdfF. FA1090, however, lacks the gonococcal genetic island (GGI) that is present in the majority of strains. The GGI in strain MS11 has been partially characterized, and it encodes a type IV secretion system (T4SS) involved in DNA release. In this study we investigated the role of iron acquisition and GGI-encoded gene products in gonococcal survival within cervical epithelial cells. We demonstrated that intracellular survival of MS11 was dependent on acquisition of iron from the host cell, but unlike the findings for FA1090, expression of the Ton complex was not required. Survival was not dependent on a putative TonB-like protein encoded in the GGI but instead was directly linked to T4SS structural components in a manner independent of the ability to release or internalize DNA. These data suggest that expression of selected GGI-encoded open reading frames confers an advantage during cervical cell infection. This study provides the first link between expression of the T4SS apparatus and intracellular survival of gonococci.

Increased expression of the type IV secretion system in piliated Neisseria gonorrhoeae variants

Neisseria gonorrhoeae produces a type IV secretion system that secretes chromosomal DNA. The secreted DNA is active in the transformation of other gonococci in the population and may act to transfer antibiotic resistance genes and variant alleles for surface antigens, as well as other genes. We observed that gonococcal variants that produced type IV pili secreted more DNA than variants that were nonpiliated, suggesting that the process may be regulated. Using microarray analysis, we found that a piliated strain showed increased expression of the gene for the putative type IV secretion coupling protein TraD, whereas a nonpiliated variant showed increased expression of genes for transcriptional and translational machinery, consistent with its higher growth rate compared to that of the piliated strain. These results suggested that type IV secretion might be controlled by either traD expression or growth rate. A mutant with a deletion in traD was found to be deficient in DNA secretion. Further mutation and complementation analysis indicated that traD is transcriptionally and translationally coupled to traI, which encodes the type IV secretion relaxase. We were able to increase DNA secretion in a nonpiliated strain by inserting a gene cassette with a strong promoter to drive the expression of the putative operon containing traI and traD. Together, these data suggest a model in which the type IV secretion system apparatus is made constitutively, while its activity is controlled through regulation of traD and traI.

Mannose-binding lectin is present in human semen and modulates cellular adhesion of Neisseria gonorrhoeae in vitro

Mannose-binding lectin (MBL) is an innate immune molecule present in blood and some mucosal tissues, which can influence microbial attachment and inflammatory responses of host cells during infection. In this study MBL was found to be present at a low concentration in semen samples in the range 1.2-24.9 ng/ml. Co-incubation of bacteria with semen resulted in the binding of MBL to the bacterial surface. Neisseria gonorrhoeae is a common cause of genitourinary infection. MBL bound to N. gonorrhoeae with strain-to-strain variation in the intensity of binding and nature of the bacterial receptor. Pretreatment with MBL concentrations similar to those found in human serum modulated the adhesion of N. gonorrhoeae strain FA1090 but not strain MS11 to epithelial cells. This effect was dose-dependent. This work demonstrates that MBL is present in human semen and modifies cellular responses to N. gonorrhoeae in a concentration-dependent manner.

Pilin antigenic variation occurs independently of the RecBCD pathway in Neisseria gonorrhoeae

Type IV pilus expression has been strongly implicated in the virulence of Neisseria gonorrhoeae, the causative agent of gonorrhea. In Neisseria, these pili undergo frequent antigenic variation (Av), which is presumed to allow reinfection of high-risk groups. Pilin Av is the result of RecA-mediated recombination events between the gene encoding the major pilin subunit (pilE) and multiple silent pilin locus (pilS) copies, utilizing a RecF-like recombination pathway. The role of RecBCD in pilin Av has been controversial. Previous studies measuring pilin Av in recB and recD mutants in two independent strains of N. gonorrhoeae (MS11 and FA1090) by indirect methods yielded conflicting results. In addition, these two laboratory strains have been suggested to express very different DNA repair capabilities. We show that the FA1090 and MS11 parental strains have similar abilities to repair DNA damage via UV-induced DNA damage, nalidixic acid-induced double-strand breaks, and methyl methanesulfonate-induced alkylation and that RecB and RecD are involved in the repair of these lesions. To test the role of the RecBCD pathway in pilin Av, the rate and frequency of pilin Av were directly measured by sequencing the pilE locus in randomly selected piliated progeny of both MS11 and FA1090 in recB and recD mutants. Our results definitively show that recB and recD mutants undergo pilin Av at rates similar to those of the parents in both strain backgrounds, demonstrating that efficient pilin Av is neither enhanced nor inhibited by the RecBCD complex.

Opposing selective forces for expression of the gonococcal lactoferrin receptor

All isolates of Neisseria gonorrhoeae express receptors that bind human transferrin (Tf). Although lactoferrin (Lf) is abundant on mucosa and in purulent exudates, many gonococci do not express an Lf receptor. The naturally occurring Lf receptor deletion mutant FA1090 (LbpB-LbpA-) is infectious, but a Tf receptor mutant of FA1090 is unable to infect male volunteers [Cornelissen, C.N., Kelley, M., Hobbs, M.M., Anderson, J.E., Cannon, J.G., Cohen, M.S., and Sparling, P.F. (1998) Mol Microbiol 27: 611-616]. Here, we report that expression of an Lf receptor in the absence of the Tf receptor was sufficient for infection, and that expression of both Lf and Tf receptors resulted in a competitive advantage over a strain that made only the Tf receptor in mixed infection of male volunteers. We confirmed that nearly 50% of clinical isolates do not make an Lf receptor. Surprisingly, about half of geographically diverse Lf - isolates representing many different auxotypes and porin serovars carried an identical lbpB lbpA deletion. Among Lf+ strains, all produced the integral outer membrane protein LbpA, but 70% did not express the lipoprotein LbpB. Thus, there are apparently selective pressures for expression of the Lf receptor in the male urethra that are balanced by others against expression of the Lf receptor in niches other than the male urethra.

A gonococcal porA pseudogene: implications for understanding the evolution and pathogenicity of Neisseria gonorrhoeae

Members of the genus Neisseria, including the human pathogens Neisseria meningitidis and Neisseria gonorrhoeae, express at least one member of a family of related porins. N. meningitidis is the only species known to express a second porin, the meningococcal serosubtyping antigen PorA, the most divergent member of this family. Unexpectedly, a porA gene was identified in the gonococcal genome. Both the gonococcal and meningococcal porA loci were adjacent to a homologue of the Escherichia coli greA gene, although the IS1106 element downstream of porA in some meningococci was absent in the gonococcus. Almost identical porA loci were present in four unrelated gonococcal isolates and clinical specimens from patients with gonorrhoea. Lack of PorA expression in the gonococcus resulted from mutations in the promoter region, which prevented transcription, and frameshift mutations in the coding region of the porA gene. Hybridization and amplification experiments, showing the absence of a porA gene in seven other Neisseria species, suggested that porA was acquired by a common ancestor of the gonococcus and meningococcus but inactivated in the gonococcus on speciation. This implies that, while advantageous during colonization of the upper respiratory tract, this protein has no function in, or hinders, colonization of the urogenital tract.

Transcriptional control of gonococcal pilE expression: involvement of an alternate sigma factor

The pilE gene encoding Neisseria gonorrhoeae (Gc) pilin contains two putative promoter sequences 5' to the transcription start point (tsp), a Pribnow box and an RpoN-binding consensus sequence. Sequence analysis shows that the nucleotide (nt) sequence of the pilE promoter region is completely conserved among eight different Gc isolates. Using a pilE::lacZ transcriptional fusion, we demonstrate that the RpoN sigma factor can function in Escherichia coli to increase pilE transcription when the NifA activator from Klebsiella is present in trans. In addition, over-production of the native pilin protein using RpoN and NifA is lethal to E. coli. Finally, we show that the RpoN sigma factor decreases the basal expression of pilE when an activator is not present. These results suggest that, in Gc, pilE transcription may be regulated by RpoN in conjunction with an activator and that sigma 70 can also act to direct transcription of pilE.

Scanning electron microscopy of piliated Neisseria gonorrhoeae processed with hexamethyldisilazane

Piliated Neisseria gonorrhoeae are virulent and attach readily to some human mucosal cells. The study of interactions between piliated Neisseria gonorrhoeae and surface structures of eukaryotic cells in tissue culture requires consistent high resolution imaging in scanning electron microscopy (SEM). The combination of the fixatives glutaraldehyde, osmium, tannic acid, and uranyl acetate improves preservation of pili and other delicate structures. Following the critical point drying (CPD) process, pili bundles remained intact, but charging produced image distortion in most of the specimens. The use of hexamethyldisilazane (HMDS) with air drying substantially reduced charging and image distortion. Less contrast and greater resolution of pili bundles and surface structures of bacteria or tissue culture cells were obtained at magnifications of 10,000 or higher. As an alternative to CPD, HMDS processing of cell culture monolayers was simple and was more efficient when a large number of samples was processed.

The opacity proteins of Neisseria gonorrhoeae strain MS11 are encoded by a family of 11 complete genes

Variants of Neisseria gonorrhoeae MS11 show distinct colony morphologies because of the expression of a class of surface components called opacity (Opa, PII) proteins. Southern analyses combined with molecular cloning of genomic DNA from a single variant of MS11 has identified 11 opa genes contained in separate loci. These opa genes code for distinct opacity proteins which are distinguishable at their variable domains. The opa gene analyses were also extended to divergent variants of MS11. These studies have shown that, during in vitro and in vivo culture, 10 of the 11 opa genes did not undergo significant change in their primary sequence. However, in these variants, one gene (opaE) underwent non-reciprocal inter-opa recombinations to generate newer Opa variants. Phylogenic analysis of the opa gene sequences suggests that the opa gene family have evolved by a combination of gene duplication, gene replacement and partial inter-opa recombination events.

Morphological differences in Neisseria meningitidis pili

Disease and carrier isolates of Neisseria meningitidis were examined for their ability to adhere to human buccal epithelial cells and human cell lines and to hemagglutinate human erythrocytes, properties thought to be associated with the presence of pili. Seventy percent (7 of 10) of carrier isolates were found to be highly adherent to human buccal epithelial cells and to agglutinate human A, B, O, Rh-, and Rh+ erythrocytes. In contrast, 60% of the disease isolates adhered poorly to human buccal epithelial cells and 80% failed to agglutinate human erythrocytes. No adherence of either disease or carrier isolates was observed when several human cell lines were tested. When the meningococcal strains were examined by electron microscopy, 7 of 10 disease isolates were found to possess large bundles of aggregated pili (alpha-type pili), while 7 of 10 carrier isolates were found to have numerous unaggregated pili (beta-type pili). A monoclonal antibody against meningococcal pili and one against gonococcal pili reacted with 6 of 10 piliated carrier isolates and 4 of 10 piliated disease isolates. These results suggest that meningococci, like gonococci, possess different types of pili which differ in morphological, antigenic, and binding properties. In addition, antigenic and morphological differences between pili from carrier and disease isolates were observed as well as differences in adherence and hemagglutinating properties.

Arrangement of pili in colonies of Neisseria gonorrhoeae

The morphology and arrangement of pili in the P++ colony phenotype of Neisseria gonorrhoeae were examined by a variety of electron microscopic techniques. The apparent structure and organization of gonococcal pili varied depending upon the method of specimen preparation. Pili as thin, individual, unbranched structures were demonstrated by negative staining and in sections of epoxy-embedded specimens. Pili forming thick structures which branch, subdivide, and rejoin to form an irregular lattice were demonstrated in specimens processed by the critical-point drying method and by rapid freezing and low temperature sublimination. We propose that in gonococcal colonies of the P++ phenotype, pili exist as individual threadlike structures only on the bacterial surfaces; as the pili leave the bacterial surfaces, they form thick bundles which branch, subdivide, and rejoin to form a supporting framework interconnecting the colony members. This arrangement of pili is usually disrupted by the commonly used method of negative staining and cannot be clearly detected within epoxy-embedded specimens. These data are summarized in a model depicting the organization of pili in the P++ colony phenotype of N. gonorrhoeae.

Penetrability of the outer membrane of Neisseria gonorrhoeae in relation to acquired resistance to penicillin and other antibiotics

Acquired antibiotic resistance in Neisseria gonorrhoeae is principally associated with three genetic markers, penA, mtr, and penB. penA is a specific marker for penicillin resistance, whereas mtr and penB are nonspecific in conferring resistance to penicillin and several other antibiotics as well. It has been suggested that the nonspecific markers may cause a general decrease in the penetrability of the gonococcal outer membrane. To investigate this, antibiotic penetration of the outer membrane was studied in two isogenic strains-FA19 (susceptible parent) and FA140 (containing penA, mtr, and penB)-and also in a clinical isolate with multiple resistance. The method involved brief treatment of exponential cells with ethylenediaminetetraacetic acid at 5 degrees C to disrupt the outer membrane barrier. The 50% inhibitory concentrations of antibiotics for treated and normal cells were measured turbidimetrically, and from their ratios outer membrane penetration barriers were calculated. Small barriers were observed for actinomycin D and benzylpenicillin, and these were very similar in the susceptible and resistant strains. Also, in FA140 no significant barriers for rifampin, erythromycin, and tetracycline were detected. These results suggest that mechanism(s) other than reduced outer membrane penetrability underlie acquired resistance due to penA, mtr, and penB.

The biology of the gonococcus

Gonorrhea has been known since antiquity. Today, this disease is the most commonly reported infectious disease in the U.S. The natural environment of the etiological agent, Neisseria gonorrhoeae, is man. In this host, the organism usually parasitizes mucosal surfaces populated by columnar epithelial cells. Under certain conditions, the gonococcus may disseminate or spread to adjacent organs. The gonococcus is well adapted to its environment and is a successful parasite. Until recently, gonococci were uniformly sensitive to penicilin. However, a plasmid encoding beta-lactamase has been identified in some isolates. Most strains exhibit specific requirements for various amino acids, vitamins, purines, and pyrimidines. Only glucose, pyruvate, and lactate are utilized as sources of energy. Glucose is dissimilated by a combination of the Entner-Doudoroff and pentose phosphate pathways. A tricarboxylic acid cycle is also present and active under certain conditions. Structurally, the cell envelope of the gonococcus resembles that of a typical Gram-negative bacterium. Gonococci are highly autolytic, especially in older cultures or after depletion of the energy source. Autolysis is not due solely to peptidoglycan hydrolysis, but appears to involve a destabilization of the outer membrane as well. Cell surface components such as pili, lipopolysaccharide, outer membrane proteins, and a capsule are associated with the virulence and pathogenicity of this organism.

Effect of ethylenediaminetetraacetate upon the surface of Escherichia coli

The effect of ethylenediaminetetraacetate (EDTA) on the envelope of two strains of Escherichia coli (B and Cla) was studied with freeze-fracturing methods. Untreated cells showed the outer membrane's outer surface with a fine texture of randomly spaced depressions of about 4.5-nm diameter; small areas with symmetrical arrangements of structural surface elements were also observed. The outer membrane's fracture plane revealed a random distribution of particles on its "concave" plane, only occasionally interrupted by particle-free areas. The "convex" aspect of the outer membrane's fracture plane showed only a few scattered particles. The cleavage plane of the inner membrane was often interrupted by many localized elevated plateaus, at which the cleaving process had, for short distances, switched to the outer membrane. The effects of EDTA treatment were mainly seen in the structure of the freeze-etched outer membrane: (i) the pits as well as the symmetrical surface elements of the outer membrane's outer surface had disappeared; (ii) a number of plateaus (about 20 to 50/cell) were seen at which a cleavage plane within the inner membrane had switched to the hydrophobic portion of the outer membrane (outer membrane's fracture plane). These plateaus were also visible in untreated cells; however, EDTA treatment apparently caused an increased exposure of plateaus. Surface areas, exposed by freeze-etching, revealed the underlying plateaus as elevations in the surface contour of the cell, suggesting a slower etching rate in the zones of the plateaus relative to the rest of the outer membrane. Well-defined, particle-free patches in the outer membrane's fracture plane, concave, were more frequent and larger in size after EDTA treatment than in the controls. In the presence of glycerol, the cells often cleaved in the outer membrane's fracture plane, but isolated plateaus were rarely observed. After metabolic poisoning of cells for 15 to 25 min at 37 degrees C, the plateaus had widened. These data suggest that the material of the plateaus has a slow rate of lateral diffusion. Placement of EDTA-treated cells in fresh medium at 37 degrees C caused, after 3 to 5 min, the reoccurrence of the pitted surface structure. We propose that the plateaus represent localized zones, at which newly synthesized lipopolysaccharide has been inserted.

Factors affecting genetic transformation of Neisseria gonorrhoeae

Piliated gonococci were competent in genetic transformation in all stages of growth in minimal and enriched media, but nonpiliated cells were almost totally incompetent. Uptake of deoxyribonucleic acid into a deoxyribonuclease-insensitive state was observed only in competent piliated cells. Competence was not affected by washing of competent cells or treatment of competent cells with proteolytic enzymes. Expression of competence required presence of any of several different monovalent or divalent cations, as well as a utilizable source of energy. Efforts to produce genotypically or phenotypically competent derivatives of nonpiliated cells were unsuccessful. These experiments are consistent with the idea that pili may play a role in the irreversible uptake of transforming deoxyribonucleic acid by the gonococcus, but fail to provide evidence for other types of competence factors.

Scanning electron microscopic study of virulent and avirulent colonies of Neisseria gonorrhoeae

A preparation procedure for scanning electron microscopy was developed by which gononcoccal colonies can be studied directly on the agar surface. After glutaraldehyde fixation directly in petri dishes, small agar pieces were cut out and dehydrated stepwise in increasing concentrations of ethanol. The blocks were thereafter transferred to a critical-point drying apparatus, via steps of increased gradients up to 100% amyl acetate. By this method five different gonococcal colony types could be distinguished analogous to light microscopic observations made by others. At higher magnifications an abundance of intercellular strands was found between the cells in virulent type 1 and 2 colonies, but not in the avirulent types 3 through 5. These strands seemed to anchor the cells to each other and to the agar surface. The presence of such structures probably explains the highly convex surface of virulent colonies and explains why colonies of avirulent strains exhibit a radial extension and a flat upper surface. The nature of these filamentous intercellular strands is discussed.

Studies on Gonococcus infection. X. Pili and leukocyte association factor as mediators of interactions between gonococci and eukaryotic cells in vitro

Two independent gonococcal surface components, pili and leukocyte association factor, appear to mediate in vitro interactions of Neisseria gonorrhoeae with tissue culture cells and human peripheral blood leukocytes, respectively.

Pili on meningococci from primary cultures of nasopharyngeal carriers and cerebrospinal fluid of patients with acute disease

The nasopharynx of known meningococcal carriers without signs of acute meningococcal disease as well as cerebrospinal fluid from patients with acute meningococcal disease were cultured on Thayer-Martin agar. Pili were observed in negatively stained preparations of over 80% cells from all primary cultures of both nasopharnyx and cerebrospinal fluids. Although pili were abundant on cells from all primary cultures, all pili were lost on serial subculture in the laboratory. This loss of pili from the cell surface on laboratory subculture was not accompanied by a concomitant loss of cell wall blebs.

Genetic transformation of biosynthetically defective Neisseria gonorrhoeae clinical isolates

Deoxyribonucleic acid (DNA) and chemically defined media were used in transformation tests of 51 strains of Neisseria gonorrhoeae which exhibited various biosynthetic defects when isolated from patients. These auxotrophic gonococci had one or more nutritional requirements involving proline, methionine, arginine, hypoxanthine, uracil, and thiamine pyrophosphate (THPP). DNA from a clinical isolate which did not require these compounds for growth on defined medium transformed each of the auxotrophic markers of all 51 recipient populations. Ten isolates had defects involving the synthesis of THPP; four strains (designated Thp(-)) had a growth requirement that was satisfied only by THPP, whereas the requirement of six strains (designated Thi(-)) was satisfied by either thiamine or THPP. DNA from Thp(-) donors elicited transformation of Thp(-) as well as Thi(-) recipients. Reciprocally, DNA from a Thi(-) donor transformed both Thi(-) and Thp(-) recipients. Furthermore, DNA from other auxotrophic gonococci had transforming activity for some phenotypically similar auxotrophic recipients. The findings indicate the existence of various nonidentical genetic defects which block reactions in the biosynthesis of proline, methionine, arginine, hypoxanthine, and THPP. Routine cultures from patients with gonorrhea were the source of these auxotrophic strains of N. gonorrhoeae; the various nutritional requirements were identified by a recently described system of gonococcal auxotyping. The transformation test results verify the hereditary basis of the auxotypes, establish that many different mutations exist in potentially virulent gonococci, and illustrate the value of these auxotrophic mutants for studies of the genetic structure and evolution of natural populations of gonococci.

Ultrastructure of pili and annular structures on the cell wall surface of Neisseria meningitidis

In a survey of negatively stained preparations of the prototype strains of Neisseria meningitidis, pili were detected on three strains. However, these pili were detected on fewer than 5% of cells in populations of these three strains. Those individual cells with pili were seldom observed to contain more than two or three pili per cell. In contrast, nearly all cells of the nonprototype group B strain ATCC 13090 had numerous pili on their surfaces. When viewed in frozen-etched replicas, a few pili were observed lying on the cell surface of this latter strain. An annular structure was also found in frozen-etched replicas. This structure usually consisted of a series of concentric rings that were always found on the flat side of these bean-shaped cells. It is concluded that such structures represent a differentiated portion of the cell wall, which is involved in cross-wall formation during synthesis of the cell septum in dividing cells.

Studies on gonococcus infection. VI. Electron microscopic study on in vitro phagocytosis of gonococci by human leukocytes

The ingestion of Neisseria gonorrhoeae by isolated human leukocytes in vitro has been studied by electron microscopy. Use of the modified critical-point drying method allows clear visualization of progressive ingestion of both pilated and nonpilated gonococci by leukocytes. Incomplete ingestion of pilated gonococci with protrusion of tangled masses of pili from sites of phagocytosis are also found.

Location of the fracture faces within the cell envelope of Acinetobacter species strain MJT-F5-5

The cell wall of the gram-negative bacterium Acinetobacter species strain MJT/F5/5 shows in thin section an external "additional" layer, an outer membrane, an intermediate layer, and a dense layer. Negatively stained preparations showed that the additional layer is composed of hexagonally arranged subunits. In glycerol-treated preparations, freeze-etching revealed that the cell walls consist of four layers, with the main plane of fracture between layers cw 2 and cw 3. The surface of [Formula: see text] 2 consisted of densely packed particles, whereas [Formula: see text] 3 appeared to be fibrillar. In cell envelopes treated with lysozyme by various methods, the removal of the dense layer has detached the outer membrane and additional layer from the underlying layers, as shown in thin sections. When freeze-etched in the absence of glycerol, these detached outer membranes with additional layers fractured to reveal both the faces [Formula: see text] 2 and [Formula: see text] 3 with their characteristic surface structures, and, in addition, both the external and internal etched surfaces were revealed. This experiment provided conclusive evidence that the main fracture plane in the cell wall lies within the interior of the outer membrane. This and other evidence showed that the corresponding layers in thin sections and freeze-etched preparations are: the additional layer, cw 1; the outer membrane, cw (2 + 3); and the intermediate and dense layers together from cw 4. Because of similarities in structure between this Acinetobacter and other gram-negative bacteria, it seemed probable that the interior of the outer membrane is the plane most liable to fracture in the cell walls of most gram-negative bacteria.

Differential attachment by piliated and nonpiliated Neisseria gonorrhoeae to human sperm

Piliated type 1 Neisseria gonorrhoeae attached to 50% of human sperm after incubation of mixtures in vitro for 15 min at 35 C. In contrast, nonpiliated type 4 N. gonorrhoeae attached to only 23% of sperm. Similar results were obtained with three different strains of N. gonorrhoeae. Treatment with heat or formaldehyde to kill bacteria did not affect the amount of attachment by piliated or nonpiliated types. Escherichia coli and N. subflava, other species of piliated bacteria, attached to about 40% of sperm, and the nonpiliated species N. meningitidis and N. catarrhalis attached to a comparable number of sperm, as did type 4 N. gonorrhoeae. Prior incubation of type 1 N. gonorrhoeae with purified antibody prepared against gonococcal pili reduced the percentage of sperm with attached bacteria to the same level as that for nonpiliated type 4 gonococci. Similar treatment of other piliated organisms or of nonpiliated Neisseria did not affect the attachment of the bacteria to sperm.

Freeze-etching of the cell envelope of an Acinetobacter species which carries a regular array of surface subunits

Freeze-etching was applied to preparations, with and without glycerol, of Acinetobacter sp. strain MJT/F5/199A, consisting of intact cells after normal growth or after incubation with chloramphenicol, spheroplasts, and isolated cell walls and outer membranes. Etched preparations show that a regular array of subunits forms the surface of normal cells. Near the zones of constriction in dividing cells, blebs and irregularities are seen, and some blebs, consisting of both surface subunits and outer membrane, are released from the cells. The cross-fractured cell envelope shows four layers which are related to the structures seen in section as follows: cw1, which is not visible in section, contains the surface subunits; cw2 consists of all or part of the outer membrane; cw3 includes the intermediate and dense, peptidoglycan-containing layers; within these cell wall layers is the plasma membrane. Internal fracture of the plasma membrane occurs under all conditions tested, but the fracture plane in the cell wall is only revealed in chloramphenicol-treated cells or normal cells freeze-fractured with glycerol present; the characteristic fracture faces are not seen in spheroplasts or isolated outer membranes. The concave fracture face cw2 consists of densely packed granules, while the convex face cw3 is fibrillar. The probable location of this fracture plane is discussed. After incubation with chloramphenicol, the outer surface of the cells is obscured by extracellular material, the dense peptidoglycan-containing layer is increased in thickness, and the cytoplasm contains rounded bodies bounded by one or more unit membranes.

Quantitative determination of antibody to gonococcal pili. Changes in antibody levels with gonococcal infection

Gonococcal pili, pure by the criteria of electron microscopic examination and polyacrylamide gel electrophoresis in sodium dodecyl sulfate, have been prepared by repeated cycles of precipitation with 0.1 M MgCl(2), followed by dissolution in 0.01 M Tris pH 8, 0.01 M NaN(3). Using a fluorescein-conjugated antibody prepared to pili from a single strain, pilar antigen(s) was found to be present in each of 18 strains of gonococci tested, and absent from strains of pilated meningococci, nonpathogenic Neisseria sp., and Escherichia coli. Purified pili, labeled with (125)I were used in an antigen binding assay to quantitatively measure antibody to pili in rabbit sera and in 561 human sera. The range of antibody activity for 133 persons unlikely to have experienced gonorrhea was 0.1-1.6 mug/ml with a geometric mean of 0.5 mug/ml. This geometric mean antibody activity was significantly lower than the geometric mean for asymptomatically infected males (1.0 mug/ml, P < 0.002), males with symptomatic gonococcal anterior urethritis (1.6 mug/ml, P < 0.001), or asymptomatically infected females (4.2 mug/ml, P < 0.001). Antibody appeared in elevated levels (> 1.6 mug/ml) 2-3 wk after infection and returned toward control levels 1 or more months after treatment. Antibody levels higher than 1.6 mug/ml were found in 26 (50%) of 52 males with gonococcal anterior urethritis, in 10 (33%) of 30 males with asymptomatic urethral infection and in 50 (89%) of 56 asymptomatically infected females. In a high-risk group of 103 females for whom culture results and antibody to pili were compared, 58 (57%) had elevated antibody levels to pili and 86% of the infected females were within this seropositive group. The antigen binding assay may provide a means to detect asymptomatic gonococcal infection in women.

Variation in colonial morphology of Neisseria gonorrhoeae after growth on media containing antimicrobial agents

Stable colonial types 1, 2, 3, and 4 were prepared from eight strains of Neisseria gonorrhoeae. Four of the strains, termed laboratory strains, had been transferred over 100 times; three strains, termed clinical strains, were transferred only three to five times after isolation from patients, and one stabilized clinical strain was transferred purposefully 30 times after isolation from a patient. Colonial types of the three categories were grown on four media containing the following agents at the level used in diagnostic media: (i) vancomycin, colistin, and nystatin; (ii) these antibiotics plus trimethoprim lactate; (iii) trimethoprim lactate alone; and (iv) a control with no antimicrobial agents. When grown on media containing the antimicrobial agents, colonial types 1, 2, and 3 of all strains showed specific and consistent changes that precluded accurate identification of the types. In general, the colonies were smaller, more dense to transmission of light, and more granular than colonies grown on control medium. More colonies showed these type changes in the clinical strains and on media containing trimethoprim lactate. Colonies of type 4 showed little or no change. The changes in colonial morphology of types 1, 2, and 3 were pronounced enough to make colony typing difficult if the antimicrobial agents, particularly trimethoprim lactate, were present in media.

Studies on gonococcus infection. IV. Pili: their role in attachment of gonococci to tissue culture cells

Attachment of Neisseria gonorrhoeae to amnion cells in tissue culture is facilitated if the gonococci bear pili. This has been determined by studying the number of pilated, colony type 2 gonococci associated with amnion cells after incubation in vitro as compared with the number of nonpilated, colony type 4 gonococci present with amnion cells under the same conditions. These data are supported by light microscope findings. Electron microscope studies provide visualization of fine structure of gonococcal attachment. Gonococci are also found within amnion cells in this in vitro system.