Neisseria gonorrhoeae PilC expression provides a selective mechanism for structural diversity of pili

A Thermosensitive, Phase-Variable Epigenetic Switch: pap Revisited

It has been more than a decade since the last comprehensive review of the phase-variable uropathogen-associated pyelonephritis-associated pilus (pap) genetic switch. Since then, important data have come to light, including additional factors that regulate pap expression, better characterization of H-NS regulation, the structure of the Lrp octamer in complex with pap regulatory DNA, the temperature-insensitive phenotype of a mutant lacking the acetyltransferase RimJ, evidence that key components of the regulatory machinery are acetylated, and new insights into the role of DNA binding by key regulators in shaping both the physical structure and regulatory state of the papI and papBA promoters. This review revisits pap, integrating these newer observations with older ones to produce a new model for the concerted behavior of this virulence-regulatory region.

RpoN and the Nps and Npa two-component regulatory system control pilE transcription in commensal Neisseria

Over 20 genes are involved in the biogenesis and function of the Neisseria Type IV pilus (Tfp). In the pathogenic species, RpoD and the integration host factor (IHF) protein regulate expression of pilE, encoding the Tfp structural subunit. We previously reported that in commensal species, pilE transcription is regulated by RpoN, IHF, and activator Npa. Npa has many hallmarks of response regulators in two-component regulatory systems, leading us to search for its response regulator partner. We report that Npa partners with sensor kinase Nps to control pilE transcription. Among the genes involved in Tfp biogenesis and function, only pilE is controlled by RpoN and Npa/Nps. We summarize our findings in a model, and discuss the implications of the differential regulation of pilE the context of Neisseria Tfp biogenesis.

Biology of the Gonococcus: Disease and Pathogenesis

Neisseria gonorrhoeae infection is a major public health problem worldwide. The increasing incidence of gonorrhea coupled with global spread of multidrug-resistant isolates of gonococci has ushered in an era of potentially untreatable infection. Gonococcal disease elicits limited immunity, and individuals are susceptible to repeated infections. In this chapter, we describe gonococcal disease and epidemiology and the structure and function of major surface components involved in pathogenesis. We also discuss the mechanisms that gonococci use to evade host immune responses and the immune responses following immunization with selected bacterial components that may overcome evasion. Understanding the biology of the gonococcus may aid in preventing the spread of gonorrhea and also facilitate the development of gonococcal vaccines and treatments.

Structural and functional studies of the Pseudomonas aeruginosa minor pilin, PilE

Many bacterial pathogens, including Pseudomonas aeruginosa, use type IVa pili (T4aP) for attachment and twitching motility. T4aP are composed primarily of major pilin subunits, which are repeatedly assembled and disassembled to mediate function. A group of pilin-like proteins, the minor pilins FimU and PilVWXE, prime pilus assembly and are incorporated into the pilus. We showed previously that minor pilin PilE depends on the putative priming subcomplex PilVWX and the non-pilin protein PilY1 for incorporation into pili, and that with FimU, PilE may couple the priming subcomplex to the major pilin PilA, allowing for efficient pilus assembly. Here we provide further support for this model, showing interaction of PilE with other minor pilins and the major pilin. A 1.25 Å crystal structure of PilEΔ1-28 shows a typical type IV pilin fold, demonstrating how it may be incorporated into the pilus. Despite limited sequence identity, PilE is structurally similar to Neisseria meningitidis minor pilins PilXNm and PilVNm, recently suggested via characterization of mCherry fusions to modulate pilus assembly from within the periplasm. A P. aeruginosa PilE-mCherry fusion failed to complement twitching motility or piliation of a pilE mutant. However, in a retraction-deficient strain where surface piliation depends solely on PilE, the fusion construct restored some surface piliation. PilE-mCherry was present in sheared surface fractions, suggesting that it was incorporated into pili. Together, these data provide evidence that PilE, the sole P. aeruginosa equivalent of PilXNm and PilVNm, likely connects a priming subcomplex to the major pilin, promoting efficient assembly of T4aP.

Characterization of motility and piliation in pathogenic Neisseria

Background

The type IV pili (Tfp) of pathogenic Neisseria (i.e., N. gonorrhoeae and N. meningitidis) are essential for twitching motility. Tfp retraction, which is dependent on the ATPase PilT, generates the forces that move bacteria over surfaces. Neisseria motility has mainly been studied in N. gonorrhoeae whereas the motility of N. meningitidis has not yet been characterized.

Results

In this work, we analyzed bacterial motility and monitored Tfp retraction using live-cell imaging of freely moving bacteria. We observed that N. meningitidis moved over surfaces at an approximate speed of 1.6 μm/s, whereas N. gonorrhoeae moved with a lower speed (1.0 μm/s). An alignment of the meningococcal and gonococcal pilT promoters revealed a conserved single base pair variation in the −10 promoter element that influence PilT expression. By tracking mutants with altered pilT expression or pilE sequence, we concluded that the difference in motility speed was independent of both. Live-cell imaging using total internal reflection fluorescence microscopy demonstrated that N. gonorrhoeae more often moved with fewer visible retracting filaments when compared to N. meningitidis. Correspondingly, meningococci also displayed a higher level of piliation in transmission electron microscopy. Nevertheless, motile gonococci that had the same number of filaments as N. meningitidis still moved with a lower speed.

Conclusions

These data reveal differences in both speed and piliation between the pathogenic Neisseria species during twitching motility, suggesting a difference in Tfp-dynamics.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0424-6) contains supplementary material, which is available to authorized users.

The role of variable DNA tandem repeats in bacterial adaptation

DNA tandem repeats (TRs), also designated as satellite DNA, are inter- or intragenic nucleotide sequences that are repeated two or more times in a head-to-tail manner. Because TR tracts are prone to strand-slippage replication and recombination events that cause the TR copy number to increase or decrease, loci containing TRs are hypermutable. An increasing number of examples illustrate that bacteria can exploit this instability of TRs to reversibly shut down or modulate the function of specific genes, allowing them to adapt to changing environments on short evolutionary time scales without an increased overall mutation rate. In this review, we discuss the prevalence and distribution of inter- and intragenic TRs in bacteria and the mechanisms of their instability. In addition, we review evidence demonstrating a role of TR variations in bacterial adaptation strategies, ranging from immune evasion and tissue tropism to the modulation of environmental stress tolerance. Nevertheless, while bioinformatic analysis reveals that most bacterial genomes contain a few up to several dozens of intra- and intergenic TRs, only a small fraction of these have been functionally studied to date.

Contribution of interactions between complement inhibitor C4b-binding protein and pathogens to their ability to establish infection with particular emphasis on Neisseria gonorrhoeae

Complement activation and resulting opsonisation with C3b form key arms of the innate immune defense against infections. However, a wide variety of pathogens subvert complement attack by binding host complement inhibitors, which results in diminished opsonophagocytosis and killing of bacteria by lysis. Human C4b-binding protein (C4BP) binds Neisseria gonorrhoeae and Streptococcus pyogenes, both uniquely human pathogens. This binding specificity is circumvented by other bacterial species, which bind C4BP from numerous mammalian hosts that they infect. Binding of C4BP to Neisseria is mediated by outer membrane porin proteins and appears to be one of the main factors mediating serum resistance. Targeting C4BP binding sites on bacterial surfaces with vaccine-induced antibodies may block binding of C4BP and enhance a common vaccine design strategy that depends on the generation of complement-dependent bactericidal and opsonophagocytic antibody activities.

The role of phenotypic variation in rhizosphere Pseudomonas bacteria

Colony phase variation is a regulatory mechanism at the DNA level which usually results in high frequency, reversible switches between colonies with a different phenotype. A number of molecular mechanisms underlying phase variation are known: slipped-strand mispairing, genomic rearrangements, spontaneous mutations and epigenetic mechanisms such as differential methylation. Most examples of phenotypic variation or phase variation have been described in the context of host-pathogen interactions as mechanisms allowing pathogens to evade host immune responses. Recent reports indicate that phase variation is also relevant in competitive root colonization and biological control of phytopathogens. Many rhizospere Pseudomonas species show phenotypic variation, based on spontaneous mutation of the gacA and gacS genes. These morphological variants do not express secondary metabolites and have improved growth characteristics. The latter could contribute to efficient root colonization and success in competition, especially since (as shown for one strain) these variants were observed to revert to their wild-type form. The observation that these variants are present in rhizosphere-competent Pseudomonas bacteria suggests the existence of a conserved strategy to increase their success in the rhizosphere.

Down-regulation of CD46 by piliated Neisseria gonorrhoeae

Human membrane cofactor protein (CD46) protects host cells against complement attack and may function as a receptor for pathogenic Neisseriae. We assessed CD46 expression in the human cervical cell line ME-180 after exposure to Neisseria gonorrhoeae. Piliated but not nonpiliated gonococci adhered to cells and produced up to an 80% reduction in CD46 surface expression by 6 h that persisted for at least 24 h. This response required a minimum multiplicity of infection of 10 and was not prevented by antibodies to CD46. CD46 down-regulation was not attributable to intracellular retention or a global or specific shutdown of mRNA or protein synthesis. Substantial quantities of CD46 were found in the supernatants, indicating a specific shedding of this protein. Adherent gonococci lacking the pilus retraction protein PilT did not down-regulate CD46 but de-repression of pilT expression restored CD46 down-regulation. After experimental infection of human volunteers with a gonococcal variant incapable of inducing CD46 down-regulation, variants of this strain were reisolated that exhibited CD46 down-regulation. Pilus-mediated interactions of gonococci with human epithelial cells results in a pathogen-induced manipulation of the host cell environment in which a membrane protein is removed from epithelial cells by liberation into the surrounding milieu.

The Pix pilus adhesin of the uropathogenic Escherichia coli strain X2194 (O2 : K(-): H6) is related to Pap pili but exhibits a truncated regulatory region

Adhesins provide a major advantage for uropathogenic Escherichia coli in establishing urinary tract infections (UTIs). A novel gene cluster responsible for the expression of a filamentous adhesin of the pyelonephritogenic E. coli strain X2194 has been identified, molecularly cloned, and characterized. The 'pix operon' contains eight open reading frames which exhibit significant sequence homology to corresponding genes in the pap operon encoding P pili, the prevalent E. coli adhesins in non-obstructive acute pyelonephritis in humans. Although a pixB gene corresponding to the PapB regulator was identified, a papI homologue could not be found in the pix operon. Instead, a fragment of the R6 gene of the highly uropathogenic E. coli strain CFT073 was identified upstream of pixB. The R6 gene is located in a pathogenicity island containing several pilus-encoding sequences and shows homology to a transposase of Chelatobacter heintzii. In a pixA-lacZ fusion system it was demonstrated that the expression of Pix pili is regulated at the transcriptional level by the R6 gene sequence. A significantly reduced transcription was observed by deleting this fragment and by lowering the growth temperature from 37 to 26 degrees C. In contrast to other filamentous adhesin systems, Pix pili are mainly expressed in the steady state growth phase and were not repressed by the addition of glucose.

Soluble pilin of Neisseria gonorrhoeae interacts with human target cells and tissue

Pili of Neisseria gonorrhoeae are phase-variable surface structures that mediate adherence to host target cells. Each pilus is composed of thousands of major pilus subunits, pilins, pilus-associated protein PilC, and possibly other components. Piliated and nonpiliated gonococcal clones may secrete a soluble smaller pilin (S-pilin) that is cleaved after amino acid 39 of the mature pilin protein. Here, purified S-pilin was found to migrate as a 61- to 64-kDa double band on nondenaturing gels, suggesting the formation of tetrameric S-pilin proteins with two isomeric forms. In situ studies of binding to formalin-fixed tissue sections demonstrated the binding of S-pilin to human tissue but not to tissue from mouse or rat organs, showing the presence of a human-specific receptor-binding domain within the pilin polypeptide. Pretreatment of the target tissues with proteinase K decreased gonococcal binding dramatically, whereas pretreatment with neuraminidase and meta-periodate, which cleave carbon-carbon linkages between vicinal hydroxyl groups in carbohydrates, did not affect gonococcal binding. In overlay assays, purified S-pilin bound to a band with a migration pattern and size similar to those of CD46, a cellular pilus receptor. Further, binding of N. gonorrhoeae to target cells and tissues could be blocked by both CD46 antibodies and purified S-pilin. These data argue that S-pilin interacts with a protein domain(s) of the CD46 receptor on human cells.

A novel interaction between type IV pili of Neisseria gonorrhoeae and the human complement regulator C4B-binding protein

C4b-binding protein (C4BP) is an important plasma inhibitor of the classical pathway of complement activation. Several bacterial pathogens bind C4BP, which may contribute to their virulence. In the present report we demonstrate that isolated type IV pili from Neisseria gonorrhoeae bind human C4BP in a dose-dependent and saturable manner. C4BP consists of seven identical alpha-chains and one beta-chain linked together with disulfide bridges. We found that pili bind to the alpha-chain of C4BP, which is composed of eight homologous complement control protein (CCP) domains. From the results of an inhibition assay with C4b and a competition assay in which we tested mutants of C4BP lacking individual CCPs, we concluded that the binding area for pili is localized to CCP1 and CCP2 of the alpha-chain. The binding between pili and C4BP was abolished at 0.25 M NaCl, implying that it is based mostly on ionic interactions, similarly to what have been observed for C4b-C4BP binding. Furthermore, the N-terminal part of PilC, a structural component of pili, appeared to be responsible for binding of C4BP. Membrane cofactor protein, previously shown to be a receptor for pathogenic N. gonorrhoeae on the surface of epithelial cells, competed with C4BP for binding to pili only at high concentrations, suggesting that different parts of pili are involved in these two interactions. Accordingly, high concentrations of C4BP were required to inhibit binding of N. gonorrhoeae to Chang conjunctiva cells, and no inhibition of binding was observed with cervical epithelial cells.

Interactions of pathogenic neisseriae with epithelial cell membranes

The closely related bacterial pathogens Neisseria gonorrhoeae (gonococci, GC) and N. meningitidis (meningococci, MC) initiate infection at human mucosal epithelia. Colonization begins at apical epithelial surfaces with a multistep adhesion cascade, followed by invasion of the host cell, intracellular persistence, transcytosis, and exit. These activities are modulated by the interaction of a panoply of virulence factors with their cognate host cell receptors, and signals are sent from pathogen to host and host to pathogen at multiple stages of the adhesion cascade. Recent advances place us on the verge of understanding the colonization process at a molecular level of detail. In this review we describe the Neisseria virulence factors in the context of epithelial cell biology, placing special emphasis on the signaling functions of type IV pili, pilus-based twitching motility, and the Opa and Opc outermembrane adhesin/invasin proteins. We also summarize what is known about bacterial intracellular trafficking and growth. With the accelerated integration of tools from cell biology, biochemistry, biophysics, and genomics, experimentation in the next few years should bring unprecedented insights into the interactions of Neisseriae with their host.

Cholera toxin and extracellular Ca2+ induce adherence of non-piliated Neisseria: evidence for an important role of G-proteins and Rho in the bacteria-cell interaction

In this study, we characterize the interaction between non-piliated (P-) Neisseria gonorrhoeae and human epithelial cells. P- mutants lacking the pilus subunit protein PilE attach at low levels to cells. Although the binding may not lead to heavy inflammatory responses, the interaction between P- Neisseria and host cells most probably play a role in colonization and asymptomatic carriage of the pathogen. Here we show that the adherence of P N. gonorrhoeae is blocked by GDP-beta-S [guanosine 5'-O(thio)diphosphate], a non-hydrolyzable GTP analogue, and by C3 exotoxin, an inhibitor of the small G-protein Rho. G-protein activators such as cholera toxin, that activates Gs, and fluoroaluminate, a general G-protein activator, induced bacterial adherence. Furthermore, increase of the extracellular free [Ca2+] dramatically enhanced adherence of non-piliated Neisseria. The pharynx and the urogenital tract are natural entry sites of the pathogenic Neisseria species, and at both sites the epithelial cells can be exposed to wide variations in Ca2+ concentration. Taken together, these data show the importance of extracellular Ca2+ in the pathogenic Neisseria-host interaction, and reveal a novel function of cholera toxin, namely induction of bacterial adherence.

Interactions of Neisseria meningitidis with cells of the human meninges

The interaction of Neisseria meningitidis with the meninges that surround and protect the brain is a pivotal event in the progression of bacterial meningitis. Two models of the human meninges were established in vitro, using (i) sections of fresh human brain and (ii) cultures of viable cells grown from human meningiomas. Neisseria meningitidis showed a specific predilection for binding to the leptomeninges and meningeal blood vessels in human brain and not to the cerebral cortex. There was a close correlation between the adherence of different Neisseria species to leptomeninges and cultured cells. The major ligand that mediated adherence was the pilus, and pilin variation modulated the interactions. The presence of Opa protein increased the association of Cap+ meningococci that expressed low-adhesive pili, but did not influence the association of high-adhesive pili. In contrast, Opc did not influence the adherence of Cap+ meningococci, whereas loss of capsule was associated with a more intimate interaction between the bacteria and the meningioma cell that was not apparent with Cap+ meningococci. There was no evidence of internalization of meningococci by meningioma cells in vitro, an observation that is consistent with the barrier properties of the leptomeninges to N. meningitidis observed in vivo.

Interaction mechanisms of encapsulated meningococci with eucaryotic cells: what does this tell us about the crossing of the blood-brain barrier by Neisseria meningitidis?

An important feature of Neisseria meningitidis is its ability to invade the meninges. This requires that bacteria cross the blood-brain barrier (BBB), which is one of the tightest barriers of the body. N. meningitidis has, therefore, evolved very sophisticated means by which it circumvents the physical properties of this cellular barrier. Recent advances have allowed the identification of several steps that might occur in the interaction of N. meningitidis with the BBB and the transit of the bacteria to the meninges.

The evolutionary origin of the protein-translocating channel of chloroplastic envelope membranes: identification of a cyanobacterial homolog

The known envelope membrane proteins of the chloroplastic protein import apparatus lack sequence similarity to proteins of other eukaryotic or prokaryotic protein transport systems. However, we detected a putative homolog of the gene encoding Toc75, the protein-translocating channel from the outer envelope membrane of pea chloroplasts, in the genome of the cyanobacterium Synechocystis sp. PCC 6803. We investigated whether the low sequence identity of 21% reflects a structural and functional relationship between the two proteins. We provide evidence that the cyanobacterial protein is also localized in the outer membrane. From this information and the similarity of the predicted secondary structures, we conclude that Toc75 and the cyanobacterial protein, referred to as SynToc75, are structural homologs. synToc75 is essential, as homozygous null mutants were not recovered after directed mutagenesis. Sequence analysis indicates that SynToc75 belongs to a family of outer membrane proteins from Gram-negative bacteria whose function is not yet known. However, we demonstrate that these proteins are related to a specific group of prokaryotic secretion channels that transfer virulence factors, such as hemolysins and adhesins, across the outer membrane.

Molecular characterization of Mycoplasma arthritidis variable surface protein MAA2

Earlier studies implied a role for Mycoplasma arthritidis surface protein MAA2 in cytadherence and virulence and showed that it exhibited both size and phase variability. Here we report the further analysis of MAA2 and the cloning and sequencing of the maa2 gene from two M. arthritidis strains, 158p10p9 and H606, expressing two size variants of MAA2. Triton X-114 partitioning and metabolic labeling with [3H]palmitic acid suggested lipid modification of MAA2. Surface exposure of the C terminus was indicated by cleavage of monoclonal antibody-specific epitopes from intact cells by carboxypeptidase Y. The maa2 genes from both strains were highly conserved, consisting largely of six (for 158p10p9) or five (for H606) nearly identical, 264-bp tandem direct repeats. The deduced amino acid sequence predicted a largely hydrophilic, highly basic protein with a 29-amino-acid lipoprotein signal peptide. The maa2 gene was expressed in Escherichia coli from the lacZ promoter of vector pGEM-T. The recombinant product was approximately 3 kDa larger than the native protein, suggesting that the signal peptide was not processed in E. coli. The maa2 gene and upstream DNA sequences were cloned from M. arthritidis clonal variants differing in MAA2 expression state. Expression state correlated with the length of a poly(T) tract just upstream of a putative -10 box. Full-sized recombinant MAA2 was expressed in E. coli from genes derived from both ON and OFF expression variants, indicating that control of expression did not include alterations within the coding region.

Comparisons between colony phase variation of Neisseria gonorrhoeae FA1090 and pilus, pilin, and S-pilin expression

The gonococcal pilus is a primary virulence factor, providing the initial attachment of the bacterial cell to human mucosal tissues. Pilin, the major subunit of the pilus, can carry a wide spectrum of primary amino acid sequences which are generated by the action of a complex antigenic variation system. Changes in the pilin amino acid sequence can produce different pilus-dependent colony morphotypes, which have been previously shown to reflect phase variation of pili on the bacterial cell surface. In this study, we further examined the relationships between changes in pilus-dependent colony morphology, pilin sequence, pilus expression, and pilus function in Neisseria gonorrhoeae FA1090. A group of FA1090 colony variants expressed different pilin sequences and demonstrated different levels of pilin, S-pilin, and pilus expression. The analysis of these colony variants shows that they do not represent two distinct phases of pilus expression, but that changes in pilin protein sequence produce a spectrum of S-pilin production, pilus expression, and pilus aggregation levels. These different levels of pilus expression and aggregation influence not only colony morphology but also DNA transformation efficiency and epithelial cell adherence.

Characterization of the region downstream of the pilus biogenesis gene pilC1 in Neisseria gonorrhoeae

The nucleotide sequence of a 3 kb region downstream of pilC1 in Neisseria gonorrhoeae MS11 was analyzed. This region contains two open reading frames, ORF1 and ORF2, and several repetitive DNA elements. ORF1 encodes an outer membrane protein that shows homology to orf98 of Pediococcus acidilactici. PCR with primers specific for ORF1 revealed that the gene is present in all gonococcal strains tested. The other open reading frame, ORF2, is highly homologous to the putative integral membrane protein HI1680 of Haemophilus influenzae.

Frequency of pilin antigenic variation in Neisseria gonorrhoeae

Variation of the pilus of Neisseria gonorrhoeae occurs by the recombination of silent pilin DNA sequences into the pilin expression locus. We have developed a quantitative, competitive reverse transcription-PCR assay which measures the frequency of pilin antigenic variation independently of changes in gonococcal colony morphology and have determined this frequency within a gonococcal population. We have also studied the frequency of antigenic variation during growth and have concluded that growth does not dramatically influence the frequency of pilin antigenic variation, although a reproducible, twofold increase is observed upon the transition into late log/stationary phase.

Transcellular passage of Neisseria gonorrhoeae involves pilus phase variation

Piliated and nonpiliated Neisseria gonorrhoeae organisms were added on top of confluent layers of HEC-1-B cells, each maintained on a microporous Transwell-COL membrane. The bacteria released into the lower chamber were characterized with respect to the following virulence determinants: pili, which mediate adherence to target host cells; PilE, the major pilus subunit protein; and PilC, which is involved in pilus biogenesis and adherence. Even if >99% of the added bacteria of N. gonorrhoeae MS11 were piliated, bacteria recovered on the other side of the cell layer were predominantly nonpiliated. The recovered clones still expressed unassembled PilE protein, but 50% had lost PilC production. Nonpiliated gonococci, in which the 5' end of pilE had been deleted, were released in reduced numbers, and piliated recA bacteria added to the cell layer were not released at all, at time points when piliated recA+ clones were found at high numbers in the lower chamber. Our data indicate that bacteria producing unassembled PilE protein are selected for during passage through an epithelial cell layer. The finding that the pilE gene sequence was altered in the transmigrants suggests that pilin sequence variation is involved in the transcellular passage of N. gonorrhoeae.

The phase-variable pilus-associated protein PilC is commonly expressed in clinical isolates of Neisseria gonorrhoeae, and shows sequence variability among strains

PilC is a phase-variable protein associated with pilus-mediated adherence of pathogenic Neisseria to target cells. In this study, 24 strains of Neisseria gonorrhoeae with known epidemiological data were examined for expression of PilC. All strains produced PilC independently of serovar and site of isolation. To investigate whether the PilC protein is conserved or variable among gonococcal strains, the complete nucleotide sequence of pilC in four strains, isolated from either rectum, throat or blood, was determined. The deduced amino acid sequence in these strains differed from each other and from the two PilC proteins of N. gonorrhoeae MS11. These data demonstrate that PilC is commonly expressed, but the PilC sequence may vary among gonococcal strains.

A novel competence gene, comP, is essential for natural transformation of Acinetobacter sp. strain BD413

Acinetobacter sp. strain BD413 (= ATCC 33305), a nutritionally versatile bacterium, has an extremely efficient natural transformation system. Here we describe the generation of eight transformation-affected mutants of Acinetobacter sp. strain BD413 by insertional mutagenesis. These mutants were found by Southern blot analysis and complementation studies to result from single nptII marker insertions at different chromosomal loci. DNA binding and uptake studies with one mutant, T205, revealed that the transformation deficiency of this mutant results from a complete lack of DNA binding and, therefore, uptake activity. A novel competence gene essential for natural transformation, named comP, was cloned by complementation of mutant T205. The nucleotide sequence of comP was determined, and its deduced 15-kDa polypeptide displays significant similarities to type IV pilins. Analysis of the ultrastructure of a transformation-deficient comP mutant and the transformation-competent wild-type strain revealed that both are covered with bundle-forming thin fimbriae (3 to 4 nm in diameter) and individual thick fimbriae (6 nm in diameter). These results provide evidence that the pilinlike ComP is unrelated to the piluslike structures of strain BD413. Taking all data into account, we propose that ComP functions as a major subunit of an organelle acting as a channel or pore mediating DNA binding and/or uptake in Acinetobacter sp. strain BD413.

Transformation competence and type-4 pilus biogenesis in Neisseria gonorrhoeae--a review

In Neisseria gonorrhoea (Ngo), the processes of type-4 pilus biogenesis and DNA transformation are functionally linked and play a pivotal role in the life style of this strictly human pathogen. The assembly of pili from its main subunit pilin (PilE) is a prerequisite for gonococcal infection since it allows the first contact to epithelial cells in conjunction with the pilus tip-associated PilC protein. While the components of the pilus and its assembly machinery are either directly or indirectly involved in the transport of DNA across the outer membrane, other factors unrelated to pilus biogenesis appear to facilitate further DNA transfer across the murein layer (ComL, Tpc) and the inner membrane (ComA) before the transforming DNA is rescued in the recipient bacterial chromosome in a RecA-dependent manner. Interestingly, PilE is essential for the first step of transformation, i.e., DNA uptake, and is itself also subject to transformation-mediated phase and antigenic variation. This short-term adaptive mechanism allows Ngo to cope with changing micro-environments in the host as well as to escape the immune response during the course of infection. Given the fact that Ngo has no ecological niche other than man, horizontal genetic exchange is essential for a successful co-evolution with the host. Horizontal exchange gives rise to heterogeneous populations harboring clones which better withstand selective forces within the host. Such extended horizontal exchange is reflected by a high genome plasticity, the existence of mosaic genes and a low linkage disequilibrium of genetic loci within the neisserial population. This led to the concept that rather than regarding individual Neisseria species as independent traits, they comprise a collective of species interconnected via horizontal exchange and relying on a common gene pool.

Type-4 pili and meningococcal adhesiveness

The ability to interact with non-phagocytic cells is a crucial virulence attribute of the meningococcus. Pili play a major role in this process and are the only means yet discovered by which capsulated bacteria may adhere to cells. Pilus-mediated adhesion is a two-step process which requires (i) the expression of the adhesin PilC1 and (ii) the expression of an appropriate pilin variant. Some pilin variants have the ability to modify the degree of adhesiveness through the formation of bundles of pili which increases bacteria-bacteria interactions.

Questions about gonococcal pilus phase- and antigenic variation

Pathogenic organisms inhabit one of several defined locations within a host where temperature, pH, and nutrients are relatively constant. While the microorganism must adapt to different environments within the host, the host immune system is the most formidable predator that can limit the growth of a pathogen. Neisseria gonorrhoeae (the gonococcus, Gc) is the causative agent of gonorrhoea, and has evolved several systems for varying the antigenicity of different surface antigens, presumably to help evade the effects of the human immune system. The On/Off/On phase variation of surface structure expression also alters the antigenic characteristics of the bacterial cell surface. Antigenic variation of the major subunit of the pilus, pilin, occurs by unidirectional, homologous recombination between a silent locus and the expression locus. The silent loci lie from 1 to 900 kb from the expression locus in the chromosome yet all can donate their sequences to the expression locus. The genetic composition of the pilin loci of two Gc strains has been elucidated, and the types of changes that lead to altered forms of the pilus have been extensively characterized. However, little is known about the precise molecular mechanisms used to allow high-frequency, non-reciprocal, chromosomal recombination between pilin loci or about what regulates the process of maintaining chromosome fidelity.

Role of pili and the phase-variable PilC protein in natural competence for transformation of Neisseria gonorrhoeae

The Gram-negative bacterial pathogen Neisseria gonorrhoeae is naturally competent for transformation with species-related DNA. We show here that two phase-variable pilus-associated proteins, the major pilus subunit (pilin, or PilE) and PilC, a factor known to function in the assembly and adherence of gonococcal pili, are essential for transformation competence. The PilE and PilC proteins are necessary for the conversion of linearized plasmid DNA carrying the Neisseria-specific DNA uptake signal into a DNase-resistant form. The biogenesis of typical pilus fibers is neither essential nor sufficient for this process. DNA uptake deficiency of defined piliated pilC1,2 double mutants can be complemented by expression of a cloned pilC2 gene in trans. The PilC defect can also be restored by the addition of purified PilC protein, or better, pili containing PilC protein, to the mutant gonococci. Our data suggest that the two phase-variable Pil proteins act on the bacterial cell surface and cooperate in DNA recognition and/or outer membrane translocation.

Interaction of pathogenic neisseriae with nonphagocytic cells

The ability to interact with nonphagocytic cells is a crucial virulence attribute of the meningococcus and the genococcus. Like most bacterial pathogens, Neisseria meningitidis and Neisseria gonorrhoeae initiate infections by colonizing the mucosal epithelium, which serves as the site of entry. After this step, both bacteria cross the intact mucosal barrier. While N. gonorrhoeae is likely to remain in the subepithelial matrix, where it initiates an intense inflammatory reaction, N. meningitidis enters the bloodstream, and eventually the cerebrospinal fluid to cause meningitis. Both pathogens have evolved very similar mechanisms for interacting with host cells. Surface structures that influence bacterium-host interactions include pili, the meningococcal class 5 outer membrane proteins or the gonococcal opacity proteins, lipooligosaccharide, and the meningococcal capsule. This review examines what is known about the roles these structures play in bacterial adhesion and invasion, with special emphasis, on pilus-mediated adhesion. Finally, the importance of these structures in neisserial pathogenesis is discussed.

Functional implications of the expression of PilC proteins in meningococci

Multiple forms of PilC were found in Neisseria meningitidis (Nm) strains isolated from the oropharynx, blood or cerebrospinal fluid expressing either Class I or Class II pili. PilC expression was observed less frequently in case as opposed to carrier isolates. Moreover, PilC and pili were not always co-expressed. Several heavily piliated strains had no detectable PilC protein as determined by Western blotting using an antiserum previously used to detect such proteins in adhesive variants (Nassif et al., 1994). Serogroup B strain MC58 produced large numbers of pili, but expressed barely detectable amounts of PilC. A clonal variant of this strain with increased expression of PilC concurrently exhibited increased adherence to Chang conjunctival epithelial cells and human umbilical vein endothelial cells (Huvecs), but with more rapid binding to the former. No alteration in pilin sequence occurred in this variant, suggesting the involvement of PilC in increased adhesion. A Pil- backswitcher isolated from the hyper-adherent variant was PilC+ but was non-adherent, indicating that any PilC adherence function requires pilus expression. Parental variant (low PilC) produced pili in bundles that were easily detached from the bacterial surface and were frequently associated with Huvec surfaces after bacteria had been sheared off, but pili infrequently replaced bacteria during infection with the PilC-expressing variant. The hyper-adherent variant, which appeared to produce morphologically distinct pilus bundles, was able to withstand considerable shearing force and remained firmly attached to Huvecs. This raises the possibility that the observed hyper-adherence may arise from better anchorage of pili to the bacterial surface in addition to increased adhesion to some host cell surfaces.

Identification and characterization of pilG, a highly conserved pilus-assembly gene in pathogenic Neisseria

Expression of type IV pili appears to be a requisite determinant of infectivity for the strict human pathogens Neisseria gonorrhoeae and Neisseria meningitidis. The assembly of these colonization factors is a complex process. This report describes a new pilus-assembly gene, pilG, that immediately precedes the gonococcal (Gc) pilD gene encoding the pre-pilin leader peptidase. The nucleotide sequence of this region revealed a single complete open reading frame whose derived polypeptide displayed significant identities to the pilus-assembly protein PilC of Pseudomonas aeruginosa and other polytopic integral cytoplasmic membrane constituents involved in protein export and competence. A unique polypeptide of M(r) 38 kDa corresponding to the gene product was identified. A highly related gene and flanking sequences were cloned from a group B polysaccharide-producing strain of N. meningitidis (Mc). The results indicate that the pilG genes and genetic organization at these loci in Gc and Mc are extremely conserved. Hybridization studies strongly suggest that pilG-related genes exist in commensal Neisseria species and other species known to express type IV pili. Defined genetic lesions were created by using insertional and transposon mutagenesis and moved into the Gc and Mc chromosomes by allelic replacement. Chromosomal pilG insertion mutants were devoid of pili and displayed dramatically reduced competence for transformation. These findings could not be ascribed to pilin-gene alterations or to polarity exerted on pilD expression. The results indicated that PilG exerts its own independent role in neisserial pilus biogenesis.

Sequence changes in the pilus subunit lead to tropism variation of Neisseria gonorrhoeae to human tissue

Pili of Neisseria gonorrhoeae are correlated with increased bacterial attachment to epithelial cells and undergo both phase and antigenic variation. Phase variation of gonococcal pili can be brought about by recombination events in the pilin structural gene, pilE, or by the on/off switch in expression of PilC, a pilus biogenesis protein for which two loci exist. We have studied the binding to epithelial cell lines and to fixed tissue sections of N. gonorrhoeae MS11 derivatives and mutants carrying structurally defined PilE and PilC proteins. In situ binding studies of N. gonorrhoeae to formalin-fixed tissue sections resulted in a binding pattern similar to that obtained using viable epithelial cell lines of different origin. Piliated gonococcal clones, containing different pilE sequences, varied dramatically from one another in their efficiencies at binding to corneal and conjunctival tissue, but bound equally well to cervical and endometrial tissues. Further, the binding data suggested that PilC expression by itself, i.e. without pili, cannot confer bacterial binding and that expression of either PilC1 or PilC2 does not confer different binding properties to the bacterial cells. Possible receptors for piliated gonococci were expressed in human tissues, such as cervix, endometrium, cornea, intestine, stomach, mid-brain and meninges, but not in human kidney. Pretreatment of the target tissues with Proteinase K decreased the gonococcal binding dramatically, whereas pretreatment with neuraminidase and meta-periodate, which cleave carbon-carbon linkages between vicinal hydroxyl groups in carbohydrates, did not affect attachment of gonococci. These data argue that pilus-dependent attachment of N. gonorrhoeae to human tissue may be mediated by a eukaryotic receptor having protein characteristics, and that the pilus subunit sequence may play an important role in the interaction with human cornea.

Pilus-facilitated adherence of Neisseria meningitidis to human epithelial and endothelial cells: modulation of adherence phenotype occurs concurrently with changes in primary amino acid sequence and the glycosylation status of pilin

Adherence of capsulate Neisseria meningitidis to endothelial and epithelial cells is facilitated in variants that express pili. Whereas piliated variants of N. meningitidis strain C311 adhered to endothelial cells in large numbers (> 150 bacteria/cell), derivatives containing specific mutations that disrupt pilE encoding the pilin subunit were both non-piliated and failed to adhere to endothelial cells (< 1 bacterium/cell). In addition, meningococcal pili recognized human endothelial and epithelial cells but not cells originating from other animals. Variants of strain C311 were obtained that expressed pilins of reduced apparent M(r) and exhibited a marked increase in adherence to epithelial cells. Structural analysis of pilins from two hyper-adherent variants and the parent strain were carried out by DNA sequencing of their pilE genes. Deduced molecular weights of pilins were considerably lower compared with their apparent M(r) values on SDS-PAGE. Hyper-adherent pilins shared unique changes in sequence including substitution of Asn-113 for Asp-113 and changes from Asn-Asp-Thr-Asp to Thr-Asp-Ala-Lys at residues 127-130 in mature pilin. Asn residues 113 and 127 of 'parental' pilin both form part of the typical eukaryotic N-glycosylation motif Asn-X-Ser/Thr and could potentially be glycosylated post-translationally. The presence of carbohydrate on pilin was demonstrated and when pilins were deglycosylated, their migration on SDS-PAGE increased, supporting the notion that variable glycosylation accounts for discrepancies in apparent and deduced molecular weights. Functionally distinct pilins produced by two fully piliated variants of a second strain (MC58) differed only in that the putative glycosylation motif Asn-60-Asn-61-Thr-62 in an adherent variant was replaced with Asp-60-Asn-61-Ser-62 in a non-adherent variant. Fully adherent backswitchers obtained from the non-adherent variant always regained Asn-60 but retained Ser-62. We propose, therefore, that functional variations in N. meningitidis pili may be modulated in large part by primary amino acid sequence changes that ablate or create N-linked glycosylation sites on the pilin subunit.

The complete general secretory pathway in gram-negative bacteria

The unifying feature of all proteins that are transported out of the cytoplasm of gram-negative bacteria by the general secretory pathway (GSP) is the presence of a long stretch of predominantly hydrophobic amino acids, the signal sequence. The interaction between signal sequence-bearing proteins and the cytoplasmic membrane may be a spontaneous event driven by the electrochemical energy potential across the cytoplasmic membrane, leading to membrane integration. The translocation of large, hydrophilic polypeptide segments to the periplasmic side of this membrane almost always requires at least six different proteins encoded by the sec genes and is dependent on both ATP hydrolysis and the electrochemical energy potential. Signal peptidases process precursors with a single, amino-terminal signal sequence, allowing them to be released into the periplasm, where they may remain or whence they may be inserted into the outer membrane. Selected proteins may also be transported across this membrane for assembly into cell surface appendages or for release into the extracellular medium. Many bacteria secrete a variety of structurally different proteins by a common pathway, referred to here as the main terminal branch of the GSP. This recently discovered branch pathway comprises at least 14 gene products. Other, simpler terminal branches of the GSP are also used by gram-negative bacteria to secrete a more limited range of extracellular proteins.

Interaction of two variable proteins (PilE and PilC) required for pilus-mediated adherence of Neisseria gonorrhoeae to human epithelial cells

Pili confer the initial ability of Neisseria gonorrhoeae to bind to epithelial cells. Pilin (PilE), the major pilus subunit, and a minor protein termed PilC, reportedly essential for pilus biogenesis, undergo intra-strain phase and structural variation. We demonstrate here that at least two different adherence properties are associated with the gonococcal pili: one is specific for erythrocytes, which is virtually unaffected by PilE variation, and another is specific for epithelial cells, and is modulated in response to the variation of PilE. Based on this finding, mutants of a recA- strain were selected that had lost the ability to bind to human cornea epithelial cells (A-) but retained the ability to form pili (P+) and to agglutinate human erythrocytes (H+). The adherence-negative mutants failed to produce detectable levels of PilC1 or PilC2 proteins, representing piIC phase variants generated in the absence of RecA. The A- pilC phase variants were indistinguishable from their A+ parents and spontaneous A+ revertants with regard to the amount of PilE produced and its electrophoretic mobility, the degrees of piliation and haemagglutination, and the pilE nucleotide sequence. These data demonstrate a central role for PilC in pilus-mediated adherence of N. gonorrhoeae to human epithelial cells and further indicate that neither PilC1 nor PilC2 is obligatory for the assembly of gonococcal pili.