Gonococcal pili. Primary structure and receptor binding domain

Canary in the Coal Mine: How Resistance Surveillance in Commensals Could Help Curb the Spread of AMR in Pathogenic Neisseria

Antimicrobial resistance (AMR) is widespread within Neisseria gonorrhoeae populations. Recent work has highlighted the importance of commensal Neisseria (cN) as a source of AMR for their pathogenic relatives through horizontal gene transfer (HGT) of AMR alleles, such as mosaic penicillin binding protein 2 (penA), multiple transferable efflux pump (mtr), and DNA gyrase subunit A (gyrA) which impact beta-lactam, azithromycin, and ciprofloxacin susceptibility, respectively. However, nonpathogenic commensal species are rarely characterized. Here, we propose that surveillance of the universally carried commensal Neisseria may play the role of the "canary in the coal mine," and reveal circulating known and novel antimicrobial resistance determinants transferable to pathogenic Neisseria. We summarize the current understanding of commensal Neisseria as an AMR reservoir, and call to increase research on commensal Neisseria species, through expanding established gonococcal surveillance programs to include the collection, isolation, antimicrobial resistance phenotyping, and whole-genome sequencing (WGS) of commensal isolates. This will help combat AMR in the pathogenic Neisseria by: (i) determining the contemporary AMR profile of commensal Neisseria, (ii) correlating AMR phenotypes with known and novel genetic determinants, (iii) qualifying and quantifying horizontal gene transfer (HGT) for AMR determinants, and (iv) expanding commensal Neisseria genomic databases, perhaps leading to the identification of new drug and vaccine targets. The proposed modification to established Neisseria collection protocols could transform our ability to address AMR N. gonorrhoeae, while requiring minor modifications to current surveillance practices. IMPORTANCE Contemporary increases in the prevalence of antimicrobial resistance (AMR) in Neisseria gonorrhoeae populations is a direct threat to global public health and the effective treatment of gonorrhea. Substantial effort and financial support are being spent on identifying resistance mechanisms circulating within the gonococcal population. However, these surveys often overlook a known source of resistance for gonococci-the commensal Neisseria. Commensal Neisseria and pathogenic Neisseria frequently share DNA through horizontal gene transfer, which has played a large role in rendering antibiotic therapies ineffective in pathogenic Neisseria populations. Here, we propose the expansion of established gonococcal surveillance programs to integrate a collection, AMR profiling, and genomic sequencing pipeline for commensal species. This proposed expansion will enhance the field's ability to identify resistance in and from nonpathogenic reservoirs and anticipate AMR trends in pathogenic Neisseria.

Repurposed Drugs That Block the Gonococcus-Complement Receptor 3 Interaction Can Prevent and Cure Gonococcal Infection of Primary Human Cervical Epithelial Cells

Novel therapies that avert the problem of Neisseria gonorrhoeae with acquired antibiotic resistance are urgently needed. Gonococcal infection of the human cervix is initiated by an interaction between a galactose modification made to its surface appendages, pili, and the I-domain region of (host) complement receptor 3 (CR3). By targeting this crucial gonococcal–I-domain interaction, it may be possible to prevent cervical infection in females. To this end, we identified the I-domain galactose-binding epitope of CR3 and characterized its galactose lectin activity. Moreover, we identified two drugs, carbamazepine and methyldopa, as effective host-targeted therapies for gonorrhea treatment. At doses below those currently used for their respective existing indications, both carbamazepine and methyldopa were more effective than ceftriaxone in curing cervical infection ex vivo. This host-targeted approach would not be subject to N. gonorrhoeae drug resistance mechanisms. Thus, our data suggest a long-term solution to the growing problem of multidrug-resistant N. gonorrhoeae infections.

In the absence of a vaccine, multidrug-resistant Neisseria gonorrhoeae has emerged as a major human health threat, and new approaches to treat gonorrhea are urgently needed. N. gonorrhoeae pili are posttranslationally modified by a glycan that terminates in a galactose. The terminal galactose is critical for initial contact with the human cervical mucosa via an interaction with the I-domain of complement receptor 3 (CR3). We have now identified the I-domain galactose-binding epitope and characterized its galactose-specific lectin activity. Using surface plasmon resonance and cellular infection assays, we found that a peptide mimic of this galactose-binding region competitively inhibited the N. gonorrhoeae-CR3 interaction. A compound library was screened for potential drugs that could similarly prohibit the N. gonorrhoeae-CR3 interaction and be repurposed as novel host-targeted therapeutics for multidrug-resistant gonococcal infections in women. Two drugs, methyldopa and carbamazepine, prevented and cured cervical cell infection by multidrug-resistant gonococci by blocking the gonococcal-CR3 I-domain interaction.

Neisseria gonorrhoeae Becomes Susceptible to Polymyxin B and Colistin in the Presence of PBT2

Neisseria gonorrhoeae (N. gonorrhoeae) causes the sexually transmitted disease gonorrhea, which has a global incidence of 106 million cases per year. No vaccine is available to prevent the disease, and the emergence of multidrug resistant (MDR) strains makes N. gonorrhoeae an immediate public health threat. Here, we show that an ionophore, PBT2, can reverse the intrinsic resistance of N. gonorrhoeae to polymyxin B and colistin. These antibiotics administered in combination with PBT2 may be an effective path to treat MDR gonococcal infections.

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.

Pili mediated intercellular forces shape heterogeneous bacterial microcolonies prior to multicellular differentiation

Microcolonies are aggregates of a few dozen to a few thousand cells exhibited by many bacteria. The formation of microcolonies is a crucial step towards the formation of more mature bacterial communities known as biofilms, but also marks a significant change in bacterial physiology. Within a microcolony, bacteria forgo a single cell lifestyle for a communal lifestyle hallmarked by high cell density and physical interactions between cells potentially altering their behaviour. It is thus crucial to understand how initially identical single cells start to behave differently while assembling in these tight communities. Here we show that cells in the microcolonies formed by the human pathogen Neisseria gonorrhoeae (Ng) present differential motility behaviors within an hour upon colony formation. Observation of merging microcolonies and tracking of single cells within microcolonies reveal a heterogeneous motility behavior: cells close to the surface of the microcolony exhibit a much higher motility compared to cells towards the center. Numerical simulations of a biophysical model for the microcolonies at the single cell level suggest that the emergence of differential behavior within a multicellular microcolony of otherwise identical cells is of mechanical origin. It could suggest a route toward further bacterial differentiation and ultimately mature biofilms.

Vaginal Lactobacilli Reduce Neisseria gonorrhoeae Viability through Multiple Strategies: An in Vitro Study

The emergence and spread of antimicrobial resistance in Neisseria gonorrhoeae (GC) underline the need of “antibiotic-free” strategies for the control of gonorrhea. The aim of this study was to assess the anti-gonococcal activity of 14 vaginal Lactobacillus strains, belonging to different species (L. crispatus, L. gasseri, L. vaginalis), isolated from healthy pre-menopausal women. In particular, we performed “inhibition” experiments, evaluating the ability of both lactobacilli cells and culture supernatants in reducing GC viability, at two different contact times (7 and 60 min). First, we found that the acidic environment, associated to lactobacilli metabolism, is extremely effective in counteracting GC growth, in a pH- and time-dependent manner. Indeed, a complete abolishment of GC viability by lactobacilli supernatants was observed only for pH values < 4.0, even at short contact times. On the contrary, for higher pH values, no 100%-reduction of GC growth was reached at any contact time. Experiments with organic/inorganic acid solutions confirmed the strict correlation between the pH levels and the anti-gonococcal effect. In this context, the presence of lactate seemed to be crucial for the anti-gonococcal activity, especially for pH values in the range 4.4–5.3, indicating that the presence of H⁺ ions is necessary but not sufficient to kill gonococci. Moreover, experiments with buffered supernatants led to exclude a direct role in the GC killing by other bioactive molecules produced by lactobacilli. Second, we noticed that lactobacilli cells are able to reduce GC viability and to co-aggregate with gonococci. In this context, we demonstrated that released-surface components with biosurfactant properties, isolated from “highly-aggregating” lactobacilli, could affect GC viability. The antimicrobial potential of biosurfactants isolated from lactobacilli against pathogens has been largely investigated, but this is the first report about a possible use of these molecules in order to counteract GC infectivity. In conclusion, we identified specific Lactobacillus strains, mainly belonging to L. crispatus species, able to counteract GC viability through multiple mechanisms. These L. crispatus strains could represent a new potential probiotic strategy for the prevention of GC infections in women.

Type IV pilus secretins have extracellular C termini

Type IV pili (T4Ps) are surface appendages used by Gram-negative and Gram-positive pathogens for motility and attachment to epithelial surfaces. In Gram-negative bacteria, such as the important pediatric pathogen enteropathogenic Escherichia coli (EPEC), during extension and retraction, the pilus passes through an outer membrane (OM) pore formed by the multimeric secretin complex. The secretin is common to Gram-negative assemblies, including the related type 2 secretion (T2S) system and the type 3 secretion (T3S) system. The N termini of the secretin monomers are periplasmic and in some systems have been shown to mediate substrate specificity. In this study, we mapped the topology of BfpB, the T4P secretin from EPEC, using a combination of biochemical and biophysical techniques that allowed selective identification of periplasmic and extracellular residues. We applied rules based on solved atomic structures of outer membrane proteins (OMPs) to generate our topology model, combining the experimental results with secondary structure prediction algorithms and direct inspection of the primary sequence. Surprisingly, the C terminus of BfpB is extracellular, a result confirmed by flow cytometry for BfpB and a distantly related T4P secretin, PilQ, from Pseudomonas aeruginosa. Keeping with prior evidence, the C termini of two T2S secretins and one T3S secretin were not detected on the extracellular surface. On the basis of our data and structural constraints, we propose that BfpB forms a beta barrel with 16 transmembrane beta strands. We propose that the T4P secretins have a C-terminal segment that passes through the center of each monomer.

IMPORTANCE

Secretins are multimeric proteins that allow the passage of secreted toxins and surface structures through the outer membranes (OMs) of Gram-negative bacteria. To date, there have been no atomic structures of the C-terminal region of a secretin, although electron microscopy (EM) structures of the complex are available. This work provides a detailed topology prediction of the membrane-spanning domain of a type IV pilus (T4P) secretin. Our study used innovative techniques to provide new and comprehensive information on secretin topology, highlighting similarities and differences among secretin subfamilies. Additionally, the techniques used in this study may prove useful for the study of other OM proteins.

Neisseria gonorrhoeae pilin glycan contributes to CR3 activation during challenge of primary cervical epithelial cells

Expression of type IV pili by Neisseria gonorrhoeae plays a critical role in mediating adherence to human epithelial cells. Gonococcal pilin is modified with an O-linked glycan, which may be present as a di- or monosaccharide because of phase variation of select pilin glycosylation genes. It is accepted that bacterial proteins may be glycosylated; less clear is how the protein glycan may mediate virulence. Using primary, human, cervical epithelial (i.e. pex) cells, we now provide evidence to indicate that the pilin glycan mediates productive cervical infection. In this regard, pilin glycan-deficient mutant gonococci exhibited an early hyper-adhesive phenotype but were attenuated in their ability to invade pex cells. Our data further indicate that the pilin glycan was required for gonococci to bind to the I-domain region of complement receptor 3, which is naturally expressed by pex cells. Comparative, quantitative, infection assays revealed that mutant gonococci lacking the pilin glycan did not bind to the I-domain when it is in a closed, low-affinity conformation and cannot induce an active conformation to complement receptor 3 during pex cell challenge. To our knowledge, these are the first data to directly demonstrate how a protein-associated bacterial glycan may contribute to pathogenesis.

The PilC adhesin of the Neisseria type IV pilus - binding specificities and new insights into the nature of the host cell receptor

Type IV pili of Neisseria gonorrhoeae and Neisseria meningitidis mediate the first contact to human mucosal epithelial cells, an interaction which is also critical for the interaction with vascular endothelial cells. The PilC proteins have been characterized as the principal pilus-associated adhesin. Here we show that PilC2 exhibits a defined cell and tissue tropism, as it binds to human epithelial and endothelial cell lines, but not to human T cells or fibroblasts. Piliated gonococci and PilC2 exhibit similar patterns of binding to human epithelial and endothelial cells, supporting the function of PilC as the key pilus adhesin. Although CD46 has previously been suggested to be a pilus receptor, several observations indicate that neisserial type IV pili and the pilus adhesin PilC2 interact with epithelial cells in a CD46 independent manner. Biochemical approaches were used to characterize the nature of host cell factors mediating binding of piliated gonococci and PilC2 protein. Our data indicate that the putative host cell receptor for gonococcal pili and the PilC2 pilus adhesin is a surface protein. Glycostructures were found to not be involved in binding. Moreover, we observed the uptake of purified PilC2 protein together with its receptor via receptor-mediated endocytosis and subsequent receptor re-exposure on the cell surface. Our data support the existence of a specific pilus receptor and provide intriguing information on the nature of the receptor.

The role of pilin glycan in neisserial pathogenesis

The pilus of pathogenic Neisseria is a polymer composed mainly of the glycoprotein, pilin. Recent investigations significantly enhanced characterization of pilin glycan (Pg) from N. gonorrhoeae (gonococcus, GC) and N. meningitidis (meningococcus, MC). Several pilin glycosylation genes were discovered recently from these bacteria and some of these genes transfer sugars previously unknown to be present in neisserial pili. Due to these findings, glycans of GC and MC pilin are now considered more complex. Furthermore, various Pg can be expressed by different strains and variants of GC, as well as MC. Intra-species variation of Pg between different groups of GC or MC can partly be due to polymorphisms of glycosylation genes. In pilus of pathogenic Neisseria, alternative glycoforms are also produced due to phase-variation (Pv) of pilin glycosylation genes. Most remarkably, the pgtA (pilin glycosyl transferase A) gene of GC can either posses or lack the ability of Pv. Many GC strains carry the phase-variable (Pv+) pgtA, whereas others carry the allele lacking Pv (Pv-). Mostly, the GC isolates from disseminated gonococcal infection (DGI) carry Pv+ pgtA but organisms from uncomplicated gonorrhea (UG) contain the Pv- allele. This data suggests that Pv of pgtA facilitates DGI, whereas constitutive expression of the Pv- pgtA may promote UG. Additional implications of Pg in various physiological and pathogenic mechanisms of Neisseria can also be envisaged based on various recent data.

Gonococcal phospholipase d modulates the expression and function of complement receptor 3 in primary cervical epithelial cells

CR3-mediated endocytosis is a primary mechanism by which Neisseria gonorrhoeae elicits membrane ruffling and cellular invasion of the cervical epithelia. Our data indicate that, upon infection of cervical epithelia, N. gonorrhoeae specifically releases proteins, including a phospholipase D (PLD) homolog, which facilitate membrane ruffling. To elucidate the function of gonococcal PLD in infection of the cervical epithelia, we constructed an N. gonorrhoeae PLD mutant. By comparative association and/or invasion assays, we demonstrated that PLD mutant gonococci are impaired in their ability to adhere to and to invade primary cervical cells. This defect can be rescued by the addition of supernatants obtained from wild-type-infected cell monolayers but not by exogenously added Streptomyces PLD. The decreased level of total cell association (i.e., adherence and invasion) observed for mutant gonococci is, in part, attributed to the inability of these bacteria to recruit CR3 to the cervical cell surface with extended infection. Using electron microscopy, we demonstrate that gonococcal PLD may be necessary to potentiate membrane ruffling and clustering of gonococci on the cervical cell surface. These data may be indicative of the inability of PLD mutant gonococci to recruit CR3 to the cervical cell surface. Alternatively, in the absence of gonococcal PLD, signal transduction events required for CR3 clustering may not be activated. Collectively, our data indicate that PLD augments CR3-mediated gonococcus invasion of and survival within cervical epithelia.

The role of lipooligosaccharide in Neisseria gonorrhoeae pathogenesis of cervical epithelia: lipid A serves as a C3 acceptor molecule

The use of primary, human, ecto- and endocervical epithelial cell cultures has increased our understanding of the pathogenesis of gonococcal infection in women. Primary cervical epithelial cells express complement (C') receptor type 3 (CR3) and C' proteins required for alternative pathway (AP) activity. Gonococcus -induced membrane ruffling and cellular invasion of primary cervical epithelia is mediated by CR3 and requires co-operative CR3 binding by gonococcus-bound iC3b, porin and pilus. We have extended these studies to identify the site of C3 deposition upon gonococci within the cervical microenvironment. By immunoprecipitation and ELISA we demonstrate that covalent and non-covalent associations occurred between gonococcal LOS and C' protein C3. Sialylation or LOS truncation did not alter the gonococcus-CR3 interaction. By Western blot analysis we observed comparable C3 opsonization patterns among a panel of LOS truncation mutants, sialylated wild-type gonococci, or wild-type bacteria that were not sialylated. Quantitative association/invasion assays performed in the presence or absence of LOS competimers support C3b deposition on the lipid A core structure. Our findings demonstrate a role for lipid A as a C3 acceptor site and suggest that multiple factors govern C3b deposition and its subsequent conversion to iC3b on the surface of the gonococcus within the cervical microenvironment.

A co-operative interaction between Neisseria gonorrhoeae and complement receptor 3 mediates infection of primary cervical epithelial cells

Little is known about the pathogenesis of gonococcal infection within the lower female genital tract. We recently described the distribution of complement receptor 3 (CR3) on epithelia of the female genital tract. Our studies further indicate that CR3-mediated endocytosis serves as a primary mechanism by which N. gonorrhoeae elicits membrane ruffling and cellular invasion of primary, human, cervical epithelial cells. We have extended these studies to describe the nature of the gonococcus-CR3 interaction. Western Blot analysis demonstrated production of alternative pathway complement components by ecto- and endocervical cells which allows C3b deposition on gonococci and its rapid conversion to iC3b. Anti-iC3b and -factor I antibodies significantly inhibited adherence and invasion of primary cervical cells, suggesting that iC3b covalently bound to the gonococcus serves as a primary ligand for CR3 adherence. However, gonococcal porin and pili also bound to the I-domain of CR3 in a non-opsonic manner. Binding of porin and pili to CR3 were required for adherence to and invasion of cervical epithelia. Collectively, these data suggest that gonococcal adherence to CR3 occurs in a co-operative manner, which requires gonococcal iC3b-opsonization, porin and pilus. In conjunction, these molecules facilitate targeting to and successful infection of the cervical epithelium.

Implications of phase variation of a gene (pgtA) encoding a pilin galactosyl transferase in gonococcal pathogenesis

The pilin glycoprotein (PilE) is the main building block of the pilus of Neisseria gonorrhoeae (gonococcus [GC]). GC pilin is known to carry a disaccharide O-glycan, which has an alphaGal attached to the O-linked GlcNAc by a 1-3 glycosidic bond. In this report, we describe the cloning and characterization of the GC gene, pilus glycosyl transferase A (pgtA), which encodes the galactosyl transferase that catalyzes the synthesis of this Gal-GlcNAc bond of pilin glycan. A homopolymeric tract of Gs (poly-G) is present in the pgtA gene of many GC strains, and this pgtA with poly-G can undergo phase variation (Pv). However, in many other GC, pgtA lacks the poly-G and is expressed constitutively without Pv. Furthermore, by screening a large number of clinical isolates, a significant correlation was observed between the presence of poly-G in pgtA and the dissemination of GC infection. Poly-G was found in pgtA in all (24 out of 24) of the isolates from patients with disseminated gonococcal infection (DGI). In contrast, for the vast majority (20 out of 28) of GC isolated from uncomplicated gonorrhea (UG) patients, pgtA lacked the poly-G. These results indicate that Pv of pgtA is likely to be involved in the conversion of UG to DGI.

Neisseria gonorrhoeae PilV, a type IV pilus-associated protein essential to human epithelial cell adherence

Type IV pili (Tfp) of Neisseria gonorrhoeae, the Gram-negative etiologic agent of gonorrhea, facilitate colonization of the human host. Tfp are assumed to play a key role in the initial adherence to human epithelial cells by virtue of the associated adhesin protein PilC. To examine the structural and functional basis for adherence in more detail, we identified potential genes encoding polypeptides sharing structural similarities to PilE (the Tfp subunit) within the N. gonorrhoeae genome sequence database. We show here that a fiber subunit-like protein, termed PilV, is essential to organelle-associated adherence but dispensable for Tfp biogenesis and other pilus-related phenotypes, including autoagglutination, competence for natural transformation, and twitching motility. The adherence defect in pilV mutants cannot be attributed to reduced levels of piliation, defects in fiber anchoring to the bacterial cell surface, or to unstable pilus expression related to organelle retraction. PilV is expressed at low levels relative to PilE and copurifies with Tfp fibers in a PilC-dependent fashion. Purified Tfp from pilV mutants contain PilC adhesin at reduced levels. Taken together, these data support a model in which PilV functions in adherence by promoting the functional display of PilC in the context of the pilus fiber.

Detection of shared magnetic antigenic determinants on whole Moraxella bovis pili by use of antisera to cyanogen bromide-cleaved M. bovis pilus protein

OBJECTIVE

To determine the ability of antisera against cyanogen bromide-cleaved pili from 4 strains of Moraxella bovis to react with whole or nondenatured pili.

SAMPLE POPULATION

Antisera to 4 strains of M. bovis produced by New Zealand White rabbits.

PROCEDURE

Pili from 4 strains of M. bovis were collected and purified. Pilus proteins (pilin) were cleaved, using cyanogen bromide. Whole pilus and cyanogen bromide-cleaved pilin were injected into rabbits. Antisera were serially diluted, reacted with 4 strains of M. bovis, and examined by immunoelectron microscopy and indirect immunofluorescence.

RESULTS

Antisera to whole pili aggregated and distorted pili from homologous strains, but pili from heterologous strains were unaffected. Antisera to cleaved pilin fragments resulted in partial aggregation and thickening of homologous and heterologous pili, suggestive of heterospecific antibodies. Attachment of antibodies to pili was detected by indirect immunofluorescence, indicating a strong reaction of antisera to whole pili with homologous pili. Weak cross-reactions were evident with certain heterologous strains. In contrast, antisera to cleaved pilin fragments reacted strongly with pili from homologous and heterologous strains.

CONCLUSIONS AND CLINICAL RELEVANCE

We detected shared antigenic determinants on pili from various strains of M. bovis that were not immunogenic in intact pili. These sites were immunogenic after cleavage of pilus protein with cyanogen bromide, and antisera produced to protein fragments reacted with whole pili from heterologous strains of the organism. Vaccines produced from cyanogen bromide-treated pili may induce broader immunity against infectious bovine keratoconjuctivitis than that provided by currently available vaccines.

Immunoblot analysis of cyanogen bromide-cleaved Moraxella bovis pilin reveals presence of shared antigenic determinants on pili from heterologous strains

Moraxella bovis pilus proteins, collected and purified from four strains of M. bovis, were cleaved with cyanogen bromide. Two major fragments were produced. Antisera were produced in rabbits to the pilin protein fragments and to whole uncleaved pili from these strains. Immunoblots of whole and cyanogen bromide-cleaved pilin were reacted with the homologous and heterologous antisera to whole pili and cleaved pilin. Antisera to whole pili reacted strongly with homologous pilin. Weaker and inconsistent reactions were detected with heterologous pilin. Antisera produced to cyanogen bromide-cleaved pilin proteins reacted strongly with homologous and heterologous pilin fragments and uncleaved pilin proteins. These findings demonstrate the presence of conserved antigenic determinants on pili from heterologous strains that are non-immunogenic in the intact pilus but are immunogenic after treatment with cyanogen bromide. Cyanogen bromide-treated pilus preparation might have potential as a vaccine because antibodies are induced against heterologous strains of M. bovis, whether these cross-reactive antibodies are protective remains to be determined.

[Gonorrhea]

Gonorrhea is a common bacterial infection caused by Neisseria gonorrhoeae, a Gram-negative diplococcus that is transmitted almost exclusively by sexual contact or perinatally. It primarily affects the mucous membranes of the lower genital tract and less frequently those of the rectum, oropharynx, and conjunctivae. Ascending genital infection in women leads to the predominant complication, acute salpingitis, one of the most common causes of female infertility in the world. Since the 1990s, a remarkable surge of information ensued regarding the pathogenesis of gonorrhea and its agent. Gonorrhea has proven difficult to control in most populations and remains a prime example of the influence that social, behavioral, and demographic factors can have on the epidemiology of an infectious disease. The management of gonorrhea and other sexually transmitted infections requires both treatment of the patient as an individual and of his or her sexual partner(s) as a public health measure to interrupt the onward spread of infection and prevent long-term complications.

Iron availability regulates DNA recombination in Neisseria gonorrhoeae

The pilus of Neisseria gonorrhoeae (the gonococcus Gc), the causative agent of gonorrhoea, promotes attachment of the gonococcus to the host epithelium and is essential for the establishment of disease. The ability of N. gonorrhoeae to infect previously exposed individuals is partially due to pilus antigenic variation. In addition, variation of the pilus has been proposed to function in the adaptation of the gonococcus to host environments. Previously, we described the development of a competitive reverse transcriptase (RT)-PCR assay that quantifies the frequency of pilin antigenic variation within a gonococcal population. Using this assay, the effect of different biologically relevant environmental conditions on the frequency of pilin antigenic variation was tested. Of the environmental conditions examined in vitro, only limited iron affected a significant change in the frequency of antigenic variation. Further investigation revealed that an observed increase in pilin antigenic variation reflected an increase in other DNA recombination and DNA repair processes within iron-starved cultures. In addition, this low iron-induced increase was determined to be independent of changes in RecA expression and was observed in a Fur mutant strain. As gonococci encounter conditions of low iron during infection, these data suggest that iron-limitation signals for increased recombinational events that are important for gonococcal pathogenesis.

Roles of PilC and PilE proteins in pilus-mediated adherence of Neisseria gonorrhoeae and Neisseria meningitidis to human erythrocytes and endothelial and epithelial cells

Unlike other type 4 pili, the neisserial pili consist of at least two distinct proteins, the highly variable major subunit PilE forming the pilus fiber and the tip-associated adhesin PilC. PilC protein purified either from gonococci or from Escherichia coli interacted with different human epithelial cell lines, primary epithelial and endothelial cells. The binding of PilC protein efficiently prevented the attachment of piliated Neisseria gonorrhoeae and Neisseria meningitidis to these cell types. Fluorescent beads coated with pili prepared from piliated wild-type N. gonorrhoeae also adhered to these cells, in contrast to beads coated with pili prepared from a piliated PilC-deficient mutant. In the latter case, the binding of fluorescent beads was restored after pretreatment of the pilus-loaded beads with purified PilC. Piliated wild-type N. gonorrhoeae, the piliated PilC-deficient mutant, and N. gonorrhoeae pili assembled in Pseudomonas aeruginosa agglutinated human erythrocytes, while nonpiliated gonococci did not. Consistently, purified PilC did not agglutinate or bind to human erythrocytes, suggesting that N. gonorrhoeae PilE is responsible for pilus-mediated hemagglutination.

Crystallographic structure reveals phosphorylated pilin from Neisseria: phosphoserine sites modify type IV pilus surface chemistry and fibre morphology

Understanding the structural biology of type IV pili, fibres responsible for the virulent attachment and motility of numerous bacterial pathogens, requires a detailed understanding of the three-dimensional structure and chemistry of the constituent pilin subunit. X-ray crystallographic refinement of Neisseria gonorrhoeae pilin against diffraction data to 2.6 A resolution, coupled with mass spectrometry of peptide fragments, reveals phosphoserine at residue 68. Phosphoserine is exposed on the surface of the modelled type IV pilus at the interface of neighbouring pilin molecules. The site-specific mutation of serine 68 to alanine showed that the loss of the phosphorylation alters the morphology of fibres examined by electron microscopy without a notable effect on adhesion, transformation, piliation or twitching motility. The structural and chemical characterization of protein phosphoserine in type IV pilin subunits is an important indication that this modification, key to numerous regulatory aspects of eukaryotic cell biology, exists in the virulence factor proteins of bacterial pathogens. These O-linked phosphate modifications, unusual in prokaryotes, thus merit study for possible roles in pilus biogenesis and modulation of pilin chemistry for optimal in vivo function.

Mutational analysis of plasmid R64 thin pilus prepilin: the entire prepilin sequence is required for processing by type IV prepilin peptidase

The thin pili of IncI1 plasmid R64, which is required for conjugation in liquid media, belong to the type IV pilus family. They consist of a major subunit, the pilS product, and a minor component, one of the seven pilV products. The pilS product is first synthesized as a 22-kDa prepilin, processed to a 19-kDa mature pilin by the function of the pilU product, and then secreted outside the cell. The mature pilin is assembled to form a thin pilus with the pilV product. To reveal the relationship between the structure and function of the pilS product, 27 missense mutations, three N-terminal deletions, and two C-terminal deletions were constructed by PCR and site-directed mutagenesis. The characteristics of 32 mutant pilS products were analyzed. Four pilS mutant phenotype classes were identified. The products of 10 class I mutants were not processed by prepilin peptidase; the extracellular secretion of the products of two class II mutants was inhibited; from 11 class III mutants, thin pili with reduced activities in liquid mating were formed; from 9 class IV mutants, thin pili with mating activity similar to that of the wild-type pilS gene were formed. The point mutations of the class I mutants were distributed throughout the prepilin sequence, suggesting that processing of the pilS product requires the entire prepilin sequence.

Isolation and characterization of three Streptococcus pneumoniae transformation-specific loci by use of a lacZ reporter insertion vector

Although more than a dozen new proteins are produced when Streptococcus pneumoniae cells become competent for genetic transformation, only a few of the corresponding genes have been identified to date. To find genes responsible for the production of competence-specific proteins, a random lacZ transcriptional fusion library was constructed in S. pneumoniae by using the insertional lacZ reporter vector pEVP3. Screening the library for clones with competence-specific beta-galactosidase (beta-Gal) production yielded three insertion mutants with induced beta-Gal levels of about 4, 10, and 40 Miller units. In all three clones, activation of the lacZ reporter correlated with competence and depended on competence-stimulating peptide. Chromosomal loci adjacent to the integrated vector were subcloned from the insertion mutants, and their nucleotide sequences were determined. Genes at two of the loci exhibited strong similarity to parts of Bacillus subtilis com operons. One locus contained open reading frames (ORFs) homologous to the comEA and comEC genes in B. subtilis but lacked a comEB homolog. A second locus contained four ORFs with homology to the B. subtilis comG gene ORFs 1 to 4, but comG gene ORFs 5 to 7 were replaced in S. pneumoniae with an ORF encoding a protein homologous to transport ATP-binding proteins. Genes at all three loci were confirmed to be required for transformation by mutagenesis using pEVP3 for insertion duplications or an erm cassette for gene disruptions.

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.

Consequences of the loss of O-linked glycosylation of meningococcal type IV pilin on piliation and pilus-mediated adhesion

Pili, which are assembled from protein subunits called pilin, are indispensable for the adhesion of capsulated Neisseria meningitidis (MC) to eukaryotic cells. Both MC and Neisseria gonorrhoeae (GC) pilins are glycosylated, but the effect of this modification is unknown. In GC, a galactose alpha-1,3-N-acetyl glucosamine is O-linked to Ser-63, whereas in MC, an O-linked trisaccharide is present between residues 45 and 73 of pilin. As Ser-63 was found to be conserved in pilin variants from different strains, it was replaced by Ala in two MC variants to test the possible role of this residue in pilin glycosylation and modulation of pili function. The mutated alleles were stably expressed in MC, and the proteins they encoded migrated more quickly than the normal protein during SDS-PAGE. As controls, neighbouring Asn-61 and Ser-62 were replaced by an Ala with no effect on electrophoretic mobility. Silver staining of purified pilin obtained from MC after oxidation with periodic acid confirmed the loss of glycosylation in the Ser-63-->Ala pilin variants. Mass spectrometry of HPLC-purified trypsin-digested peptides of pilin and Ser-63-->Ala pilin confirmed that peptide 45-73 has the molecular size of a glycopeptide in the wild type. In strains producing non-glycosylated pilin variants, we observed that (i) no truncated S pilin monomer was produced; (ii) piliation was slightly increased; and (iii) presumably as a consequence, adhesiveness for epithelial cells was increased 1.6- to twofold in these derivatives. In addition, pilin monomers and/or individual pilus fibres, obtained after solubilization of a crude pili preparation in a high pH buffer, were reassociated into insoluble aggregates of pili more completely with non-glycosylated variants than with the normal pilin. Taken together, these data eliminate a major role for pilin glycosylation in piliation and subsequent pilus-mediated adhesion, but they demonstrate that glycosylation facilitates solubilization of pilin monomers and/or individual pilus fibres.

Discovery of a novel protein modification: alpha-glycerophosphate is a substituent of meningococcal pilin

Pili, which are filamentous protein structures on the surface of the meningitis-causing organism Neisseria meningitidis, are known to be post-translationally modified with substituents that affect their mobility in SDS/PAGE and which might play a crucial role in adherence and bloodstream invasion. Tryptic digests of pili were analysed by fast atom bombardment and electrospray MS to identify putative modifications. Serine-93 was found to carry a novel modification of alpha-glycerophosphate. This is the first time that alpha-glycerophosphate has been observed as a substituent of a prokaryotic or eukaryotic protein.

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.

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.

Identification, localization, and distribution of the PilT protein in Neisseria gonorrhoeae

A monoclonal antibody (MAb) directed against a highly conserved protein of Neisseria gonorrhoeae with a molecular size of 40 kDa was isolated and characterized. The protein antigen detected by this MAb was detected by enzyme-linked immunosorbent assay and immunoblotting in all strains of N. gonorrhoeae tested across a wide range of serovars. The 40-kDa protein was found to be expressed at relatively low levels and localized to both the cytosolic and cytoplasmic membrane fractions. Screening of a lambda gt11 expression library derived from gonococcal genomic DNA with the anti-40-kDa MAb and DNA sequence analysis suggested that the 40-kDa protein and the product of the gonococcal pilT gene were identical. Immunoblotting analysis of gonococcal mutants carrying defined mutations in the pilT gene confirmed that the 40-kDa protein was indeed PilT. The N-terminal sequence derived by microsequencing of the protein purified from gonococci led to the correction of the previously published pilT gene sequence. Sequencing of the pilT gene from three different strains revealed an extremely high degree of conservation at both the amino acid and DNA levels.

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.

Cloning and characterization of the bundle-forming pilin gene of enteropathogenic Escherichia coli and its distribution in Salmonella serotypes

bfp, the structural gene of the major repeating bundle-forming pilus (BFP) subunit, was cloned from the enteroadherent factor (EAF) plasmid of enteropathogenic Escherichia coli (EPEC) strain B171 (O111:NM). The bfp open reading frame encoded a 193-amino-acid protein; comparison of this sequence with the biochemically determined N-terminal amino acid sequence showed that the mature pilin protein is comprised of 180 amino acids, that this sequence is similar to other members of the type IV pilin family, and that it is preceded by a 13-amino-acid signal peptide. Expression of the cloned bfp structural gene in an EPEC strain that had been cured of the EAF plasmid yielded a 21,000 dalton protein that co-migrated with the BFP precursor protein. Thus, other genes, probably carried by the EAF plasmid, are required for the maturation of the bfp product and for the production of extracellular pilus filaments. Use of bfp as a hybridization probe showed that homologous sequences are present in all tested EPEC strains and in 13 of 16 tested Salmonella serotypes. Fifty per cent of these bfp probe-sensitive salmonellae exhibited the localized-adherence (LA) phenotype when incubated with tissue culture cell monolayers, a trait previously associated with EAF plasmid-containing EPEC strains. Scanning electron micrographs of a bfp probe-sensitive, LA-positive Salmonella dublin strain showed that it grows as adherent colonies on infected monolayers and that within these colonies, BFP-like fibres form inter-bacterial linkages. For EAF plasmid-containing EPEC strains and for several Salmonella serotypes, BFP expression may lead to the development of adherent colonies on epithelial surfaces early in the infective process.

Q pili enhance the attachment of Moraxella bovis to bovine corneas in vitro

Moraxella bovis, the causative agent of infectious bovine keratoconjunctivitis, exhibits several virulence factors, including pili, haemolysin, leukotoxin, and proteases. The pili are filamentous appendages which mediate bacterial adherence. Prior studies have shown that Q-piliated M. bovis Epp63 are more infectious and more pathogenic than I-piliated and non-piliated isogenic variants, suggesting that Q pili per se, or traits associated with Q-pilin expression, promote the early association of Q-piliated bacteria with bovine corneal tissue. In order to better evaluate the role of Q pili in M. bovis attachment, several M. bovis strains and a recombinant P. aeruginosa strain which elaborates M. bovis Q pili but not P. aeruginosa PAK pili, were evaluated using an in vitro corneal attachment assay. For each strain tested, piliated organisms attached better than non-piliated bacteria. M. bovis Epp63 Q-piliated bacteria adhered better than either the I-piliated or non-piliated isogenic variants. Finally, recombinant P. aeruginosa organisms elaborating M. bovis Q pili adhered better than the parent P. aeruginosa strain which did not produce M. bovis pili. These results indicate that the presence of pili, especially Q pili, enhances the attachment of bacteria to bovine cornea in vitro.

Neisseria gonorrhoeae prepilin export studied in Escherichia coli

The pilE gene of Neisseria gonorrhoeae MS11 and a series of pilE-phoA gene fusions were expressed in Escherichia coli. The PhoA hybrid proteins were shown to be located in the membrane fraction of the cells, and the prepilin product of the pilE gene was shown to be located exclusively in the cytoplasmic membrane. Analysis of the prepilin-PhoA hybrids showed that the first 20 residues of prepilin can function as an efficient export (signal) sequence. This segment of prepilin includes an unbroken sequence of 8 hydrophobic or neutral residues that form the N-terminal half of a 16-residue hydrophobic region of prepilin. Neither prepilin nor the prepilin-PhoA hybrids were processed by E. coli leader peptidase despite the presence of two consensus cleavage sites for this enzyme just after this hydrophobic region. Comparisons of the specific molecular activities of the four prepilin-PhoA hybrids and analysis of their susceptibility to proteolysis by trypsin and proteinase K in spheroplasts allow us to propose two models for the topology of prepilin in the E. coli cytoplasmic membrane. The bulk of the evidence supports the simplest of the two models, in which prepilin is anchored in the membrane solely by the N-terminal hydrophobic domain, with the extreme N terminus facing the cytoplasm and the longer C terminus facing the periplasm.

Systematic fractionation of serum antibodies using multiple antigen homologous peptides as affinity ligands

The fractionation of polyclonal antibodies on multiple peptide ligands is described. The method is an application of a procedure for the synthesis of large numbers of peptides on individual polyethylene pins (Geysen et al., 1987). In this application, each pin-bound peptide is used as an affinity support. Antibodies bound to the peptides are then eluted, using buffers of either high or low pH. Each eluted antibody is then tested for specific binding to peptides or proteins, using ELISA procedures. A rabbit antiserum raised to gonococcal pilin was fractionated on a complete set of octapeptides homologous with the sequence of the pilin protein. Antibodies eluted from some of the peptides bound to pilin in solution. In a second example three hyperimmune sera raised to three different potyviruses were fractionated on their respective homologous peptide sequences. Testing the eluted antibodies on the three virus coat proteins revealed peptides which bound cross-reacting antibodies. Thus the method can be used to confirm direct peptide binding evidence for sequential epitopes. These peptides can then be used in affinity chromatography to increase the specificity of polyclonal sera. This can be achieved either by elution of the specific antibody from the peptide or by removal of cross-reacting antibodies from the whole serum by absorption on peptide.

Porphyromonas (Bacteroides) gingivalis fimbrillin: size, amino-terminal sequence, and antigenic heterogeneity

Bacterial fimbriae mediate cell adhesion and are important in colonization. Fimbrial proteins from strains of Porphyromonas (Bacteroides) gingivalis isolated from different individuals were compared for their size, amino-terminal sequence, and antigenic diversity. Two major protein components of the crude fimbrial preparations differed in apparent molecular mass, ranging from 41 to 49 kDa for the fimbrillin monomer and from 61 to 78 kDa for the other major protein. The amino-terminal sequence of the antigenically related group of proteins of the fimbrillin monomer in the 41- to 49-kDa range showed significant homology; however, minor sequence heterogeneity was observed, mainly in residues 4 to 6. One of the observed amino-terminal sequences, AFGVGDDESKVAKLTVMVYNG, resembled the deduced sequence of P. gingivalis 381 (D.P. Dickinson, M. K. Kubiniec, F. Yoshimura, and R.J. Genco, J. Bacteriol. 170:1658-1665, 1988). Fimbriae from all the strains of P. gingivalis showing this sequence contained a fimbrillin monomer of 43 kDa and showed a strong reaction with both polyclonal and monoclonal antibodies directed to the fimbriae from P. gingivalis 2561 (381). Fimbriae from strains showing amino-terminal sequence variations in residues 4 to 6 (i.e., substitution of VGD with either E or NAG) were more diverse in their molecular sizes. Most of these variant fimbriae showed weak reactions with the polyclonal antibodies and no reaction with the monoclonal antibodies induced to the fimbriae of strain 2561. No correlation could be established between the molecular size and immunological reactivity of the fimbrillin monomer of P. gingivalis strains. Strains 9-14K-1 and HG 564 not only showed markedly different sequences from the other three amino-terminal sequences but also did not react with either polyclonal or monoclonal antibodies to the fimbriae of strain 2561. Strains W50, W83, and AJW 5 failed to show any immunological reactivity with the antibodies to fimbrillin or fimbriae of strain 2561. Fimbriae from different strains revealed different immunologic reactions with rabbit antisera to each of the synthetic peptides of residues 59-78 (peptide I), 79-100 (peptide J), and 91-108 (peptide E) of strain 381. These results suggest that P. gingivalis fimbrillin subunits have size, sequence, and antigenic heterogeneity among the strains and that these differences may be important in the function and immune reactivities of the fimbriae.

Pathogenic neisseriae--a model of bacterial virulence and genetic flexibility

The outcome of the early stages of a neisserial infection is determined by receptor-mediated events that culminate in the attachment and invasion of human mucosal tissues. The factors participating in this process, including pili, opacity proteins (Opa), and perhaps lipopolysaccharide (LPS), are subject to complex genetic controls that allow these factors to be produced in multiple forms. Antigenic variation allows the pathogenic Neisseriae to evade the human immune response, and facilitates their interaction with a variety of different cells and tissues of the human host. One of the major genetic mechanisms causing antigenic variation is transformation, which allows virulence genes to be exchanged and recombined between independent Neisseria strains within multiply infected individuals. A number of other factors, such as IgA protease, alpha-factor, and the meningococcal capsule are also implicated in pathogenesis and render the pathogenic Neisseriae an excellent model for the investigation of bacterial virulence.

The pili of Aeromonas hydrophila: identification of an environmentally regulated "mini pilin"

Ultrastructural studies of Aeromonas hydrophila strain AH26 revealed two distinctive pilus types: "straight" pili appear as brittle, rod-like filaments, whereas "flexible" pili are supple and curvilinear. Straight pili are produced constitutively under all tested conditions of growth. In contrast, the expression of flexible pili is regulated by physical and chemical variables, being produced at 22 vs. 37 degrees C, in a liquid vs. a solid medium, and when the availability of free-iron is reduced by the presence of deferoxamine mesylate. Both pilus proteins were purified and biochemically and functionally characterized. The major repeating subunit of the straight pilus is a 17,000-mol wt polypeptide with amino acid sequence homology with Escherichia coli type 1 and Pap pili. The flexible pilus filament is a homopolymer composed of a novel 46 amino acid polypeptide. Resistance of the flexible pilus filament to disaggregation using various chemical treatments was demonstrated; its stability as a polymer and its apparent mechanical strength seem to be conferred by a 20 amino acid hydrophobic, COOH-terminal domain. Purified straight pili lack hemagglutinating function. In contrast, purified flexible pili cause the agglutinin of human, guinea pig, ovine, bovine, and avian erythrocytes, although this property could only be demonstrated in the presence of divalent cations and was most evident at 4 vs. 22 degrees C. Taken together, these results suggest that the pathogenic and ecological roles of the flexible pilus are related to this species' existence as a free-living organism in aquatic environments and its ability to cause infections, both in cold-blooded vertebrates and the human intestine.

Vibrio cholerae O395 tcpA pilin gene sequence and comparison of predicted protein structural features to those of type 4 pilins

Vibrio cholerae O1 expresses a pilus that is coordinately regulated with cholera toxin production and hence termed TCP, for toxin-coregulated pilus. Insertion of Tn5 IS50L::phoA (TnphoA) into the major pilin subunit gene, tcpA, has previously been shown to render the strain avirulent as a result of its inability to colonize. One such insertion was isolated and used as a probe to screen for clones containing the intact tcpA gene. The DNA sequence of tcpA was determined by using the intact gene and several tcpA-phoA gene fusions. The deduced protein sequence agreed completely with that previously determined for the TcpA N terminus and with the size of the mature pilin protein. The reported homology with N-methylphenylalanine (type 4) pilins near the N terminus was extended and shown to include components of the atypical leader peptide as well as overall predicted structural similarities in other regions of the pilins. In contrast to the modified N-terminal phenylalanine residue found in all characterized type 4 pilins, the corresponding position in tcpA contains a Met codon, thus implying that the previously uncharacterized amino acid corresponding to the N-terminal position of the mature TcpA pilin is a modified form of methionine. Except for this difference, mature TcpA has the overall predicted structural motifs shared among type 4 pilins.

The role of direct oligonucleotide repeats in gonococcal pilin gene variation

Previous studies indicate that gonococcal pilin phase and antigenic variation occur by intragenomic pilin gene recombination, the outcome of which resembles that of gene conversion. During such transitions, the expressed complete pilin gene (pilE) acquires a novel sequence corresponding to that of a silent pilin gene (pilS). In the present study, we find that internal deletions of pilE can produce pilus-/pilus+ phase transitions: direct oligonucleotide repeats in the pilin-encoding portion of pilE bracket the deleted segments. A novel, orthodox pilE is formed upon repair of the internal deletions, with pilS sequence probably acting as a template for repair. Such deletion/repair of pilE is suggested as a principal mechanism underlying gonococcal pilus variation.

Identification, cloning, and sequencing of piv, a new gene involved in inverting the pilin genes of Moraxella lacunata

Moraxella lacunata is a bacterium that is a causative agent of human conjunctivitis and keratitis. We have previously cloned the Q and I pilin (formerly called beta and alpha pilin) genes of Moraxella bovis and determined that an inversion of 2 kilobases (kb) of DNA determines which pilin gene is expressed. Using an M. bovis pilin gene as a hybridization probe to screen a lambda ZAP library of M. lacunata DNA, we have isolated a clone that not only contains the entire type 4 pilin gene inversion region of M. lacunata but inverts the 2-kb region on a plasmid subclone (pMxL1) in Escherichia coli. Deletion derivatives of pMxL1 yielded some plasmids that still had the entire inversion region but were phase locked into one or the other of the two potential orientations. Similarly, insertions of a 2-kb streptomycin-resistant element (omega) within some regions outside of the inversion also resulted in phase-locked plasmids. These deletions and insertions thus localize a probable invertase necessary for the inversion event. The region was sequenced, and an open reading frame with over 98% DNA sequence homology to an open reading frame that we previously found in M. bovis and called ORF2 appeared to be a strong candidate for the invertase. This conclusion was confirmed when a plasmid containing the M. bovis ORF2 supplied, in trans, the inversion function missing from one of the M. lacunata phase-locked inversion mutants. We have named these putative invertase genes piv(ml) (pilin inversion of M. lacunata) and piv(mb) (pilin inversion of M. bovis). Despite previously noted sequence similarities between the M. bovis sites of inversion and those of the Hin family of invertible segments and a 60-base-pair region within the inversion with 50% sequence similarity to the cin recombinational enhancer, there is no significant sequence similarity of the Piv invertases to the Hin family of invertases.

Antigenic stability of fimbriae of Bacteroides nodosus

Successful vaccination of sheep against footrot and attempts to eradicate the disease depend on there being a limit to the antigenic diversity of the causative bacterium, Bacteroides nodosus. Fimbrial antigenic variation was therefore investigated in vivo, both under conditions of chronic infection and under the pressure of a vaccine-induced immune response, to ascertain whether this represented an obstacle to such goals. Material was available from 5 experiments and although B. nodosus appeared to have undergone changes in its fimbrial antigens in one of these, the possibility that superinfection was responsible for the variation detected could not be ruled out because all sheep in this case were maintained at pasture. Overall, the results provided no evidence of fimbrial antigenic shift in B. nodosus in vivo and in conclusion, the survival of the organism in the sheep's foot, both in long-term natural infection and following vaccination, must therefore be related to factors other than the ability to undergo antigenic variation in order to evade the host's immune response.

Restriction fragment length polymorphism in the cytadhesin P1 gene of human clinical isolates of Mycoplasma pneumoniae

Genomic DNA obtained from Mycoplasma pneumoniae clinical isolates spanning a 30-year period was analyzed for the presence of polymorphism in their P1 cytadhesin genes. All clinical isolates expressed a 170-kilodalton P1 protein that reacted with anti-P1 monoclonal antibodies. However, Southern blot analysis of specific M. pneumoniae isolates with subclones of the P1 structural gene revealed the presence of restriction fragment length polymorphism, permitting the classification of their P1 genes into two distinct categories.

Pili (fimbriae) of Branhamella species

PURPOSE

Pili (fimbriae) have frequently been found to be involved in the attachment of bacteria to mucosal epithelial cells, an important initial step in the disease process. The purpose of this study was to determine if Branhamella catarrhalis expresses type 4 pili.

MATERIALS AND METHODS

Piliated B. catarrhalis phenotypic characteristics of colony morphology, agar corrosion, twitching motility, competence for deoxyribonucleic acid (DNA) transformation, autoagglutination, and pellical formation were observed. DNA was isolated from Branhamella spp. and used in genomic Southern hybridizations with a Moraxella bovis pilin gene as a probe. Electron microscopy of negatively stained bacteria was carried out to visualize pili.

RESULTS

B. catarrhalis has several (but not all) of the phenotypic characteristics that are related to the presence of type 4 (MePhe) pili in closely related Moraxella spp., including competence for DNA transformation, autoagglutination, pellicle formation, colony morphology, and pitting of agar. The one phenotype we have not found that is generally characteristic of type 4 piliated bacteria is twitching motility. Genomic Southern hybridization analysis using a cloned M. bovis Q pilin gene as a probe reveals DNA homologous to the Q pilin gene in B. catarrhalis, Branhamella ovis, Branhamella caviae, and Branhamella cuniculi. Examination of B. catarrhalis strain ATCC25240 by electron microscopy reveals two different kinds of pili. One kind appears similar to other type 4 pili, whereas a second class is short pili extending outward from all portions of the bacteria.

CONCLUSION

Phenotypic, electron-microscopic, and hybridization data are all consistent with type 4 pili being present on some B. catarrhalis strains.

Morphogenetic expression of Moraxella bovis fimbriae (pili) in Pseudomonas aeruginosa

Type 4 fimbriae (pili) are found in a wide variety of gram-negative bacteria and are composed of small structural subunits which share significant sequence homology among different species, especially at their amino-terminal ends. Previous studies demonstrating morphogenetic expression of Bacteroides nodosus fimbriae from cloned subunit genes in Pseudomonas aeruginosa suggested that there is a common mechanism for type 4 fimbriae assembly and that the structural subunits are interchangeable (J. S. Mattick et al., J. Bacteriol. 169:33-41, 1987). Here we have examined the expression of Moraxella bovis fimbrial subunits in P. aeruginosa. M. bovis subunits were assembled into extracellular fimbriae in this host, in some cases as a homopolymer but in others as a mosaic with the indigenous subunit, indicating structural equivalence. This result contrasts with other studies in which recombinant P. aeruginosa expressing different subunits produced fimbriae composed almost exclusively of one subunit or the other (T. C. Elleman and J. E. Peterson, Mol. Microbiol. 1:377-380, 1987). Both observations can be explained by reversibility of subunit-subunit interactions at the site of assembly, with the forward equilibrium favoring chain extension between compatible subunits.