Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Neisseria gonorrhoeae (Gonorrhea)
Reviewed for currency November 8, 2020
Gonorrhea is a common bacterial infection that is transmitted almost exclusively by sexual contact or perinatally and primarily affects the mucous membranes of the urethra and cervix and, less frequently, those of the rectum, oropharynx, and conjunctivae. Ascending genital infection in women leads to endometritis and salpingitis—collectively called pelvic inflammatory disease (PID)—the predominant complication and one of the most common causes of female infertility. Other complications include acute epididymitis; ophthalmitis; disseminated infection with arthritis, dermatitis, and sometimes endocarditis; and transmission to the neonate with attendant conjunctivitis (ophthalmia neonatorum).
Gonorrhea is one of the oldest known human illnesses, and references to sexually acquired urethritis can be found in ancient Chinese writings, the biblical Old Testament (Leviticus), and other works of antiquity. Galen ( ad 130) introduced the term gonorrhea (“flow of seed”), implying interpretation of urethral exudate as semen. The causative organism was described by Neisser in 1879 and was first cultivated in 1882 by Leistikow and Löffler. Untreated infections were understood to resolve spontaneously over several weeks or months, but reinfection was recognized. Many therapies were tried, but not until the advent of the sulfonamides in the 1930s and penicillin in 1943 was truly effective treatment available. Since that time, therapy has changed radically in the face of antimicrobial resistance. We are now facing a major crisis as resistance to the last of the “single-dose” third-generation cephalosporin therapies is increasing. Growth of fundamental knowledge about the organism and the host response to infection was slow for 80 years, but a surge of new information began in the 1970s, and the molecular biology of the gonococcus and the pathogenesis of gonorrhea have been well elucidated. Public health control efforts have met with variable success, and gonorrhea remains the second most common reportable disease in the United States (following sexually transmitted chlamydial infection), a prime example of the influence that social, behavioral, and demographic factors can have on the epidemiology of an infectious disease despite highly effective antimicrobial therapy.
The Organism
Description
Neisseria gonorrhoeae is a non–spore-forming, gram-negative coccus that characteristically grows in pairs (diplococci) with adjacent sides flattened. It does not have flagella but can move based on extension and retraction of its type IV pilus. This process is known as “twitching” and is the basis of the colony types seen in piliated and nonpiliated gonococcal strains. The gonococcus closely resembles the related pathogen Neisseria meningitidis and several species of nonpathogenic Neisseria. All Neisseria spp. rapidly oxidize dimethyl- p -phenylenediamine or tetramethyl- p -phenylenediamine, the basis of the oxidase test. Traditionally, gonococci are differentiated from other Neisseria by their ability to grow on selective media; to use glucose but not maltose, sucrose, or lactose; and to reduce nitrites, and also by their inability to grow well at reduced temperature or on simple nutrient agar.
Growth and Cultivation
Gonococci do not tolerate drying, and patient samples to be used for cultivation should be inoculated immediately onto an appropriate growth or transport medium. Growth is best for most strains at 35°C to 37°C, and many freshly isolated strains have a relative or absolute requirement for atmospheric carbon dioxide in concentrations of approximately 5%. All strains are strictly aerobic under usual growth conditions, but the organism grows anaerobically when nitrite is provided as an electron acceptor. Colonies appear in 24 to 48 hours, but on most media viability is rapidly lost after 48 hours because of autolysis.
Gonococci are inhibited by many fatty acids, and it is necessary to incorporate starch or other substances that absorb fatty acids into most growth media. All strains have complex growth requirements, including requirements for several vitamins, amino acids, iron, and other factors. For clinical purposes, a satisfactory medium is chocolate agar enriched with glucose and other defined supplements. Isolation of gonococci from sites that normally contain high concentrations of the normal microbiota, especially the pharynx, rectum, and cervix, may be difficult because of overgrowth of the hardier normal microbiota—a problem that is largely overcome with use of media containing antimicrobial agents that inhibit most nonpathogenic Neisseria and other species but permit growth of most strains of N. gonorrhoeae and N. meningitidis. Chocolate agar that contains vancomycin, colistin, nystatin, and trimethoprim (modified Thayer-Martin medium) is widely used for this purpose in the United States; a similarly constituted translucent selective medium (New York City medium) is also commonly used. Specimens from sites that usually do not harbor indigenous microbiota (e.g., blood, synovial fluid, and cerebrospinal fluid) should be cultured on antibiotic-free media.
Surface Structures
The envelope of N. gonorrhoeae is similar in basic structure to that of other gram-negative bacteria. As the interface between the gonococcus and host, the cell surface has been intensively studied ( Fig. 212.1 ), and specific surface components have been related to adherence, tissue and cellular penetration, cytotoxicity, and evasion of host defenses both systemically and at the mucosal level.
Type IV Pili
Type IV pili (fimbria) are strong flexible filaments extending from the gonococcal cell surface. They can be several microns in length and 50 to 80 angstroms in width. Pili traverse the outer membrane of the gonococcus through an integral outer membrane protein known as PilQ. Mature pili are composed of repeating protein subunits (pilin) with a molecular weight of 19 ± 2.5 kDa. Through the activity of the proteins PilT and possibly PilB, gonococci can depolymerize and repolymerize the pilus strand, causing the bacteria to “twitch.” The pilin subunit has regions of considerable interstrain antigenic similarity, especially near the amino terminus, but areas of extreme antigenic variability are also present. A single strain of N. gonorrhoeae is capable of producing pili with differing antigenic compositions. This has compromised the usefulness of pilus-based vaccines against gonorrhea. The presence or absence of pili result in varied colonial forms, which can be distinguished when N. gonorrhoeae is grown on translucent agar. Fresh clinical isolates initially form colony types P + and P ++ (formerly called T1 and T2), and the organisms have numerous pili extending from the cell surface ( Fig. 212.2 ); P − colonies (formerly T3 and T4) lack pili. Piliated gonococci are better able than organisms from P − colonies to attach to human mucosal surfaces and are more virulent in animal and organ culture models and in human inoculation experiments than nonpiliated variants. Expression of pili is a function of the pil gene complex. A spontaneous shift between P + or P ++ colonies to P − colony types, known as phase variation, occurs after 20 to 24 hours of growth in vitro. This is caused by errors in DNA replication, resulting in pilE genes that are out of frame and fail to produce functional protein.
In addition to mediating attachment, pili contribute to resistance to killing by neutrophils. In the fallopian tube mucosa model ( Fig. 212.3 ), pili facilitate attachment to nonciliated epithelial cells, which initiates a process of entry and transport through these cells into intercellular spaces near the basement membrane or directly into the subepithelial space while concurrently nearby ciliated mucosal cells lose their cilia and are sloughed. CD46 was considered to be the main pilin receptor, but the issue is currently uncertain and the identity of the pilus receptor is an area of active study. Other pilus-associated proteins are likely to be important to adhesion to host cells, particularly PilC. Other factors also mediate attachment, notably opacity (Opa) proteins, lipooligosaccharide (LOS), and porins (Por).
Outer Membrane
Like all gram-negative bacteria, the gonococcus possesses a cell envelope composed of three distinct layers: an inner cytoplasmic membrane, a middle peptidoglycan cell wall, and an outer membrane. The outer membrane contains LOS, phospholipid, and a variety of proteins (see Fig. 212.1 ). Porin, formerly designated protein I, has a molecular weight of 32 to 36 kDa and is closely associated in the membrane with LOS. Porin provides channels that allow aqueous solutes to pass through the otherwise hydrophobic outer membrane and is believed to play an important role in pathogenesis. Porin is the product of a gene designated porB. Porin proteins occur in two major antigenic classes, designated PorB1A and PorB1B, each of which is composed of many distinct genetic variants. Variations in Por sequence or antigenic types form the basis for the most commonly used gonococcal typing systems. Strains expressing PorB1A and occasionally PorB1B are associated with genotypic resistance of N. gonorrhoeae to the bactericidal effect of normal (nonimmune) human serum and, perhaps as a direct result, with an enhanced propensity to cause bacteremia. Porin-related serum resistance is due to binding to loops on the porin protein of the complement downregulatory components C4bp or factor H (fH). PorB1A also appears to directly promote invasion of epithelial cells, which also helps explain the propensity for bacteremic dissemination.
Opa proteins are outer membrane proteins with molecular weights of 20 to 28 kDa. They are members of a family of proteins, each produced from its own opa gene. The amino-acid sequence of the Opa proteins varies somewhat, primarily because of differences in two hypervariable regions in each protein. Expression of Opa varies because of high-frequency variations in opa DNA that result in translational frame shifting. An individual strain of N. gonorrhoeae can express none or up to 11 Opa variants but usually not more than 3 at a time. Gonococci isolated from mucosal sites usually express Opa and their colonies are opaque, but most isolates obtained from the cervix during menstruation and isolates from normally sterile sites, such as fallopian tubes, blood, and synovial fluid, generally lack Opa and form translucent colonies. Many Opa proteins increase adherence between gonococci and to a variety of eukaryotic cells, including phagocytes. Certain Opa variants appear to promote invasion of epithelial cells. Two classes of Opa receptor on eukaryotic cells have been identified: heparin-related compounds and CD66, or carcinoembryonic antigen–related cell adhesion molecules (CEACAMs). Certain Opa proteins are able to bind to CEACAM receptors on B and T cells, resulting in downregulation of immune responses. This may help account for the poor immune response to natural infection.
Reduction-modifiable protein (Rmp) has a molecular weight of 30 to 31 kDa; is present in all gonococci in close association with porin and LOS; and shows little, if any, interstrain antigenic variation. Rmp can stimulate blocking antibodies that reduce serum bactericidal activity against N. gonorrhoeae, which may potentiate infection after sexual exposure to an infected partner. Several other outer membrane proteins have been identified, including multiple iron-repressible proteins, some of which are shared with N. meningitidis. Two of the iron-repressible proteins (85 and 110 kDa) constitute a specific receptor for human transferrin, and two others form a receptor for human lactoferrin. The transferrin receptor is required for successful experimental urethral infection, but the role of the lactoferrin receptor is unclear; it does not influence infectivity. Two additional proteins constitute a receptor for hemoglobin. Other proteins are expressed only during anaerobic growth. The ability of N. gonorrhoeae to grow anaerobically after removing available oxygen from the microenvironment contributes to its ability to survive in the cervical and vaginal microaerobic environment. Immunoglobulin A1 (IgA1) proteases, present in N. gonorrhoeae and N. meningitidis but not in nonpathogenic Neisseria, are assumed to protect the organism from secretory IgA antibody at mucosal surfaces, but this role has not been proven.
Gonococcal LOS is composed of lipid A and a core oligosaccharide that, in contrast to the polysaccharide of most gram-negative bacteria, lacks O-antigen side chains. Sialylation of LOS core sugars in vitro or in vivo masks epitopes on both LOS and porin and contributes to resistance to bactericidal antibodies. LOS possesses endotoxic activity and contributes to ciliary loss and the death of mucosal cells in the fallopian tube explant model (see Fig. 212.3 ). LOS core sugars undergo high-frequency phase and antigenic variation in vitro and in vivo, which may contribute to the pathogenesis of infection, including resistance to bacterial anti-LOS antibodies present in normal serum and invasion of epithelial cells.
The peptidoglycan layer of N. gonorrhoeae may also contribute to the inflammatory response. The gonococcus has lytic transglycosylases, which produce and release highly inflammatory peptidoglycan monomers. These peptidoglycan fragments are toxic in the fallopian tube explant system and cause complement consumption in vitro. In addition, peptidoglycan fragments have been found in the apparently sterile synovial fluid of patients with gonococcal arthritis-dermatitis syndrome. Gonococci produce a surface polyphosphate that may have capsule-like functions, such as creating a hydrophilic, negatively charged cell surface. However, a carbohydrate capsule analogous to that of N. meningitidis or Streptococcus pneumoniae is not produced.
Strain Typing
Studies of the clinical manifestations and epidemiology of gonorrhea have been greatly enhanced by the development of reproducible methods for typing N. gonorrhoeae. These methods are not available in clinical laboratories, however. They consist of characterization of gonococcal strains based on two primary methods: auxotyping and serotyping using monoclonal antibodies to variable epitopes on the porin protein. In the future, with the introduction of rapid and inexpensive whole-genome sequencing, strains will be compared through use of these methods.
Genetics
Plasmids
Many gonococci possess a 24.5-mDa conjugative plasmid and can thereby conjugally transfer other non–self-transferable plasmids with high efficiency; chromosomal genes are not mobilized. Many gonococci carry a plasmid (Pc r ) that specifies production of a TEM-1 type of β-lactamase (penicillinase). The two most common Pc r plasmids have molecular weights of 3.2 and 4.4 mDa and are closely related to each other and to similar plasmids found in certain Haemophilus spp., including Haemophilus ducreyi. In fact, it is suspected that gonococci first acquired Pc r plasmids from H. ducreyi. Pc r plasmids are commonly mobilized to other gonococci by the conjugative plasmid.
Gonococci with plasmid-mediated high-level resistance to tetracycline, with minimum inhibitory concentrations (MICs) of 16 mg/L or greater, carry the 24.5-mDa conjugative plasmid into which the tetM transposon has been inserted. The tetM determinant also confers tetracycline resistance to a variety of other bacteria, including some Streptococcus and Mycoplasma spp. and various genital organisms such as Gardnerella vaginalis and Ureaplasma urealyticum. Because of its location on the conjugative plasmid, high-level tetracycline resistance is readily transferred among gonococci. The tetM determinant functions by encoding a protein that protects ribosomes from the effect of tetracycline. Finally, all gonococci contain a small (2.6 mDa) cryptic plasmid of unknown function.
Chromosomal Mutations and Transformation
Mutations in biosynthetic pathways are common, presumably reflecting the ready availability in vivo of essential nutrients such as amino acids, purines, and pyrimidines at infected mucosal sites. Nevertheless, N. gonorrhoeae is not highly mutable in that it lacks error-prone repair systems and is relatively resistant to external mutagenic stimuli such as ultraviolet light. Instead, gonococci have evolved efficient systems for phase and antigenic variation of surface components (pili, Opa, and LOS) that do not depend on such mutagenic pathways.
Gonococci also use transfer of naked DNA between cells (transformation) to promote genetic variability. The piliated variants of virtually all clinical isolates of N. gonorrhoeae are highly competent in transformation, but loss of the ability to express pili is always accompanied by a dramatic reduction in transformation competence. Uptake of transforming DNA is limited to homologous (i.e., gonococcal) DNA, which reflects recognition of a unique nucleotide sequence by a surface receptor. No bacteriophages have been found in N. gonorrhoeae.
Chromosomal resistance of N. gonorrhoeae to β-lactam antibiotics and the tetracyclines results from interactions among a series of individual mutations, some of which (e.g., the mtr determinant) alter the net accumulation of antimicrobial agents inside the cell. The mtr locus has been shown to be an efflux pump similar to other membrane transporters. The penA locus alters penicillin-binding protein 2 to reduce its affinity for penicillin. For epidemiologic purposes, chromosomal resistance is defined when the MIC is such that clinical failures are common with the maximum practical therapeutic dose, which corresponds to MICs of 2 mg/L or greater for both tetracycline and penicillin G. Clinically significant resistance to the fluoroquinolones, indicated by MICs of ciprofloxacin of 1 mg/L or higher (up to 16 mg/L), result from the additive effects of multiple chromosomal mutations involving the genes gyrA and gyrB, which code for DNA gyrases, and parC and parE, which code for topoisomerases.
Pathobiology of Gonococcal Infection
An unusual characteristic of gonococcal infection is that the pathobiologic events that result in infection in men and women are different, leading to different clinical expression of the infection.
Infection of the Male Urethra
N. gonorrhoeae infection in men occurs as an acute urethritis, which develops from the concomitant inflammatory response directed at infecting gonococci. One hallmark of gonococcal disease in men is the frequent presence of a purulent urethral discharge, which is associated with polymorphonuclear leukocyte (PMN) influx and shedding of urethral epithelial cells. Human volunteer studies indicate that there is an incubation period from the time of infection to the onset of clinical symptoms of disease; during this time, gonococci cannot be cultured from the urethra for up to 40 hours after the initiation of infection. In vitro infection assays and microscopic analysis of patient exudates indicate that gonococci can enter urethral epithelial cells and are released, and then infected epithelial cells are shed from the mucosal surface to the urethral lumen. Experimental infection in men has also demonstrated that high concentrations of the chemokine interleukin (IL)-8 and cytokines IL-6 and tumor necrosis factor-α (TNF-α) are present within the urethral lumen with progressive gonococcal infection. Recent evidence demonstrates that LOS elicits TNF-α, IL-1β, IL-6, and IL-8 secretion from urethral epithelial cells. Release of cytokines and chemokines from the urethral epithelium may therefore initiate the inflammatory response associated with gonococcal urethritis by triggering PMN influx. PMN influx in conjunction with cytokine release from the urethral epithelium subsequently potentiates the clinical symptoms associated with disease. Unless intercepted by effective antimicrobial therapy, this process is cyclic during the course of infection, with extension into the upper male genital tract. The acquired immune response in humans is ineffective in slowing disease progression or preventing reinfection.
Microscopic examination of urethral exudates from men documented to have culture-proven gonorrhea indicates that gonococci are found within PMNs and urethral epithelial cells. The interaction of gonococci with PMNs is dependent on the presence of Opa, but it does not require pili. Opa proteins can be divided into two broad classes represented by Opa 50 (i.e., proteins that recognize host cell heparin sulfate proteoglycans [HSPGs]) and Opa 52 (i.e., proteins that recognize CEACAMs ). Experimental infection of men with Opa − gonococci results in a shift to an Opa + phenotype. An Opa + phenotype is also prevalent in clinical isolates obtained from men with naturally acquired gonococcal infection. Primary male urethral epithelial cells do not express CEACAMs, and an N. gonorrhoeae strain FA1090Δ Opa mutant is not impaired in its ability to cause infection in a human experimental model. This would suggest that the role of Opa proteins in gonococcal urethritis in men resides in their ability to facilitate a gonococcal-PMN interaction. (Accordingly, generating such an inflammatory response may give a survival advantage to the gonococcus because it can replicate within the PMN. )
CEACAMs can serve as coreceptors for other cell surface receptors (e. g., integrins) present on professional phagocytic cells. Engagement of CEACAMs sends a priming signal within PMNs that activates adhesion receptors without triggering a respiratory burst or the release of inflammatory mediators. An Opa-CEACAM interaction may therefore enhance gonococcal survival within these cells.
Porin also contributes to the intracellular survival of gonococci within PMNs. Gonococcal porin is unique among gram-negative bacterial porins in its ability to translocate to and insert into a targeted host cell membrane. Within a host cell membrane, porin forms an anion-selective, voltage-gated channel that is modulated through its interaction with adenosine triphosphate or guanosine triphosphate. Insertion of porin into the PMN membrane inhibits degranulation by causing a change in membrane potential without triggering the respiratory burst within these cells. Further modulation of host cell function attributed to porin involves its ability to inhibit phagosome maturation and to downregulate cell surface receptors important to immune function (e.g., FcγRII, FcγRIII, and complement receptors 1 [CR1] and 3 [CR3]). A single primary receptor has not been identified that uniquely modulates the interaction of the gonococcus with PMNs; however, this association occurs independently of CR3. Fcγ-, CEACAMs, HSPGs, and integrin receptors present on the PMN cell surface may all play a role in gonococcal adherence and/or its internalization.
The initial site of gonococcal disease in men is the urethral epithelial cell ( Fig. 212.4A ). Disease occurs as a sequential process in which an initial interaction occurs between gonococcal pilus and the urethral epithelium. Pilus expression is required for efficient infection of the urethral epithelium. An intimate association between the urethral epithelium and the gonococcus (see Fig. 212.4B ) is achieved through the interaction of the asialoglycoprotein receptor (ASGP-R) and gonococcal LOS, a major constituent of the gonococcus cell membrane. Engagement of the ASGP-R by the gonococcus results in pedestal formation beneath the bacterium. Endocytosis ensues primarily because of actin- and clathrin-dependent processes. Endocytosis mediated by the ASGP-R results in endosomal fusion and acidification, which results in clathrin-coat disassembly and uncoupling of the ASGP-R–ligand complex. After gonococcal internalization, ASGP-R is recycled to the urethral cell surface, where it is available to bind more gonococci. Small proportions of infecting gonococci enter urethral epithelial cells by a macropinocytic mechanism, although membrane ruffling is not observed. The intracellular fate of the gonococcus is unclear. The gonococcus does induce antiapoptotic events that prolong the life of the epithelial cell.
Adherence of the gonococcus to the ASGP-R is dependent on the presence of an exposed galactose on the terminal lacto- N -neotetraose (LNnT) moiety on LOS. This moiety mimics human paragloboside and provides one means by which the gonococcus escapes immune recognition. In addition, the LNnT epitope can serve as a sialic acid acceptor. The presence of sialic acid on gonococcal LOS confers (unstable) resistance to the bactericidal action of normal human serum (i.e., serum resistance). The importance of the LNnT moiety to gonococcal pathogenesis in men can be inferred from human experimental infection studies and from clinical data obtained from men with naturally acquired gonorrhea, which demonstrate that LNnT is selected for in vivo. Gonococci bearing the LNnT moiety on their LOS exhibit enhanced infectivity in human volunteer studies. It is thought that serum resistance conferred by LOS sialylation allows a greater proportion of gonococci to survive the innate immune factors in the urethral lumen during disease. Consequently, a lower infectious dose is required to establish disease because a greater proportion of the infection inoculum survives and proliferates.
Sialylation of gonococcal LOS occurs when it is intracellular. LOS sialylation is mediated by gonococcus-encoded sialyltransferase present in the gonococcal outer membrane. The gonococcus lacks the ability to synthesize cytidine 5′-monophosphate N -acetylneuraminic acid (CMP-NANA) and must parasitize this substrate from its human host. Sialylation of the LNnT epitope impairs the ability of gonococci to invade primary urethral epithelial cells and epithelial cell lines, to cause disease in human volunteers, and to be phagocytized by neutrophils. Gonococcal infectivity is restored with sialic acid removal by neuraminidase or by the replication of gonococci within the lumen of the urethra in the absence of host-derived CMP-NANA. Within the lower female genital tract, sialylated gonococci may become modified to enhance disease transmission to men—that is, neuraminidases produced by the vaginal microbiota (e.g., G. vaginalis ) may remove sialic acid from sialylated gonococci. Cervical epithelia also produce neuraminidase ; however, the specificity of this enzyme to cleave endogenous or exogenous substrates exhibits cyclic variability. The level of sialic acid found within the microenvironment of the cervix also exhibits cyclic variation. Neuraminidase and the ASGP-R are also present on human sperm. Sialylated gonococci in proximity to sperm cells may become desialylated through the action of neuraminidase present on these cells. Subsequent gonococcal adherence to the ASGP-R on sperm can then, in turn, facilitate disease transmission.
Infection of the Lower Female Genital Tract
The majority of gonococci transmitted from men to their partners have sialylated LOS. However, the presence or absence of sialic acid on LOS does not influence the interaction of the gonococcus with the cervical epithelium.
In contrast to the overt inflammatory response generated with gonococcal infection of the male urethra, 50% to 80% of women with lower genital tract N. gonorrhoeae infection are asymptomatic and 70% to 90% of women with disseminated infection lack signs of genital tract involvement. Analysis of cervical secretions obtained from uninfected women and from women infected with the gonococcus reveal that an antibody response is not generated with uncomplicated infection. These findings are consistent with the ability of the gonococcus to evade and to subvert host immune function.
Within the lower female genital tract, the cervical epithelium provides a source of alternative pathway (AP) complement (C′) activity, albeit at a level only comparable to approximately 10% of that observed for human serum. These components are produced by the cervical epithelia. Within minutes of infection of cervical epithelia, C′ protein C3b is deposited on the lipid A portion of gonococcal LOS and is rapidly inactivated to iC3b ( Fig. 212.5A ). These data are supported by the predominance of iC3b (in comparison to C3b) on the surface of clinically isolated gonococci. The affinity of C′ fH for sialylated LOS and for porin of a PI.A isotype may augment C3b inactivation. However, C3b inactivation occurs in a kinetically similar manner on gonococci of either a PI.A or a PI.B isotype and on sialylated gonococci or on gonococci that are not sialylated.
CR3 serves as the primary receptor for N. gonorrhoeae adherence to and invasion of the ectocervix and endocervix (see Fig. 212.5A ). Binding of gonococcal pilus to the I-domain of CR3 probably allows the gonococcus to overcome the electrostatic repulsion between its own cell surface and that of the cervical cell. The twitching action of the gonococcal pilus with reduction in pilus length may act to juxtapose the gonococcus at the cervical cell surface, where C′ concentrations would be expected to allow efficient opsonization for the subsequent intimate adherence of iC3b on the organism surface and gonococcal porin to the I-domain. Thus, binding of the gonococcus requires the cooperative action of iC3b bound to the gonococcal surface in conjunction with gonococcal porin and pilus. Engagement of CR3 results in a complex signaling cascade in which a vinculin- and ezrin-enriched focal complex formation occurs before membrane ruffle formation. A signal transduction cascade that is dependent on the activation of wortmannin-sensitive kinases (i.e., phosphatidylinositol 3-kinase or mitogen-activated protein kinases) and Rho GTPases initiates ruffling (see Fig. 212.5B ). Gonococci are then internalized within macropinosomes.
On infection of cervical epithelia, gonococci release a phospholipase D homologue that gains access to the cervical intracellular environment nonspecifically through macropinocytosis of gonococci. Gonococcal phospholipase D (NgPLD) appears to promote infection of the cervical epithelium in several ways. Data indicate that this secreted gonococcal protein augments signaling events that trigger CR3 mobilization to the cervical cell surface. This ensures gonococcal receptor availability and, consequently, efficient targeting to and association with the cervical cell surface. NgPLD also modulates cervical cell signal transduction events, leading to membrane ruffling. Mutant gonococci that lack functional NgPLD activity do not elicit membrane ruffling, and they are impaired in their ability to associate with and to invade primary human cervical cells.
As with invasion of male urethral epithelial cells, the intracellular fate of gonococci within the cervical epithelium is unclear. Ligand binding to the I-domain of CR3 does not invoke a proinflammatory response in professional phagocytic cells. Consequently, the ability of gonococci to subvert cervical cell signal transduction cascades and the complement system in such a manner to allow a cooperative mechanism of CR3-mediated adherence to and invasion of the cervical epithelium may also enhance its survival within the lower female genital tract. Gonococcal invasion in the absence of a respiratory burst increases the number of gonococci that survive intracellularly, whereas inactivation of the C′ system enhances gonococcal survival extracellularly. Consequently, subversion of host cell signal transduction and the C′ system by the gonococcus within the lower female genital tract allows this bacterium to obtain a carrier-like state. Ascending infection of the uterus and fallopian tubes may occur as a consequence of hormonal changes that alter the mucosal epithelia, molecules available for gonococcal use, and/or virulence factors expressed by the gonococcus. In this regard, menses is associated with an increased risk to women for gonococcal PID and disseminated infection. C3 production by the cervical epithelium exhibits cyclic variability, and the highest levels of C3 are detected during menses. A correlation can also be made between the presence or absence of Opa and the site of gonococcal isolation. Opa − (or transparent) gonococci predominate in the fallopian tubes and in the cervix at the time of menses. Conversely, Opa + gonococci predominate in the male urethra and the cervix at the time of ovulation.
Infection of the Upper Female Genital Tract
Approximately 45% of women with gonococcal cervical infection will develop an ascending infection, the prerequisite to PID. Ascent to the upper female genital tract may be facilitated by the ability of gonococci to exhibit twitching motility in conjunction with hormonal changes, which influence the expression of C′ and molecules serving as gonococcal receptors within the female genital tract. Human expression of CR3 progressively decreases in an ascending manner from the ectocervix to the fallopian tubes. Conversely, expression of the lutropin receptor (LHr) increases in an ascending manner from the endometrium to the fallopian tubes, and expression is upregulated during menses. The LHr serves as a receptor for gonococcal invasion of fallopian tube epithelia. The interaction of the gonococcus with the LHr increases the invasive character of the gonococcus for the fallopian tube epithelia and therefore is said to occur in a contact-inducible manner. Human chorionic gonadotropin (hCG), a ligand for LHr, can competitively inhibit the LHr-gonococcus interaction. These data suggest that gonococci possess a surface molecule that mimics hCG. Recently, Spence and coworkers reported that this hCG-like molecule is the ribosomal protein L12. L12 is shown to facilitate gonococcal transcytosis through the fallopian tube epithelia. The LHr is also present on the human uterus, placenta, decidua, and fetal membranes. Hypothetically, a gonococcus-LHr interaction occurring on decidua and placental membranes could result in severe complications of disease and may in part contribute to the increased risk for spontaneous abortion associated with N. gonorrhoeae infection.
Gonococcal adherence via the LHr occurs selectively on nonciliated cells ; however, it is the ciliated cells of the fallopian tube epithelia that are subsequently shed. If the infection is left untreated, complete loss of ciliary action can occur. Cytotoxicity of ciliated cells is attributed to gonococcal peptidoglycan and LOS either directly or indirectly through the induction of increased production of the inflammatory cytokine TNF. The loss of ciliated cells within the fallopian tube provides the gonococcus access to subepithelial tissues. Access to subepithelial tissue is also obtained with invasion of nonciliated cells, after which gonococci are transcytosed to the basal lateral surface of these cells and released. In Hec1B cells, the gonococcal protein L12 mediates transcytosis to the basal lateral surface ; however, this has yet to be demonstrated in a fallopian tube organ model. Sialylation of intracellular gonococci (before their exocytosis) might prime these organisms for disseminated infection because of the increased serum resistance.
The Human Immune Response to Neisseria gonorrhoeae
Adaptive Immune Response
Studies of human immune responses to N. gonorrhoeae have yielded little information about the factors necessary for development of a protective response. It has been recognized for years that recurrent infection by the gonococcus after successful treatment is a common event. The diary of Boswell describing his frequent recurrences of infection are testimony to this fact. Some of his infections persisted for months before clearing. Modern studies have shown that the gonococcus can persist in asymptomatic women and men for over 6 months, and probably considerably longer. Using a human experimental model of infection, by challenging subjects with the homologous organism after clearing the first infection with antibiotics, investigators showed that recurrent infection can occur. The inability of the human to readily clear the infection and the reports of long-term carriage and reinfection by the same organism all are indicative of very poorly protective immune response to the gonococcus. Studies of the human immune response focused on the evolution of serum antibodies to pilus and porin after infection but yielded little evidence that a protective response could be developed or sustained. Despite these observations, a study was undertaken to protect individuals from gonorrhea through use of a pilus-based vaccine. Analysis of the results of this study indicated that the vaccination was completely ineffective. Since the application of molecular techniques to the gonococcus and the elucidation of the genome, a number of organism factors have been found that allow the organism to evade the host immune response. These include the ability of a large number of surface structures to undergo phase and antigenic variation, the molecular mimicry by surface glycolipids of human antigens, and horizontal gene exchange. In addition, studies in mice suggest that the gonococcus may directly inhibit the adaptive and innate immune responses. The mechanisms for these effects are not known. Analysis of individuals receiving the four-component meningococcal B vaccine suggested that limited protection against gonococcal infection occurred. These results need to be confirmed in future studies.
Innate Immune Response
More recent interest in the innate immune response has stimulated similar studies on N. gonorrhoeae. The hallmark of symptomatic gonococcal infection in humans is a robust exudative PMN response. In spite of this, the gonococcus persists and the inflammatory response can continue untreated for prolonged periods. Studies have shown that gonococci can delay fusion with primary granules in the neutrophil and that lytic enzymes involved in peptidoglycan biosynthesis impart resistance to killing of the gonococcus by lysozyme and human neutrophils. The gonococcus appears to be immune to the effects of antimicrobial peptides as a result of a variety of mechanisms from novel pumps to modifications of lipid A structures by phosphoethanolamine. Studies have shown that in vivo the sialylated LOS enhances deposition of fH, an inhibitor of the alternative pathway of complement to the bacterial surface. This results in the degradation of C3 to inactive C3b and loss of complement lytic activity. Taken together, the findings of previous studies of the immune response, both adaptive and innate, reveal an organism that is remarkably adapted to the human host.
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here