The aetiological agent of gonorrhoea essay

Neisseria gonorrhoeae (also known as the gonococcus) is the aetiological agent of gonorrhoea, a sexually
transmitted infection (STI) that remains a major global
public health concern. WHO surveillance of clinical
strains of N. gonorrhoeae has identified strains that are
resistant to most available antibiotics, highlighting the
imminent possibility of widespread untreatable gonorrhoea (BOX 1). Treatment recommendations by the
WHO are aimed at both elimination of the organism
and prevention of further spread of antimicrobialresistant gonorrhoea1
. With a worldwide incidence of
over 78 million cases each year, uncontrolled transmission and limited treatment options in both low-income
countries and poorer communities in developed countries, untreatable gonorrhoea will result in increases in
the incidence of and complications from infection14.
N. gonorrhoeae mainly colonizes the genital mucosa
but it can also colonize the ocular, nasopharyngeal and
anal mucosa57. Pathology largely results from damage that is caused by the activation of innate immune
responses at the sites of colonization, as N. gonorrhoeae
does not express potent exotoxins. Complications
from untreated, ascending, genital-tract infections in
women can include pelvic inflammatory disease, infertility and ectopic pregnancy8
. Maternal transmission
to children during birth can also lead to neonatal
. Untreated N. gonorrhoeae infection can
also lead to disseminated gonococcal infection,
potentially giving rise to infectious arthritis and
N. gonorrhoeae belongs to the genus Neisseria, of
which N. gonorrhoeae and Neisseria meningitidis (also
known as the meningococcus) are the two pathogenic
species, with the latter being a leading cause of bacterial
meningitis11. In addition, at least eight non-pathogenic
commensal Neisseria spp. make up a substantial proportion of the human nasal and oropharyngeal flora12.
Other Neisseria spp. are able to colonize a range of
non-human mammalian and non-mammalian hosts,
such as non-human primates, dogs, cats, herbivorous
mammals, dolphins, birds and insects13. Phylogenetic
analyses show that N. gonorrhoeae and N. meningitidis
evolved from a common ancestor but now represent
separate lineages that normally occupy distinct niches:
the genital mucosa and nasopharyngeal mucosa, respectively 1417. Although N. meningitidis can withstand
dehydration, survive outside the human host for periods of time and spread via respiratory droplet transmission, N. gonorrhoeae is unviable if dehydrated or
exposed to non-physiological temperatures. The events
that led to the evolution of two separate organisms that
are highly similar in their core genome and physiology
and yet cause markedly distinct diseases in different
locations of the human body are not yet understood.
As both the commensal and pathogenic Neisseria spp.
occupy the same niches, it is often difficult to differentiate colonization factors from the virulence factors that
are necessary to elicit host damage.
As N. gonorrhoeae colonizes genital, rectal and oral
mucosa, it expresses a repertoire of factors that enable
Department of Microbiology
Immunology, Northwestern
University Feinberg School
of Medicine, Chicago,
Illinois 60611, USA.
Correspondence to H.S.S.
[email protected]
Published online 12 Feb 2018
Bacterial secreted proteins
that damage host cells.
Pelvic inflammatory disease
A clinical syndrome where
infected fallopian tube tissues
are damaged by the host
inflammatory response to
Ectopic pregnancy
A sequela of pelvic
inflammatory disease that
occurs when a fertilized egg
implants anywhere other than
the uterine lining, such as in
the fallopian tube, which risks
organ damage and blood loss.
Neisseria gonorrhoeae host
adaptation and pathogenesis
Sarah Jane Quillin and H Steven Seifert
Abstract | The host-adapted human pathogen Neisseria gonorrhoeae is the causative agent of
gonorrhoea. Consistent with its proposed evolution from an ancestral commensal bacterium,
N. gonorrhoeae has retained features that are common in commensals, but it has also developed
unique features that are crucial to its pathogenesis. The continued worldwide incidence of
gonorrhoeal infection, coupled with the rising resistance to antimicrobials and the difficulties in
controlling the disease in developing countries, highlights the need to better understand the
molecular basis of N. gonorrhoeae infection. This knowledge will facilitate disease prevention,
surveillance and control, improve diagnostics and may help to facilitate the development of
effective vaccines or new therapeutics. In this Review, we discuss sex-related symptomatic
gonorrhoeal disease and provide an overview of the bacterial factors that are important for the
different stages of pathogenesis, including transmission, colonization and immune evasion, and
we discuss the problem of antibiotic resistance.
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Purulent exudate
The hallmark symptom of
gonorrhoea; a liquid genital
secretion composed of
neutrophils and
Neisseria gonorrhoeae.
replication and survival in these environmental niches
and factors that modulate and evade the host immune
system. Understanding the mechanisms through which
N. gonorrhoeae interacts with and evades the host
immune system is necessary to facilitate better infection prevention, diagnostic development, surveillance
and the development of vaccines and new treatments. In
this Review, we discuss the prevalence of asymptomatic
infection in both sexes, the main stages of gonococcal
pathogenesis, from transmission, colonization, adaptation to environmental conditions and immune evasion, and the rise in antimicrobial resistance. Although
N. gonorrhoeae and N. meningitidis share many genetic
and physiological features, this Review will focus on the
host adaptation and pathogenesis of N. gonorrhoeae.
Symptomatic and asymptomatic infections
Differences in the developmental and embryological
origins of cells lining the urogenital tracts of men and
women have endowed these microenvironments with
different surface molecules that act as receptors and
co-receptors for N. gonorrhoeae and lead to differences in
the mechanisms by which N. gonorrhoeae survives in the
male and female urogenital niches18. In addition,
the prevailing dogma in the field is that female genital infections are mostly asymptomatic and male genital
infections are mostly symptomatic1820. However, there
are many studies showing that asymptomatic infections are common in both sexes2125. The long-held and
highly repeated supposition that infections in women
are mostly asymptomatic and those in men are symptomatic is mainly based on the fact that overt symptoms
(that is, immune cell influx and inflammation) in men
are easier to diagnose owing to a purulent exudate from
the penis and resultant painful urination. Clinical manifestations in women are more likely to go unnoticed, as
inflammation does not occur in the same niche as urination and thus is less likely to be painful. Moreover,
symptoms of gonorrhoeal infection in women are more
likely to be non-specific, as the vaginal discharge that is
caused by neutrophil influx may be mistaken for bacterial vaginosis, yeast infection, hormonal variation in
vaginal secretions or normal variability in secretions26.
Data on the antibody, cytokine and chemokine composition and general magnitude of the inflammatory
response in women are sparse and inconclusive. One
clinical study of responses to N. gonorrhoeae in individuals and one study of the response of immortalized vaginal and cervical epithelial cells in culture have resulted
in different views. Analyses of the cervical mucus of
infected and uninfected women found a lack of strong
immunoglobulin A1 (IgA1) induction, a slight reduction of IgG levels and an absence of pro-inflammatory
interleukin-1 (IL-1), IL-6 and IL-8 cytokines in infected
individuals compared with uninfected individuals27
suggesting that there was no fulminant inflammatory
response to infection. By contrast, analyses of immortalized vaginal and cervical epithelial cells infected with
N. gonorrhoeae in vitro showed increased levels of IL-1,
IL-6 and IL-8, suggesting that there is an inflammatory
cytokine response during infection28. Owing to the widespread prevalence of asymptomatic infections in men
and women, it is also plausible that a detectable antibody
or cytokine response in genital secretions may not result
in detectable physiological symptoms. We suggest that
the idea that infections in women are normally asymptomatic reflects differences between the anatomy of the
urogenital tracts of men and women. To fully understand the epidemiology of gonorrhoea, surveillance and
diagnostic tests need to be improved for both sexes25 to
enable faster responses to gonorrhoea and less expensive
treatments (BOX 1).
Transmission is often the most understudied stage
of infections, and this is also true for N. gonorrhoeae
infections. A successful pathogen must be able to be
efficiently transmitted to new hosts, and, as an obligate human colonizer, N. gonorrhoeae cannot survive
outside the host. Transmission between hosts relies on
sexual networks to spread the pathogen from the core,
high-risk population in which the majority of infections
occur to the fringe, medium-risk group that transmits
N. gonorrhoeae back to the core group and to the members partners. High-risk populations include individuals with multiple sexual partners who have unprotected
sex29. Individuals are also often unaware that they are
Box 1 | Diagnosis, incidence and epidemiology of Neisseria gonorrhoeae
Historically, gonorrhoea was diagnosed by a Gram stain of the purulent exudate from
a patient showing Gram-negative diplococci among polymorphonuclear leukocytes
(neutrophils). This method of diagnosis is still used in the developing world and in
remote clinics, but in modern facilities, diagnosis is made using a variety of
nucleic-acid-based assays (for example, nucleic acid amplification tests that identify
Neisseria gonorrhoeae-specific nucleic acid signatures or, less commonly,
antibody-based assays)29. Often, diagnosis is confirmed by culture, which requires
growing isolates from clinical exudates on N. gonorrhoeae growth medium and
examining growth and bacterial morphology. Primary clinical specimens are isolated
on non-selective chocolate agar as well as on selective agar containing antimicrobial
compounds (such as vancomycin, colistin, trimethoprim lactate, nystatin, and
anisomycin or amphotericin B) that stop the growth of other bacteria and fungi151.
Recently, whole-genome sequencing has been employed to study N. gonorrhoeae
epidemiology and the spread of resistance, and it has been discovered that genital
Neisseria meningitidis infections are often misdiagnosed as N. gonorrhoeae infections
when nucleic-acid-based assays are used152,153.
In the developing world, where gonorrhoea is most prevalent, limited resources for
public health surveillance, limited self-reporting of sexually transmitted infections
(STIs) and barriers to accessing complete medical records can prevent accurate
assessment of the burden of gonorrhoea in the population. Nonetheless, in 2012, the
WHO reported 78 million cases of gonorrhoea occurring worldwide in people ages
1549, which roughly corresponds to a prevalence of 0.6% among men and 0.8% among
women. As the number of asymptomatic infections and the number of people who do
not seek treatment are unknown, it is likely that the actual number of infections is much
higher. The highest prevalence for disease is estimated to occur within the western
Pacific and African regions. The WHO has developed guidelines for effective diagnosis,
treatment and dissemination of information regarding diagnosis and treatment,
with the goal of reaching target populations including adults, adolescents aged
1019, people living with HIV, sex workers, men who have sex with men, and
transgender people. Untreated individuals with gonorrhoea are at risk of infertility,
pelvic inflammatory disease and, rarely, disseminated gonococcal infection as well as
being at risk of transmitting the disease. Gonorrhoea is considered a non-ulcerative STI,
like chlamydia and trichomoniasis, and along with other non-ulcerative STIs, people
with gonorrhoea have a higher risk of transmitting HIV to their partners owing to
increased genital shedding of the virus in people who are coinfected with HIV154,155.
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Collections of bacterial cells
that exist as discrete
Antigenic variation
A reversible process by which a
microorganism provides many
different versions of a gene
product at a frequency higher
than the normal mutation rate.
Phase variation
A stochastic form of genetic
change that varies gene
expression on/off or up/down.
part of a larger sexual network. N. gonorrhoeae attaches
to sperm30,31 and is easily transmitted from men to their
partners through ejaculates, as they contain a high number of bacteria32. However, how the efficiency of transmission from women to their partners is maintained is
less apparent. The surface of N. gonorrhoeae must be
free of sialic acid to successfully bind and enter urethral
epithelial cells of men, and so it is thought that bacterial sialidases, which are secreted by the cervicovaginal
microbiota of women, must first desialylate N. gonorrhoeae lipooligosaccharide (LOS) to enable efficient
transmission from women to men33.
Establishment of infection
Adherence, colonization and invasion. Following
transmission, N. gonorrhoeae establishes contact with
the mucosal epithelium to replicate and ultimately be
transmitted to new hosts. N. gonorrhoeae is primarily
a mucosal colonizer, attaching to various epithelial surfaces. The primary event establishing infection and the
first step in pathogenesis (FIGS 1,2) is the bacterial adherence to the epithelium of the mucosa, which is mediated through distinct bacterial surface structures that
include type IV pili, opacity (Opa) proteins, LOS and
the major outer membrane protein porin (also known
as PorB). During initial infection, following initial host
cell interaction, N. gonorrhoeae attachment and subsequent colonization depends largely on type IV pili forming microcolonies on the epithelial cell surface34. Type IV
pili are outer membrane structures that are crucial for
mediating initial cellular adherence, natural transformation competence, twitching motility and immune evasion through antigenic variation and phase variation3539.
Adherence to the epithelial surface and subsequent pilus
retraction bring the gonococci close to the cell surface.
Interactions between Opa proteins and carcinoembryonic antigen-related cell adhesion molecule
(CEACAM) receptors and other molecules, like heparin sulfate, are important for adherence, and the
OpaCEACAM interaction may be one of the major
adherence interactions32,4042. Opa proteins are abundant outer membrane proteins that mediate adherence
after initial contact by type IV pili as well as immune
evasion by multigene phase variation that results in
antigenic variation4345. Type IV pili and Opa proteins
are expressed during infection of both women and
men40,41,4648 and are considered essential for the colonization of the mucosal epithelium of the genital tract and
Nature Reviews | Microbiology
Adherence to
urogenital epithelium A neutrophil-rich, purulent
exudate facilitates transmission
Competition with resident
microbiota, nutrient acquistion
and microcolony formation
Influx of
adherence and
phagocytosis of
N. gonorrhoeae
Opa protein Microbiota
Urogenital epithelium
Type IV pili
N. gonorrhoeae
Peptidoglycan, LOS and OMVs
cause NOD and TLR activation on
epithelial cells, macrophages and
DCs; HBP causes activation of
TIFA-dependant innate immunity
in epithelial cells and macrophages
and invasion
of epithelium
Cytokine, chemokine and
inflammatory transcription
factor activation
Release of
LOS and OMVs
5 6
2 7
Figure 1 | Overview of Neisseria gonorrhoeae infection. During initial infection, Neisseria gonorrhoeae adheres to host
epithelial cells through type IV pili (step 1), which retract and enable epithelial interactions with other prominent surface
structures160,161. After initial adherence, N. gonorrhoeae replicates and forms microcolonies (step 2), and possibly biofilms34,49,
and likely competes with the resident microbiota. When colonizing the epithelium, N. gonorrhoeae is capable of invasion
and transcytosis. During these initial stages of infection, N. gonorrhoeae releases fragments of peptidoglycan,
lipooligosaccharide (LOS) and outer membrane vesicles (OMVs)111,162,163 (step 3) that activate Toll-like receptor (TLR) and
nucleotide-binding oligomerization domain-containing protein (NOD) signalling in epithelial cells, macrophages and
dendritic cells (DCs)111,164,165. NOD and TLR signalling from these cells leads to activation of inflammatory transcription
factors and the release of cytokines and chemokines (step 4). N. gonorrhoeae also releases heptose-1,7-bisphosphate (HBP),
which activates TRAF-interacting protein with FHA domain-containing protein A (TIFA) immunity115 (step 5). The release of
pro-inflammatory cytokines and chemokines by these innate immune signalling pathways creates cytokine and chemokine
gradients that recruit large numbers of polymorphonuclear leukocytes, or neutrophils, to the site of infection (step 6), where
they interact with and phagocytose N. gonorrhoeae. The influx of neutrophils makes up a purulent exudate that then
facilitates transmission (step 7). NLR, NOD-like receptor; Opa, opacity.
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Structured formations of
bacterial cells within an
extracellular matrix that stick
to one another and together on
a surface.
C4bbinding protein
(C4BP). A classical
complement pathway
regulatory protein, akin to
factor H in the alternative
pathway, that regulates
complement activation on host
Factor H
A control protein of the
alternative complement
pathway that binds C3b,
displacing activated factor Bb
and enabling cleavage and
subsequent inactivation of C3b
by factor I-induced conversion
to iC3b.
Oxidative burst
An antimicrobial response
through the release of reactive
oxygen species from host cells.
The transit of the cellular
epithelium by a bacterium.
other sites of infection (FIG. 2a). N. gonorrhoeae can form
biofilms on abiotic surfaces and epithelial cells in vitro49,50;
however, the precise role of biofilms during infection
remains to be determined5052; it is not known whether
stable mucosal colonization during infection is mediated
by microcolonies, biofilms or a combination of both.
The prominent surface factors porin and LOS
also affect colonization. Porin is a nutrient channel,
is one of the most abundant gonococcal outer membrane proteins, binds complement factors C4b-binding
protein (C4BP) and factor H and suppresses the neutrophil oxidative burst and neutrophil apoptosis53. LOS is
localized to the outer leaflet of the outer membrane
and is similar in structure to the ubiquitous bacterial
lipopolysaccharides, although LOS lacks the O-antigen
polymer54. LOS is important for adherence and invasion
of host cells; variations in LOS affect immune cell recognition, and sialylation of LOS affects serum resistance
through complement evasion and host transmission5456.
In addition to colonization of the mucosal epithelium,
N. gonorrhoeae can invade epithelial cells. Although
less is known about mucosal cell invasion than surface
colonization, it has been shown that N. gonorrhoeae
invades non-ciliated cervical epithelial cells and the urethral epithelial cells of men when LOS is desialylated18.
It is thought that the interaction between LOS and
the asialoglycoprotein receptors promotes epithelial
invasion in the urethra of men, whereas complement
receptor 3 (CR3) serves as the receptor that mediates
invasion in the lower cervical genital tract and lutropinchoriogonadotropic hormone receptor serves as the
receptor in the endometrium and in fallopian environments. This invasion of the epithelium and the resultant
transcytosis of the epithelium could lead to disseminated
gonococcal infection, but the relevance of epithelial cell
invasion and transcytosis to uncomplicated infections
is less clear. The differences in receptors that mediate
epithelial invasion highlight the complex, multifaceted
nature of tissues lining the genital tract, which is a central part of the difficulty in establishing appropriate animal and tissue culture models to study a host-restricted
pathogen (BOX 2).
a b
Type IV pili
Opa protein
Fatty acids
FarAFarB MtrCMtrDMtrE
Surface structures needed for adhesion,
invasion and immune evasion
Iron acquisition and efflux of toxic materials
c d
Enzymes quench and detoxify ROS and repair damage
to proteins and DNA
Endogenous and neutrophil-generated ROS
Transcriptional regulatory networks that respond
to changing nutrient concentrations, oxygen
concentrations, oxidative stress, DNA and protein
damage and antimicrobials
Nature Reviews | Microbiology
Regulatory networks respond to
changing environmental conditions
DNA repair
Figure 2 | Overview of Neisseria gonorrhoeae pathogenesis factors.As a host-restricted pathogen, Neisseria gonorrhoeae
encodes a relatively small repertoire of pathogenesis and colonization factors compared with other Gram-negative
bacteria166. a |N. gonorrhoeae uses an array of surface structures to adhere to host cells, occasionally invade host cells and
evade the immune system18,142,143. These surface structures include type IV pili, lipooligosaccharide (LOS), porin, and opacity
(Opa) proteins. b| Efflux pumps protect N. gonorrhoeae from antimicrobials and fatty acid stress, and membrane transporters
allow N. gonorrhoeae to co-opt nutrients from the surrounding environment66,141,167169. The pump FarAFarB controls fatty
acid transport, while the pump MtrCMtrDMtrE controls antimicrobial peptides. The membrane transport complexes
LpbALpbB, HpuAHpuB and TbpATbpB contribute to iron transport and iron homeostasis. c | A set of transcriptional
regulators, discussed in detail in the main text and FIG. 3, induce transcriptional programmes to adapt and respond to
changing environmental conditions during infection82,127,170173. The regulons that respond to iron levels, oxidative conditions
and oxygen concentration are co-regulated and interconnected. d| Protective enzymes like catalase and peptide
methionine sulfoxide reductase (MsrAMsrB) detoxify reactive oxygen species (ROS), such as superoxide anion (O2

hydrogen peroxide (H2O2) and hypochlorous acid (HOCl), that are generated endogenously and by neutrophils86.
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Nutritional immunity
The ability of the host to
sequester important nutrients
during infection.
Low molecular mass
iron-binding chemical
compounds secreted by
bacteria to chelate iron for
subsequent uptake into the
bacterial cell.
Environments where oxygen
concentration is limited but
not zero.
Growth and metabolism. Once N. gonorrhoeae adheres
to the mucosal epithelium, efficient colonization
requires extracellular bacterial replication and nutrient
acquisition from the surrounding extracellular milieu.
It has not been thoroughly determined which microenvironments are encountered during colonization, and
thus, the exact nutrient composition of each ecological
niche that N. gonorrhoeae may inhabit during urogenital, rectal and oropharyngeal infection is unknown. In
laboratory culture, N. gonorrhoeae has complex media
requirements. Specifically, bacteria cannot grow in
culture without a supplemented source of glucose, glutamine, thiamine, phosphate, iron and carbon dioxide5759. In order to meet its nutritional requirements,
N. gonorrhoeae must interact and possibly compete with
resident microbiota for available nutrients6062 (FIG. 1).
Indeed, N. gonorrhoeae must acquire nutrients like
iron, zinc and manganese (FIG. 3) that are limited by the
human host as a defence against bacterial pathogens in a
process termed nutritional immunity63,64. As Neisseria spp.
lack siderophores, N. gonorrhoeae scavenges iron directly
from host-bound complexes, obtaining metals through a
series of membrane transport complexes by transporting
them into the bacterial cell6568 (FIG. 2). Finally, the influx
of neutrophils that occurs during symptomatic colonization (FIG. 1) may promote nutrient acquisition by causing
leakage of serum components and tissue damage and by
exposing N. gonorrhoeae to intracellular nutrient pools
following phagocytosis, thus providing nutrients for
bacterial growth6974.
Regulatory networks. In order to survive and replicate,
N. gonorrhoeae must adapt to changing environmental
conditions within the host genital tract, rectum and
oropharynx (FIG. 3). The limitations of current N. gonorrhoeae experimental models have prevented gains in
specific knowledge of the pH, and nutrient and oxygen
concentrations in the varying ecological niches of the
genital mucosa; however, the range of adaptive mechanisms that the bacterium has acquired and maintained
over evolutionary time indicates the main contributing
environmental changes that may affect survival during
infection. These mechanisms regulate global transcriptional changes through transcriptional regulators and
two-component systems (FIGS 2c,3), translational regulation through regulatory small RNAs (sRNAs) and clonal
variation through phase variation.
For specific environmental variables (for example, metal availability, oxygen concentrations, reactive
oxygen species, protein misfolding, membrane stress
and the presence of antimicrobial peptides (FIG. 2bd)),
N. gonorrhoeae utilizes an array of responsive transcriptional factors to activate and repress small-scale and
large-scale adaptive transcriptional programmes (FIG. 3).
During infection, N. gonorrhoeae encounters antimicrobial peptides. Transcriptional regulators MtrR, MtrA and
MpeR contribute to regulation of an antimicrobial efflux
pump, MtrCMtrDMtrE, which exports antimicrobial
peptides7578. RNA polymerase- factor RpoH maintains
protein homeostasis, and the two-component system
MisRMisS responds to membrane stress79. Portions
of the human genital mucosa present a microaerophilic
environment, but different subcellular locations may
vary in oxygen concentration. N. gonorrhoeae is capable of either aerobic or anaerobic respiration controlled
through a truncated denitrification pathway that is regulated by copper-containing nitrite reductase (AniA) and
nitric oxide reductase subunit B (NorB)80,81. The oxygensensing fumarate and nitrate reduction regulator protein
(Fnr) is required for activation of this pathway, in addition
to the two-component system containing nitrate/nitrite
sensor protein NarQ and nitrate/nitrite response regulator NarP82. Intracellular iron homeostasis is maintained
via the ferric uptake regulation protein (Fur), and there
is substantial overlap between the regulons responsive to
iron, anaerobic conditions and oxidation, highlighting
the fine-tuned and interconnected nature of adaptive
regulatory networks in N. gonorrhoeae8386.
N. gonorrhoeae encodes a specialized repertoire of 34
putative transcriptional regulators, two-component systems and sRNAs. These systems are relatively small in
number compared with the ~200 transcriptional regulators, two-component systems and sRNAs found in
Escherichia coli. The small number of these regulons
in N. gonorrhoeae is likely due to the organisms long
evolutionary history solely colonizing human hosts,
thereby limiting the diversity of environmental conditions it may encounter and regulons it may require to
survive. In addition, a large number of N. gonorrhoeae
genes are stochastically modulated by phase variation87
likely reducing the need for transcriptional regulation.
During phase variation, Neisseria spp. modulate protein
production. Protein production may be altered through
changes in transcription efficiency or changes in translation efficiency. Transcription efficiency is altered by
varying the numbers of polynucleotide repeat sequences
in genes, thus tuning the levels of gene expression.
Box 2 | Neisseria gonorrhoeae infection models
As Neisseria gonorrhoeae has evolved to survive and persist only within the human body,
there are limitations to understanding the complexities of the environmental
challenges that the bacterium encounters during infection in humans. These limitations
may include, but are not limited to, the specific types and local concentrations of
specific nutrients (for example, iron, manganese, zinc, glucose, lactate and pyruvate)
that are present during infection, the oxygen concentration at different sites during
colonization, the heterogeneity of cytokine and chemokine responses among
individuals and their immune cell profiles, and the composition of local microbiota that
is encountered during infection. Currently, the interaction of N. gonorrhoeae with
specific types of host cells (epithelial cells, endothelial cells, neutrophils and
macrophages) can be studied ex vivo by using immortalized or primary cell lines.
However, immortalized cell lines do not always model human tissues, and the
heterogeneity of primary human cells can introduce variability into these studies114. The
most well-developed mouse model is the oestradiol-treated vaginal infection model156,
but this model is limited to addressing certain questions specifically, which factors
affect short-term colonization or survival of N. gonorrhoeae in the murine vaginal tract
owing to its lack of receptors (complement receptor 3 (CR3), CD46 and
carcinoembryonic antigen-related cell adhesion molecules (CEACAMs)) that are
required for bacterial adherence to and subsequent colonization of the mouse mucosal
epithelium and other differences in host physiology. The ongoing development
of transgenic mouse models that express human receptors will enable the study of
N. gonorrhoeae pathogenesis; however, these models may not be able to replicate all
the factors that are required for human infection157159.
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Translation efficiency is altered through changes in
the gene coding sequence, like the introduction of
stop codons or repeat sequences, thus tuning protein
production. N. gonorrhoeae is estimated to phase vary
over 100 genes that encode a variety of gene products88.
Phase variation presumably provides different advantages to the numerous subpopulations that result during
colonization. The opa gene family is the most wellcharacterized, phase-variable system both mechanistically and functionally89. Opa phase variation occurs
through changes in the number of CTCTT repeats in
the leader peptide sequence of each of 11 opa alleles
in the chromosome, resulting in altered expression of
these genes and affecting host cell adherence and neutrophil stimulation. The ability of N. gonorrhoeae to generate a repertoire of different phenotypes within a clonal
lineage promotes long-term adaptation on the bacterial
population level. In addition to phase variation of Opa
proteins, N. gonorrhoeae also encodes a phase-variable
methyltransferase, ModA13, that switches between an
on and off state of gene expression, regulating changes
in polynucleotide repeats that occur during replication. Differences in the amount of ModA13 activity
result in altered methylation patterns of promoters and
expression levels of various genes. This phenomenon is
called the ModA13 phasevarion and influences virulence factor gene expression and biofilm formation9094.
N. gonorrhoeae proteins may also be globally regulated
by post-translational modifications, and acetylation has
been shown to affect numerous pathways, anaerobic
growth and the ability to form biofilms95. Lastly, it is
known that N. gonorrhoeae expresses sRNAs. Although
many N. gonorrhoeae regulatory sRNAs have not been
characterized mechanistically, it is known that the sRNA
Heat shock
Protein misfolding
Protein homeostasis
Free iron levels
peptide levels
Zinc and
Redox homeostasis
Manganese and
zinc homeostasis
oxidation state
Iron-responsive regulon
Iron homeostasis
Nature Reviews | Microbiology
Cellular factors
Figure 3 | Overview of transcriptional regulatory factors of Neisseria gonorrhoeae. To adapt to a changing urogenital,
rectal and oropharyngeal environment during infection, Neisseria gonorrhoeae has moderately few regulatory networks.
N. gonorrhoeae has regulators that specifically respond to metal availability, antimicrobial peptides, oxygen availability,
membrane stress and protein misfolding. These systems are often overlapping, particularly at the level of iron and oxygen
availability. Transcriptional regulator MpeR is known to repress the antimicrobial efflux pump operon repressor MtrR; both
regulators mediate antimicrobial peptide efflux76,78. The two-component regulatory system composed of sensor histidine
kinase and response regulator (MisRMisS) responds to membrane perturbations and controls membrane homeostasis174.
The oxygen-sensor fumarate and nitrate reduction regulator protein (Fnr) responds to oxygen concentrations and
contributes to regulation of genes encoding copper-containing nitrite reductase (AniA) and nitric oxide reductase
subunit B (NorB), which control a denitrification system required for anaerobic respiration82,171. Iron-response master
regulator ferric uptake regulation protein (Fur) responds to fluctuating iron levels and controls iron homeostasis under
iron-replete and iron-starved conditions175. Peroxide-responsive repressor (PerR) responds to fluctuating zinc and
manganese levels, controlling metal influx through MntAMntBMntC and zinc and manganese homeostasis172.
RNA polymerase- factor RpoH responds to heat shock and protein misfolding and controls a regulon that maintains
protein-folding homeostasis173. Oxidative stress regulatory protein OxyR responds to the presence of reactive oxygen
species (ROS) and maintains redox homeostasis176.
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Alternative complement
One arm of the complement
system that is triggered by C3b
binding to a microorganism or
other surface, damaged tissue
and foreign material.
Classical complement
One arm of the complement
system triggered by antigen
antibody complexes with
immunoglobulin G (IgG) and
IgM antibodies.
Complement C3
An innate immune protein at
the centre of the alternative
and classical complement
Membrane attack
Groups of proteins that are
formed of complement
components C8 and C9 that
form pores in the membranes
of microorganisms.
Complement system
An innate immune defence
that recognizes and kills
microorganisms through
opsonization and formation of
membrane attack complexes.
The process by which host
molecules bind to the surface
of a microorganism to enhance
NrrF is transcribed in response to iron availability and
controls a small regulon96 and that another sRNA,
FnrS, controls a regulon of four genes in response to
anaerobic conditions84.
Interactions with the host immune system
Interactions with complement. The alternative
complement pathway and the classical complement pathway
are major arms of the innate immune system that converge at the level of protein complement C3, which can
lead to the deposition of opsonin C3b to facilitate bacterial phagocytosis and kill invading pathogens through
the formation of membrane attack complexes. The alternative and classical complement pathways are activated
by the presence of invading microorganisms; the former
is activated by non-specific tissue damage or microorganism binding, and the latter is activated through IgG
and IgM antibody deposition that leads to clearance by
the immune system. Activation of both pathways converges on cleavage and activation of C3, which is crucial
for maintaining a cascade that results in the assembly of
membrane attack complexes (transmembrane pores that
form on bacterial surfaces), causing lysis and cell death.
The ability of N. gonorrhoeae to evade recognition
and attack from the human complement system is a
major feature of host adaptation by this species, which
is highlighted by the observation that N. gonorrhoeae
resists the action of the human complement system but
is sensitive to animal complement systems97. Patients
with complement deficiencies have been found to have
a higher risk of systemic N. gonorrhoeae infection98,99.
In both the cervical epithelium and human serum, the
alternative and classical complement pathways respond
to N. gonorrhoeae infection by initiating the complement cascade to opsonize invading bacteria18,100102.
Studies in vitro have shown that N. gonorrhoeae interacts with several complement components103. N. gonorrhoeae evades complement-mediated killing through
two general mechanisms: by binding to and inactivating
complement cascade components and preventing membrane attack complex formation, and by presenting itself
as part of the host by expressing molecules found in the
host on the bacterial surface and binding to complement
regulatory proteins (FIG. 4a).
In the first mechanism, N. gonorrhoeae inactivates
the complement cascade through factor I. C3b binds to
gonococcal LOS through lipid A and is rapidly inactivated by factor I-mediated cleavage to iC3b, thus inactivating the complement cascade101 (FIG. 4a). In addition,
in the cervical epithelium, N. gonorrhoeae binds to the
alternative complement pathway receptor CR3 and the
receptor for iC3b, which is thought to facilitate epithelial
cell invasion100,102.
In the second mechanism, N. gonorrhoeae shields
itself from complement recognition, thus subverting
complement activation in both the cervical epithelium
and human serum. In the cervical epithelium, N. gonorrhoeae binds the alternative complement pathway
regulator factor H through sialylated LOS and porin
(FIG. 4a). Normally, factor H acts as an alternative complement pathway regulatory protein that binds sialylated
cell structures to protect cells, as host structures bound
to factor H are considered self and not targeted for
opsonization and lysis104,105. In the serum, N. gonorrhoeae
can bind the classical complement pathway regulator
C4BP, a molecule that has a similar function to factor H, to the porin106. Moreover, N. gonorrhoeae can bind
complement regulatory factor CD46 through the pilus,
though the role of this interaction in pathogenesis is not
fully defined107 (FIG. 4a). Without the ability to avoid complement recognition and killing, N. gonorrhoeae would
not be able to effectively colonize epithelial mucosa and
grow, as shown by its sensitivity to killing by animal
complement components.
Immune cell detection and signalling. Owing to the
lack of surveillance and difficulty in diagnosing asymptomatic gonorrhoea, little is known about how the
immune system responds to N. gonorrhoeae during
asymptomatic infection, though these infections likely
represent a high and likely underreported proportion of
infections. Symptomatic infection stimulates the release
of pro-inflammatory cytokines and chemokines (IL-6,
IL-8, IL-1B, IL-17, interferon- (IFN) and the cytokineexpression controlling transcription factor nuclear
factor-B (NF-B)), causing an influx of neutrophils
to the site of infection and potentially causing inflammatory damage within the epithelial mucosa74,108110
(FIG. 1). During colonization, bacterial factors like LOS
and peptidoglycan are present and capable of triggering
their cognate innate immune sensors Toll-like receptor 2 (TLR2), TLR4, nucleotide-binding oligomerization
domain-containing protein 1 (NOD1) and NOD2, all of
which induce signalling of the host innate immune system111113 (FIG. 1). In addition, detection by immune sentinel cells like macrophages and dendritic cells55,114 releases
a gradient of cytokines and chemokines, including IL-6,
IL-8, IL-1B, IL-17 and IFN, thus resulting in an influx
of neutrophils. N. gonorrhoeae has also been shown to
release heptose-1,7-bisphosphate, a metabolic intermediate that acts as a pathogen-associated molecular
pattern to trigger TRAF-interacting protein with FHA
domain-containing protein A (TIFA)-dependent innate
immunity115 (FIG. 1). Moreover, N. gonorrhoeae can survive within macrophages by modulating apoptosis and
cytokine production114 and may polarize macrophages
in a way that suppresses T cell proliferation116 (FIG. 4b).
As discussed earlier in this Review, N. gonorrhoeae
colonization may result in either symptomatic or
asymptomatic infection. Symptomatic infection occurs
when there is sufficient neutrophil influx into the site
of infection to produce a purulent exudate. Although
it is known that the presence of a purulent exudate is
a result of bacterial innate immune stimulation that
causes cytokine and chemokine signalling and neutrophil influx, it is not known whether asymptomatic
patients also recruit neutrophils to the site of infection.
It is possible that N. gonorrhoeae recruits neutrophils in
numbers that are insufficient in some people to produce
observable symptoms, but it is also possible that neutrophils are not recruited to the site of infection during
asymptomatic colonization.
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Differences in N. gonorrhoeae Opa variants and their
propensity for CEACAM binding and immune stimulation may explain the heterogeneity of symptoms
in infected individuals. OpaCAECAM interactions
determine neutrophil adhesion, phagocytosis and
stimulation of the oxidative burst. The N. gonorrhoeae
cell surface may lack Opa proteins entirely (Opa-less),
express one Opa variant only or express a combination
of many opa alleles. During human infection, there is
variability of Opa expression in N. gonorrhoeae ranging
from multiple Opa-expressing to Opa-less strains, and
Opa expression is correlated with the menstrual cycle47.
a N. gonorrhoeae prevents complement activation, opsonization and bacterial killing
c N. gonorrhoeae modulates T cell function and varies its surface components to avoid the adaptive immune system
b N. gonorrhoeae modulates the activities of macrophages, DCs and neutrophils
N. gonorrhoeae modulates apoptosis and
polarizes some macrophage lineages
Classical complement
Oxidative killing
Non-oxidative killing
TH1 and TH2 cell
T cell proliferation
Antigenic variation
of type IV pili
DC-mediated CD4+
T cell proliferation
Factor H
Sialylated LOS
Cervical epithelium
Factor I
Phase variation
of Opa protein
and LOS
Uptake and survival
in phagosome
Outer membrane
Inner membrane
Lipid A
Sialic acid
Type IV
Uptake and survival
of N. gonorrhoeae in
Figure 4 | Neisseria gonorrhoeae evades and modulates the innate and adaptive immune systems. Nature Reviews a | During | Microbiology
infection, both the alternative and classical complement pathways may be activated by Neisseria gonorrhoeae.
N. gonorrhoeae binds complement proteins to prevent opsonization and killing by membrane attack complexes18 and
sialylates its lipooligosaccharide (LOS) to hide from the complement system177. N. gonorrhoeae binds host factor H
and C4bbinding protein (C4BP), becoming serum resistant by presenting as self and by shielding itself from complement
recognition97,178N. gonorrhoeae also binds to the alternative complement pathway receptor complement receptor 3 (CR3)
and the receptor for iC3b, a process thought to aid in epithelial cell invasion100. N. gonorrhoeae binds C3b through lipid A
on its LOS, rapidly inactivating C3b by factor I-induced conversion to iC3b179. b |N. gonorrhoeae is able to survive in and
around macrophages and neutrophils during infection and modulate the immune-activating properties of dendritic cells
(DCs)114,116,122,124,136,164,180. In macrophages, N. gonorrhoeae is able to survive inside the phagosome and modulate apoptosis
and production of inflammatory cytokines114. The bacterium polarizes macrophages, resulting in macrophages that are
less capable of T cell activation116, and, similarly, DCs exposed to N. gonorrhoeae are less capable of stimulating T cell
proliferation136. The interactions of N. gonorrhoeae and neutrophils are complex and are discussed in detail in the main
text. c |N. gonorrhoeae infection does not generate immunological memory, owing to the ability of N. gonorrhoeae to
antigenically and phase vary its surface structures, including type IV pili, opacity (Opa) proteins and LOS. In addition,
N. gonorrhoeae modulates the adaptive immune response by suppressing T helper 1 (TH1) and TH2 cell proliferation and
subsequent activation by influencing cytokine production134,135. IL10, interleukin10; NETosis, cell death caused by
neutrophil extracellular traps (NETs); PDL1, programmed cell death 1 ligand 1; TGF, transforming growth factor-;
TLR, Toll-like receptor.
2 0 1 8 M a c mil l a n P u bl i s h e r s Li mit e d, p a rt o f S p ri n g e r N a t u r e. Al l ri g h t s r e s e r v e d.
It is not known whether particular Opa expression
patterns predominate during asymptomatic or symptomatic infection. Although CEACAM1, CEACAM3,
CEACAM5 and CEACAM6 bind to N. gonorrhoeae Opa
variants, only CAECAM1, CEACAM3 and CEACAM5
are expressed on neutrophils117. The OpaCAECAM3
interaction is the only one known to stimulate a bactericidal neutrophil oxidative burst. Although the
underlying mechanism for why some infections elicit
observable symptoms and some do not is unknown, it
has been proposed that a subset of the bacterial population capable of OpaCEACAM3 binding induces killing by neutrophils of a sufficient number of bacteria
to prevent massive neutrophil influx and an observable
purulent exudate. There is evidence that neutrophils use
CEACAM3 as a decoy receptor, as bacterial contact with
the CEACAM family of receptors enables colonization
of other cells types but on neutrophils enables capture of
Opa-expressing variants for subsequent phagocytosis,
neutrophil activation and killing118,119.
Neutrophil infiltration to sites of infection. Similarly,
although it is known that N. gonorrhoeae is able to
both evade and modulate host immune responses,
there is disagreement as to whether neutrophil recruitment ultimately serves to benefit the host, pathogen
or both. One hypothesis is that N. gonorrhoeae aims
to remain undetected by the immune system because
immune stimulation ultimately benefits the host. This
hypothesis is supported by the observation of prevalent asymptomatic infections in humans, the observation that asymptomatic partners are infectious and
data showing that bacterial shedding does not correlate
with neutrophil infiltration in mice120. An alternative
hypothesis is that neutrophil inflammation at sites of
infection serves primarily to benefit the pathogen,
largely through facilitating transmission. In this scenario, the neutrophil exudate carries live, replicating bacteria, analogous to a Trojan horse74. Data supporting
the argument that neutrophil inflammation benefits the
pathogen show that, although the presence of N. gonorrhoeae recruits neutrophils to the site of infection, most
bacteria (except those expressing Opa variants that
engage CEACAM3 on neutrophils) are able to survive
and replicate inside and outside of neutrophils86,121127.
In this scenario, it is possible that the transfer of infected
neutrophils between partners through sexual contact
with purulent exudate is a mechanism that the bacterium exploits for efficient transmission from women to
their partners74. Several components of seminal plasma,
like lactoferrin77
, are chemoattractants for neutrophils128. However, testing any transmission hypothesis
is difficult because human transmission studies are
unethical and there is no animal model that accurately models sexual transmission. The ambiguity of
the role of neutrophils in disease pathogenesis stems
from the heterogeneous nature of interactions between
N. gonorrhoeae and neutrophils, a lack of knowledge
regarding what causes some infections to be symptomatic whereas others show symptoms, and limitations
to experimental models. BOX 2 addresses the usefulness
and limitations of current models that are used to study
mechanisms of N. gonorrhoeae pathogenesis. As this is a
system in equilibrium, it is probable that specific host
pathogen interactions sway the equilibrium in either
direction, ultimately determining whether neutrophil influx benefits the host or the pathogen for each
individual infection.
Adaptive immunity and issues with vaccine development. It is known that individuals who have been
treated for gonorrhoea can be repeatedly infected
with no development of immunological memory. An
experimental gonococcal infection model study in men
showed that initial infection failed to provide protection against repeated infection with the identical strain
within 21 days of initial infection129. N. gonorrhoeae
evades the adaptive arm of the human immune system
by several mechanisms. N. gonorrhoeae undergoes antigenic and phase variation of the surface-exposed type IV
pili, Opa proteins and LOS to escape immunity45,48,130
(FIG. 4c). The carbohydrate structures of LOS also have a
role in immune evasion by mimicking host molecules.
Many N. gonorrhoeae LOSs show cross reactivity with
antibodies that recognize human glycosphingolipid
surface antigens, particularly on human erythrocytes,
mimicking human surface antigens and contributing to
the difficulty of vaccine development131133. In addition,
N. gonorrhoeae has been shown to actively suppress the
adaptive immune response by modulating IL-10 production from mouse iliac lymph node cells, CD4+
T cells
and genital tract explants by modulating transforming
growth factor- (TGF) cytokine production in BALB/c
mouse vaginal cells and the type 1 regulatory T cell
activity of CD4+ T cells, thus preventing T helper 1 and
T helper 2 cell development134,135. Moreover, dendritic
cells that have been exposed to N. gonorrhoeae are no
longer capable of inducing CD4+ T cell proliferation136.
Whereas N. gonorrhoeae stimulates a large innate
immune response from the human host and suppresses
the adaptive immune response, these interactions of
N. gonorrhoeae on both arms of the immune system can
extend infections and enable repeated infections in the
high-risk group of the population137.
Strategies that have searched for protective antigens by comparing infected individuals have been
unsuccessful for N. gonorrhoeae vaccine development
because natural protection is uncommon (if it exists
at all). The ability of N. gonorrhoeae to antigenically
and phase vary multiple surface proteins has reduced
the number of viable vaccine antigen candidates.
Vaccine development has also been hampered by the
lack of a global systematic vaccine antigen analysis
where many antigen candidates are consistently tested
in a high-throughput manner as compared with the
few that have been tested in disparate experiments.
Purified pili and killed whole cells have been tested,
but neither has resulted in a viable vaccine138. The
lack of viable vaccine antigen candidates combined
with limitations to the current animal models has also
impeded progress. A recently published study from
New Zealand, wherein young adults were inoculated
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Outer membrane vesicle
A membrane-bound vesicle
that is secreted from the
bacterial envelope and can
contain a variety of cellular
with a group B outer membrane vesicle meningococcal
vaccine, showed reduced rates of gonorrhoea in that
population139. This is the first time a vaccine has shown
reduced rates of gonorrhoea in those inoculated, and
follow-up studies are needed to determine whether the
vaccine confers true protection or whether the reduced
rates are not actually in response to the vaccination.
If a viable vaccine were generated, producing and distributing a substantial amount of vaccine to reduce
worldwide gonorrhoea prevalence would be a large
economic challenge and unlikely to be undertaken by
the pharmaceutical industry. The necessary follow-up
studies, production and distribution of a viable vaccine will require non-profit and government support
for funding.
Antimicrobial resistance
Without an effective vaccine, antibiotics have been the
only effective method for controlling gonorrhoea, but
the effectiveness of antibiotics is now in question. The
main molecular mechanisms that are used by bacteria to
develop antimicrobial resistance are protective alteration
of antibiotic targets, decreased influx of antibiotics into
the cell through transport proteins, increased efflux of
antibiotics out of the cell via multidrug efflux pumps and
expression of antibiotic-inactivating enzymes. Different
strains of N. gonorrhoeae have evolved numerous resistance determinants using all of these mechanisms to
inhibit killing by all major classes of antibiotics (FIG. 5a).
There has been substantial research into -lactam resistance mechanisms of N. gonorrhoeae. Transpeptidase
penicillin-binding protein 2 (Pbp2; encoded by the
penA gene) is a periplasmic transpeptidase and the
main lethal target of cephalosporins; most resistant isolates contain mosaic mutations in penA140. The pump
MtrCMtrDMtrE and its repressor MtrR contribute
to N. gonorrhoeae resistance through antimicrobial
efflux141. Variants of the major porin protein, which is
encoded by porB, contribute to resistance to -lactams,
but resistance requires a concomitant mutation in
mtrR142. Interestingly, when all known resistance determinants are transformed into a susceptible recipient
strain from a resistant strain, the transformants do not
reconstitute the full level of resistance of the donor strain,
suggesting the presence of one or more undiscovered
factors that cannot easily be transferred to a recipient
Nature Reviews | Microbiology
Sulfonamides Sulfonamides
Tetracyclines Tetracyclines
High-level -lactamase
First clinical use
Last clinical use
of resistance
Inner membrane
mtrC mtrD mtrE
Figure 5 | Antibiotic resistance in Neisseria
gonorrhoeae. a | Main resistance determinants of
Neisseria gonorrhoeae. Transpeptidase penicillin-binding
protein 2 (PBP2; encoded by penA) is a periplasmic
transpeptidase and the main lethal target of
cephalosporins; most resistant isolates contain mosaic
mutations in penA. The efflux pump MtrCMtrEMtrD and
its repressor MtrR contribute to N. gonorrhoeae resistance
through antimicrobial efflux. The major porin protein, PorB,
which is encoded by porB, is also a main resistance
determinant that cannot manifest independently, but
requires concomitant mutation in mtrR. b | Timeline of
antibiotic-resistance development. Since the treatment
of N. gonorrhoeae with sulfonamides in the 1930s,
N. gonorrhoeae has acquired genetic resistance
determinants that prevent killing by all major classes of
antibiotics that are used as first-line methods of treatment
for gonorrhoea143,145,147,181. As shown in the timeline, each
new class of antibiotics that served as a first-line treatment
for N. gonorrhoeae has been stopped, as strains gained
resistance. Recently, resistance was observed for the last
available first-line treatment for N. gonorrhoeae infection,
the extended-spectrum cephalosporins143. The ability of
N. gonorrhoeae to evolve resistance has led the WHO and
CDC to term it a superbug and to speculate that if new
therapies are not developed soon, we may face an era of
untreatable antimicrobial-resistant gonorrhoea3
2 0 1 8 M a c mil l a n P u bl i s h e r s Li mit e d, p a rt o f S p ri n g e r N a t u r e. Al l ri g h t s r e s e r v e d.
Mosaic alleles
Gene alleles that are produced
by recombination of different
gene sequences.
Multilocus sequence typing
A system to define strains of a
species by defining the DNA
sequence of alleles of a defined
series of housekeeping genes.
strain through transformation143. Moreover, expression
levels of the Mtr efflux pump, controlled by MtrR, affect
bacterial fitness during vaginal infection of female mice
through an unknown mechanism144. This is an important example of how the development of antibiotic resistance can influence fitness, as, historically, the field has
treated bacterial pathogenesis as a separate area of study
from antimicrobial resistance. In addition, a greater
understanding of N. gonorrhoeae antibiotic resistance
determinants, especially in the context of increased fitness by enhancement of colonization or pathogenesis,
is important because of the high potential for horizontal gene transfer between pathogenic and commensal Neisseria spp. within the human host, indicating a
potential reservoir for antimicrobial-resistant genes.
The human history of antibiotic development is
matched by the history of N. gonorrhoeae developing
and retaining resistance to all new effective antibiotics
(FIG. 5b). These antimicrobials include sulfonamides,
penicillins, tetracyclines, macrolides and fluoroquinolones143,145. The recent failures of treatments using cefixime
and ceftriaxone, the extended-spectrum cephalosporins
and -lactam antibiotics that are used as the last available first-line treatments for gonorrhoea, have highlighted the potential for untreatable gonorrhoea to
become a widespread public health epidemic146. Due
to a lack of quality metadata on individuals who have
been infected with N. gonorrhoeae, their sexual networks and the phenotypic and genotypic characteristics
of the gonococcal population, we can only speculate on
the reasons for antimicrobial resistance emergence and
spread. Antimicrobial resistance has likely been facilitated by unrestricted access to and over-prescription
of antimicrobials, particularly in the WHOs Western
Pacific Region147. In addition, N. gonorrhoeae is naturally competent for transformation; thus, it is able to
take up gonococcal DNA and to a lesser extent other
bacterial DNA from the environment and recombine it
efficiently with homologous sequences in the gonococcal
genome148,149. The high propensity for N. gonorrhoeae to
take up DNA from the environment adds to the likelihood that N. gonorrhoeae genes encoding antibiotic
resistance determinants will mutate and become resistant. Transformation can produce mosaic alleles in genes
that represent antimicrobial resistance determinants,
wherein two orthologous or paralogous genes combine
to form a mosaic, resistant variant gene150. In addition,
mutations can arise within a gene, conferring antibiotic
resistance to that gene product. On the basis of previous
observations, it is reasonable to predict that the spread
of cephalosporin-resistant N. gonorrhoeae will continue
to increase. Suspected multidrug-resistant strains are
genotyped by multilocus sequence typing and N. gonorrhoeae multi-antigen sequence typing (NG-MAST), but
it is important to continue hypothesis-driven molecular
research to understand the molecular mechanisms of
action for N. gonorrhoeae antibiotic resistance determinants. Indeed, this will enable the development of
novel therapeutics and heighten our understanding
of how resistance persists in a population and spreads
among strains.
In this Review, we have summarized our knowledge
of the course of N. gonorrhoeae pathogenesis from
transmission, adherence, colonization and invasion to
evasion of the innate and adaptive immune systems.
We have emphasized the difficulty in studying a hostadapted human pathogen. Owing to the multifaceted
nature of the urogenital tract, rectum and oropharynx
(which are composed of many different types of epithelial tissue), the fact that innate immune cell composition
varies by individual and the unknown concentrations
of oxygen and nutrients, substantial challenges exist to
develop tissue culture and animal models to study the
obligate human pathogen N. gonorrhoeae. Advances in
standardizing the cell culture techniques of primary tissue culture and transgenic mouse models may help to
ameliorate these challenges. In addition, we have argued
that asymptomatic infections are common in men and
women. Though the prevailing dogma currently holds
that infections in women are mainly asymptomatic
whereas infections in men are not, many studies show
asymptomatic infections are prevalent in both sexes. We
argue that the prevailing hypothesis more likely stems
from physiological and anatomical differences in the
urogenital tract between sexes, making neutrophil influx
much more obvious and gonorrhoea easier to diagnose
in men than in women.
Owing to the host-restricted life cycle of N. gonorrhoeae and the limitations of existing tissue culture
and animal model systems, many niches of the microenvironments that the gonococci inhabit and replicate
within are unknown. Particularly, the nutrient and
oxygen availability, the magnitude of innate immune
responses and the microbiota composition (which are
specific to the different sites within the male and female
genital tracts) may be markedly different, requiring
N. gonorrhoeae to adopt distinct adaptive programmes
in response to local conditions as infection progresses.
Deep sequencing of multiple clinical strains and further
characterization of common intersecting gene regulons
among strains can help elucidate which gene networks
are important for colonization and pathogenesis. In
addition, the continued development of tissue culture
systems that are composed of different cell types for
example, modelling the different tissues that line the
ascending vaginal, cervical and fallopian tube epithelia, and transgenic mouse models with humanized epithelial surface receptors can help to generate more
complex model microenvironments that are relevant
to human infection. Very little data exist on the host
and bacterial factors that contribute to infections that
do not display overt symptoms, and more sensitive
surveillance, screening and diagnostics are needed to
characterize bacterial strains and host cell factors that
contribute to asymptomatic colonization.
Despite the massive amount of data from sequencing projects showing the vast diversity of microbial
species, scientists and policy makers still tend to think
of all major groups of bacteria as being alike. N. gonorrhoeae is an example of a host-restricted organism
that has been on a singular evolutionary path, existing
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currently (and likely throughout its evolution) as a commensal-like organism in equilibrium with the host but
retaining the ability to elicit inflammation. Efficient
horizontal gene transfer mechanisms have contributed
to the rise in antimicrobial resistance. The stochastic
alterations of gene expression and antigenic properties
through phase variation and antigenic variation have
hindered the development of viable vaccine candidates. Understanding the underlying mechanisms by
which N. gonorrhoeae evades immune detection and
develops antimicrobial resistance factors will aid in the
development of new therapeutics for gonorrhoea.
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H.S.S. was supported by the US National Institutes of Health
(NIH) grant R37AI033493. S.J.Q. was partially supported
by NIH grant T32AI0007476.
Author contributions
S.J.Q. and H.S.S. contributed to researching data for the
article. S.J.Q. and H.S.S. substantially contributed to the dis
cussion of content. S.J.Q. and H.S.S. wrote the article. S.J.Q.
and H.S.S. reviewed and edited the manuscript before
Competing interests statement
The authors declare no competing interests.
Publishers note
Springer Nature remains neutral with regard to jurisdictional
claims in published maps and institutional affiliations.
Reviewer information
Nature Reviews Microbiology thanks M. A. Apicella, S. GrayOwen and M. W. Russell for their contribution to the peer
review of this work.
240 | APRIL 2018 | VOLUME 16
2 0 1 8 M a c mil l a n P u bl i s h e r s Li mi t e d, p a rt o f S p ri n g e r N a t u r e. Al l ri g h t s r e s e r v e d.

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