Sexually Transmitted Infections — Gonorrhea and Chlamydia
Contents (8)
Gonorrhea (caused by Neisseria gonorrhoeae) and chlamydia (caused by Chlamydia trachomatis) are the two most common bacterial sexually transmitted infections (STIs) in developed countries and represent a major public health burden globally. These pathogens share epidemiologic features, similar clinical presentations, and overlapping treatment considerations, yet possess distinct microbiologic characteristics and transmission patterns. Gonorrhea affects approximately 87 million new infections annually worldwide, while chlamydia causes approximately 131 million infections, with prevalence highest among sexually active individuals aged 15-24 years across all demographic groups. Both infections carry significant morbidity through complications including pelvic inflammatory disease (PID), ectopic pregnancy, infertility, and neonatal ophthalmia, making accurate diagnosis and prompt treatment essential for clinical practice. Their frequent co-infection (occurring in 20-40% of gonorrhea cases) necessitates dual-therapy approaches. Understanding the distinct epidemiology, transmission dynamics, antimicrobial resistance patterns—particularly fluoroquinolone-resistant gonorrhea and azithromycin-resistant chlamydia—and evolving treatment guidelines is critical for USMLE success and appropriate patient management.
Both N. gonorrhoeae and C. trachomatis demonstrate sophisticated mechanisms for establishing urogenital infection, evading immune responses, and causing tissue damage through inflammatory cascades.
- Bacterial adhesion and epithelial invasion: Neisseria gonorrhoeae expresses pili (filamentous appendages composed of pilin protein) that mediate initial attachment to columnar epithelial cells of the urethra, endocervix, rectum, and pharynx. These pili undergo antigenic variation through recombination, generating strain diversity that partially explains the difficulty in vaccine development. Upon adherence, the organism secretes IgA protease (a serine protease that cleaves dimeric IgA at the hinge region), which inactivates a major mucosal antibody and facilitates immune evasion. The bacterium then undergoes phase variation—reversible ON/OFF switching of pili expression—allowing some organisms to downregulate pili and express opacity proteins (Opa) that bind to complement receptors and carcinoembryonic antigen-related cellular adhesion molecules (CEACAMs), promoting cellular invasion and intracellular survival. C. trachomatis possesses outer membrane proteins (OMPs), particularly OmpA, which mediate attachment to host cells; this organism is an obligate intracellular pathogen that invades non-ciliated columnar epithelial cells via a distinct mechanism involving actin-dependent endocytosis. The chlamydial inclusion (reticulate body) prevents phagolysosome fusion through inhibition of SNARE proteins, allowing prolonged intracellular replication without bacterial lysis.
- Inflammatory cascade and tissue damage: The inflammatory response paradoxically drives much of the pathology. N. gonorrhoeae cell wall lipopolysaccharide (LPS) contains a unique lipid A structure that potently activates innate immunity through Toll-like receptor 4 (TLR4), triggering NF-κB signaling and production of IL-6, IL-8, TNF-α, and other pro-inflammatory cytokines. This robust innate immune activation recruits polymorphonuclear leukocytes (PMNs) to the epithelium; the resulting purulent exudate contains millions of PMNs attempting to phagocytose the organism but often failing due to incomplete opsonization and anti-phagocytic mechanisms. The epithelial damage observed in gonorrhea—including subepithelial microabscesses, ulceration, and desquamation—results primarily from PMN-mediated injury rather than direct bacterial toxin production. C. trachomatis similarly triggers IL-8 and other chemokine production, but the intracellular location delays and modifies the inflammatory response. Importantly, repeated or chronic chlamydial infections generate a Th1-skewed response with IFN-γ production that drives fibroblast activation and excessive collagen deposition, leading to tubal scarring and stricture formation—a key mechanism for long-term sequelae.
- Epithelial translocation and ascending infection: Following the initial focal urethritis or cervicitis, organisms (particularly C. trachomatis) ascend through the endocervix into the upper genital tract via the fallopian tubes, a process facilitated by disruption of local defenses and loss of epithelial integrity. C. trachomatis possesses a 7.5-kb cryptic plasmid encoding proteins that promote epithelial cell apoptosis and tissue damage. Persistence of chlamydial infection occurs in response to stress (antibiotic therapy, nutrient deprivation, interferon-γ) through conversion to an aberrant morphologic form—the persistent reticulate body—that exhibits reduced metabolic activity and altered gene expression, potentially explaining chronic infections and recurrences. The mucosal immunoglobulin A deficiency noted in some persistently infected individuals may reflect altered mucosal immunity induced by chlamydial antigens. N. gonorrhoeae can disseminate hematogenously in rare cases (1-3% of untreated infections), particularly during menstruation when there is disruption of the endometrial barrier; disseminated gonococcal infection (DGI) results from seeding to joints, skin, and other sites, with certain porin variants (PorB) conferring enhanced propensity for dissemination.
- Complement evasion and resistance mechanisms: N. gonorrhoeae expresses a porin (PorB) that can insert into host cell membranes and inhibit complement activation; some strains produce a polysaccharide capsule that mimics human neural tissue, facilitating molecular mimicry and potentially reducing antibody recognition. Both organisms downregulate or modify surface antigens through phase and antigenic variation, explaining the difficulty in generating sterilizing immunity. Antimicrobial resistance in N. gonorrhoeae now involves mutations in penicillin-binding proteins (PBPs), alterations in membrane permeability (mtrR mutations affecting the MtrCDE efflux pump), and plasmid-mediated resistance genes (particularly penA and other chromosomal alterations conferring reduced fluoroquinolone susceptibility). C. trachomatis resistance to macrolides involves mutations in 23S rRNA and altered ribosomal binding, a mechanism now detected in multiple geographic regions.
- Neisseria gonorrhoeae (Gonorrhea): An obligate human pathogen with no environmental reservoir, transmitted exclusively through sexual contact (genital, rectal, pharyngeal) or vertically to neonates during vaginal delivery. The organism is gram-negative, oxidase-positive, diplococci with a polypeptide capsule in clinical isolates (losing capsule rapidly in culture). Specific virulence factors include pili (pilin antigenic types), opacity proteins, IgA protease, and lipopolysaccharide. While no animal reservoir exists, strain variation and antigenic drift permit continued transmission within human populations despite immune responses.
- Chlamydia trachomatis (Chlamydia): An obligate intracellular, gram-negative bacterium with 19 serovars (A-L associated with urogenital and lymphoproliferative disease; L1-L3 with lymphogranuloma venereum [LGV]). Transmission occurs through sexual contact with infected mucous membranes or, rarely, through fomites with eye exposure. The organism lacks a cell wall peptidoglycan layer, explaining its resistance to β-lactam antibiotics and why it requires specific culture conditions (cell culture or molecular techniques rather than standard bacterial media). C. trachomatis has a unique replication cycle involving infectious elementary bodies (EBs) and metabolically active but non-infectious reticulate bodies (RBs).
- Epidemiologic risk factors (both organisms)
- Age 15-24 years: Peak incidence reflects sexual debut, reduced barrier contraceptive use, and potentially higher biological susceptibility
- Multiple sexual partners or new sexual partner: Direct relationship with transmission probability
- Inconsistent condom use: Condoms reduce transmission by approximately 95% when consistently used
- History of prior STI: Indicates behavioral patterns and may reflect incomplete partner treatment
- Sex work or exchange of sex for money/drugs: Associated with higher partner numbers and reduced access to healthcare
- Incarceration history: Confined populations show elevated prevalence
- Men who have sex with men (MSM): Particularly at risk for rectal and pharyngeal infections; higher rates of antimicrobial resistance observed in some regions
- Pregnancy: Routine screening is essential; both organisms cause neonatal complications
- Socioeconomic factors and healthcare access: Poverty, lack of insurance, and limited preventive care access correlate with higher prevalence
The clinical spectrum varies by anatomic site of infection, host factors, and organism. Approximately 50% of women and 10% of men with chlamydia are asymptomatic, highlighting the importance of screening, while gonorrhea more commonly causes symptomatic disease (approximately 90% of men with urethritis, but still 10-20% asymptomatic).
- Urethritis (anterior urethritis)
- Dysuria and urinary frequency: Dysuria results from epithelial inflammation and PMN infiltration creating mucosal ulceration; urinary frequency reflects urinary urgency from urethral irritation and bladder inflammation
- Urethral discharge: In gonorrhea, discharge is typically purulent, abundant, and greenish-yellow (reflecting the large number of PMNs and bacteria), often noted upon first morning micturition; in chlamydia, discharge tends to be mucopurulent, scant, and clear to whitish, reflecting slower PMN recruitment and intracellular location of bacteria. The distinction, while useful, is not reliable enough for diagnostic purposes
- Dysuria-pyuria dissociation: Can occur in chlamydia where intracellular location limits pyuria; important to consider when dysuria is present without significant pyuria
- Cervicitis (mucopurulent cervical discharge)
- Vaginal discharge: May be purulent or mucopurulent, often foul-smelling if secondary bacterial overgrowth occurs
- Cervical friability and erythema: Results from epithelial inflammation; petechial hemorrhages may be visible
- Dysuria and urinary frequency: Similar mechanisms to urethritis; dysuria may be exacerbated by urination over inflamed perineal tissue
- Lower abdominal pain or dyspareunia: Reflects extension of inflammation to the endometrium and lower pelvis
- Post-coital bleeding: Results from friable cervical epithelium
- Proctitis (rectal infection)
- Rectal pain, tenesmus, and purulent discharge: Particularly common in MSM; symptoms reflect rectal epithelial inflammation
- Hemorrhoid-like appearance: Rectal mucosa may appear inflamed without true hemorrhoids
- May be entirely asymptomatic: Especially with chlamydia, underscoring the importance of targeted screening in high-risk populations
- Pharyngitis (pharyngeal infection)
- Usually asymptomatic: Even when bacterial cultures yield positive results
- When symptomatic: mild sore throat, exudate: Difficult to distinguish from viral pharyngitis
- Transmission risk: Oral sex is a significant transmission route; pharyngeal gonorrhea shows particularly high treatment failure rates with certain antibiotics
- Pelvic inflammatory disease (PID)
- Lower abdominal pain, often bilateral: Results from inflammation of endometrium, fallopian tubes, and peritoneum; pain is typically worse during menses when endometrial shedding exposes mucosal vessels
- Fever and systemic symptoms: Reflect systemic inflammatory response
- Cervical motion tenderness, adnexal tenderness, and uterine tenderness: Physical exam findings from direct inflammation
- Mucopurulent cervical discharge: May persist
- Potential for bacteremia and sepsis: In severe cases
- Epididymitis (in men)
- Unilateral testicular pain and swelling: Results from inflammation extending from the vas deferens to the epididymis
- Dysuria and urethral discharge may persist
- Nodular, tender swelling along the epididymis: Distinguishable from testicular swelling in orchitis
- Disseminated gonococcal infection (DGI) — rare but important
- Migratory polyarthralgia/arthritis: The arthritis-dermatitis syndrome features frank synovitis (typically affecting knees, wrists, fingers) with pustular or vesiculopustular skin lesions (often over joints or on extremities); fewer than 50% of DGI patients have urogenital symptoms at presentation, making history of recent STI symptoms critical
- Constitutional symptoms: Fever, malaise
- Potential for gonococcal endocarditis or meningitis: Rare but life-threatening complications
- Associated with pregnancy and menstruation: States where mucosal barrier is compromised
- Neonatal complications (via maternal transmission)
- Ophthalmia neonatorum (conjunctivitis): Purulent discharge and conjunctival inflammation 2-7 days after delivery (gonorrhea) or 5-14 days (chlamydia); historically a leading cause of preventable blindness, now rare in developed countries with prophylaxis
- Chlamydial pneumonia: Occurs at 2-16 weeks of age; presents with staccato cough, tachypnea, and interstitial infiltrates (distinct from viral pneumonia in that afebrile or low-grade fever with prominent respiratory symptoms)
- Chlamydial inclusion conjunctivitis: Self-limited but can progress to chronic follicular conjunctivitis
- History and risk assessment
- Recent sexual contact and partners' STI status
- Contraceptive method (particularly barrier use)
- Symptoms duration, discharge character, dysuria pattern
- Prior STI history and treatment compliance
- Pregnancy status or planning
- Routine screening indicated for all sexually active individuals <25 years and older patients with risk factors
- Nucleic acid amplification tests (NAATs) — GOLD STANDARD
- Sensitivity and specificity: PCR, TMA, or SDA achieve >95% sensitivity and >99% specificity for both organisms; superior to culture, especially for asymptomatic infections and non-urethral sites
- Specimen types: First-void urine (most sensitive for males), endocervical swab (females), rectal swab, pharyngeal swab, or urine for both organisms in screening scenarios
- First-void urine: In men, 1st-10 mL of void captures urethral secretions; detects infection in asymptomatic individuals with >95% sensitivity
- Self-collected vaginal swabs: Acceptable and may improve compliance; equal sensitivity to clinician-collected samples
- Interpretation: A positive NAAT confirms infection; negative NAAT with high pretest probability (e.g., partner notification of exposure) warrants repeat testing or empiric treatment in some cases
- Gram stain (limited role, primarily male urethritis)
- Characteristic finding (male urethritis): >5 neutrophils per high-power field (hpf) with intracellular gram-negative diplococci in urethral exudate; highly specific (~95-99%) for gonorrhea in symptomatic men but only ~50-65% sensitive (many organisms are intracellular)
- Not acceptable for female cervicitis: Insufficient sensitivity and specificity
- Not useful for asymptomatic individuals: PMN response may be minimal
- Rapid presumptive diagnosis: Allows initiation of treatment pending culture/NAAT confirmation, but NAAT is preferred
- Cervical culture (limited current role)
- Thayer-Martin medium with selective antibiotics (vancomycin, colistin, nystatin, trimethoprim) to suppress normal flora
- Culture shows gold standard organism identification and antimicrobial susceptibility testing (important for surveillance of resistance patterns but not routinely performed)
- Now largely replaced by NAAT due to superior sensitivity, particularly for asymptomatic infections
- Still used selectively for suspected treatment failures or when antimicrobial susceptibility testing needed
- Urinalysis and pyuria assessment
- Pyuria (≥10 WBC/hpf on urinalysis or ≥25 WBC/μL on urine culture): Indicates urethritis or cystitis; present in
Immediate considerations: treat presumptively at the visit when suspicion is high rather than waiting for NAAT results, since loss to follow-up drives ongoing transmission. Hemodynamic instability, peritonitis, suspected tubo-ovarian abscess, pregnancy, inability to tolerate oral therapy, or failed outpatient therapy mandate admission for parenteral treatment (CDC 2021 STI Treatment Guidelines).
First-line therapy (CDC 2021)
- Gonorrhea — third-generation cephalosporin: ceftriaxone 500 mg IM as a single dose (1 g if ≥150 kg). Ceftriaxone monotherapy replaced the older dual-therapy regimen; azithromycin is no longer added routinely for gonorrhea itself.
- Chlamydia — tetracycline: doxycycline 100 mg PO twice daily for 7 days, now preferred over single-dose azithromycin because of superior cure rates at rectal sites.
- Co-treatment: if chlamydia has not been excluded by NAAT, give doxycycline along with ceftriaxone — reflecting the high co-infection rate already noted.
Special populations and second-line options
- Pregnancy: azithromycin 1 g PO single dose for chlamydia; doxycycline and all tetracyclines are contraindicated (fetal bone/tooth deposition). Ceftriaxone remains safe.
- Cephalosporin allergy or unavailability: gentamicin IM plus high-dose oral azithromycin; oral cefixime is an alternative for urogenital (not pharyngeal) gonorrhea. True IgE-mediated penicillin allergy rarely precludes ceftriaxone — cross-reactivity is roughly 1–3% and side-chain driven.
- PID (outpatient): ceftriaxone IM plus doxycycline plus metronidazole for 14 days; inpatient regimens use cefoxitin or cefotetan plus doxycycline.
- Disseminated gonococcal infection: parenteral ceftriaxone, typically ceftriaxone 1 g IV/IM daily, for at least a week, longer for endocarditis or meningitis.
Procedural/definitive management: septic arthritis requires joint aspiration and often repeated drainage; tubo-ovarian abscess failing antibiotics requires image-guided or surgical drainage.
Contraindicated or obsolete: fluoroquinolones for gonorrhea (resistance); azithromycin monotherapy for gonorrhea; doxycycline in pregnancy. Treat partners (expedited partner therapy where legal), advise abstinence for 7 days, retest at 3 months, and perform test-of-cure for pharyngeal gonorrhea and for chlamydia in pregnancy.
Upper genital tract and reproductive
- Pelvic inflammatory disease: ascending infection with Th1-driven tubal fibrosis; signals itself as cervical motion, uterine, or adnexal tenderness in a young woman with discharge.
- Tubo-ovarian abscess (emergency): walled-off purulent collection; persistent fever and a tender adnexal mass on exam or complex cystic mass on ultrasound despite antibiotics. Rupture causes peritonitis and septic shock.
- Tubal factor infertility and ectopic pregnancy: scarred, non-peristaltic tubes; suspect in a woman with prior PID presenting with amenorrhea plus adnexal pain — a ruptured ectopic is an emergency.
- Chronic pelvic pain: adhesive disease after repeated episodes.
- Fitz-Hugh–Curtis syndrome: transperitoneal or lymphatic spread to the liver capsule causing perihepatitis; RUQ pleuritic pain with normal or near-normal transaminases, violin-string adhesions at laparoscopy.
- Epididymitis/orchitis: retrograde canalicular spread; unilateral tender epididymis with relief on elevation (Prehn sign) — always exclude torsion first.
Systemic and immune-mediated
- Disseminated gonococcal infection (urgent): hematogenous seeding; migratory polyarthralgia with pustular lesions, or frank purulent monoarthritis requiring drainage. Gonococcal endocarditis and meningitis are life-threatening.
- Reactive arthritis: post-chlamydial, HLA-B27–associated; conjunctivitis, urethritis, oligoarthritis.
- Increased HIV acquisition: mucosal ulceration and recruitment of CD4+ target cells.
Neonatal: gonococcal ophthalmia neonatorum can perforate the cornea and blind — an ocular emergency; chlamydial pneumonia presents later with staccato cough and eosinophilia.
Treatment-related
- Doxycycline: pill esophagitis, photosensitivity, dental staining in children/fetus.
- Erythromycin in neonates: associated with infantile hypertrophic pyloric stenosis — nonbilious projectile vomiting.
- Ceftriaxone: hypersensitivity, biliary sludging, and displacement of bilirubin/calcium precipitation in neonates (use cefotaxime instead).
- Macrolides: QT prolongation; any antibiotic can precipitate C. difficile colitis.
- Single best next step for suspected urethritis/cervicitis: NAAT for both organisms from urine or a site-appropriate swab, plus presumptive treatment at the same visit — do not culture first and do not wait for results.
- The regimen examiners want: ceftriaxone 500 mg IM once for gonorrhea, plus doxycycline 100 mg PO BID × 7 days if chlamydia is not excluded. Azithromycin is no longer routine dual therapy for gonorrhea (CDC 2021).
- Pregnancy swap: azithromycin 1 g PO for chlamydia; doxycycline is contraindicated. Test-of-cure is indicated in pregnancy and for pharyngeal gonorrhea; everyone else is retested at 3 months for reinfection, not failure.
- The association examiners love: Fitz-Hugh–Curtis — RUQ pain with normal LFTs in a young woman with PID, violin-string perihepatic adhesions. Second favorite: HLA-B27 reactive arthritis after chlamydia (can't see, can't pee, can't climb a tree).
- DGI buzzwords: migratory polyarthralgia + tenosynovitis + scattered pustular lesions in a young, often menstruating woman; blood and synovial cultures are frequently negative, so send NAATs from all mucosal sites. Purulent monoarthritis needs joint drainage.
- Neonatal timing: gonococcal conjunctivitis presents within the first week and is an ocular emergency; chlamydial conjunctivitis appears in the second week, and chlamydial pneumonia at 4–12 weeks with staccato cough and eosinophilia. Erythromycin ointment prophylaxis prevents gonococcal, not chlamydial, disease.
- Common distractors to avoid: fluoroquinolones for gonorrhea (resistance — abandoned); β-lactams for chlamydia (no peptidoglycan target); treating the patient but not the partner (use expedited partner therapy where legal); withholding ceftriaxone for a remote penicillin rash when cross-reactivity is only about 1–3%.
- Screening: USPSTF recommends screening all sexually active women under 25 and older women at increased risk, regardless of symptoms.