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Gram-negativeCocci

Neisseria gonorrhoeae

Gram-negative, oxidase-positive, intracellular diplococcus that causes gonorrhea, including urethritis, cervicitis, pelvic inflammatory disease, and disseminated gonococcal infection.

Organism Card

DomainMust know
Identity
  • G−ve kidney-bean (coffee-bean) diplococci; intracellular within PMNs
  • Oxidase +, catalase +
  • Obligate human pathogen; no animal reservoir [1][2]
  • Non-motile, non-sporing; fragile organism (dies quickly outside host)
Lab discriminator
  • Ferments glucose only; does NOT ferment maltose (vs N. meningitidis which ferments both glucose AND maltose) [1][2]
  • Grows on Thayer-Martin / Martin-Lewis selective agar (chocolate agar + vancomycin/colistin/nystatin to suppress normal flora) [3]
  • Oxidase + diplococci on chocolate agar = Neisseria; maltose − = gonococcus
Reservoir / transmission
  • Strictly human; transmitted by sexual contact (genital, oral, anal)
  • Vertical transmission during delivery → ophthalmia neonatorum [5]
  • Risk: MSM, multiple sexual partners, young sexually active adults [3][4]
Key virulence
  • Pili (type IV) → adhesion to mucosal epithelium + antigenic variation (evades Ab)
  • Opa proteins → mediate intimate attachment and invasion of epithelial cells
  • LOS (lipo-oligosaccharide) → endotoxic activity; undergoes phase variation
  • IgA1 protease → cleaves secretory IgA on mucosal surfaces
  • No polysaccharide capsule (vs N. meningitidis which is capsulated) [1][2]
Clinical syndromes
  • ♂ Urethritis: purulent urethral discharge + dysuria (2–5 d incubation); usually symptomatic [3]
  • ♀ Cervicitis: often asymptomatic/mild; may → PID (salpingitis, tubo-ovarian abscess, peritonitis) → infertility/ectopic pregnancy [4][6]
  • Extragenital: pharyngitis (oral sex), proctitis (MSM), conjunctivitis [3]
  • Disseminated gonococcal infection (DGI): triad of tenosynovitis + dermatitis + migratory polyarthralgia, OR purulent monoarthritis [3][7]
  • Septic arthritis in sexually active young adults → think N. gonorrhoeae [7][8]
  • Ophthalmia neonatorum: purulent conjunctivitis within 28 d of birth; risk of corneal perforation & blindness [5]
Diagnosis
  • NAAT/PCR = optimal diagnostic method for both N. gonorrhoeae and C. trachomatis [3]
  • Preferred specimen: first-void urine (♂); endocervical swab (♀) [3]
  • Gram stain of urethral swab (♂ only): intracellular G−ve diplococci within PMNs — high Sn in symptomatic males; NOT reliable in ♀ (cervical flora contamination) [3]
  • Culture on Thayer-Martin agar: allows antibiotic sensitivity testing — essential given rising resistance [3]
  • Antigen detection is NOT available for N. gonorrhoeae [3]
  • Septic arthritis: joint fluid Gram stain + C/ST [8]
Treatment
  • IM ceftriaxone (single dose) + PO azithromycin (dual therapy to cover co-infection with Chlamydia and resistant strains) [4][6]
  • DGI/septic arthritis: IV ceftriaxone × 1 week [8]
  • PID: single dose IM ceftriaxone + course of PO doxycycline + PO metronidazole [6][9]
  • Key resistance: increasing resistance to fluoroquinolones, penicillins, tetracyclines worldwide; cephalosporin resistance emerging — always perform sensitivity testing when culture available [3]
  • Treat sexual partner(s); test for concurrent STIs (Chlamydia, syphilis, HIV) [4]
Prevention
  • Notifiable disease in Hong Kong
  • Screen and treat sexual partners (contact tracing)
  • Condom use; no vaccine available
  • Ophthalmia neonatorum prophylaxis: topical erythromycin ointment at birth
  • Screen ♀ for GC in high-risk pregnancies
Classic traps
  • Maltose fermentation: gonococcus = maltose − vs meningococcus = maltose + (highest-yield biochemical discriminator) [1]
  • Young adult + migratory polyarthralgia + skin lesions + tenosynovitis = DGI, NOT Reiter's (Reiter's has urethritis + conjunctivitis/uveitis + arthritis but follows enteric/chlamydial infection, not gonococcal) [7][10]
  • Complement (C5–C9 / terminal) deficiency → recurrent Neisseria infections (both meningitidis and gonorrhoeae) [11]
  • Gonococcal septic arthritis is often polyarticular (vs S. aureus monoarticular) [7]
  • Asymptomatic ♀ cervicitis is a common vignette trap — always consider GC in PID workup

Exam Intelligence

  1. NAAT (PCR) is the recommended optimal diagnostic method for both N. gonorrhoeae and C. trachomatis — directly from Felix Lai senior notes mirroring GC lecture content; antigen detection is NOT available for gonococcus [3].

  2. Thayer-Martin agar is the selective medium for gonococcal culture; this allows antibiotic sensitivity testing, which is critical given emerging cephalosporin resistance [3][1].

  3. Maltose fermentation negative is the single most tested biochemical discriminator: gonococcus ferments glucose only; meningococcus ferments glucose AND maltose [1][2].

  4. Disseminated gonococcal infection (DGI) presents as either (a) the classic triad of tenosynovitis + dermatitis + polyarthralgia, or (b) purulent monoarthritis. This is the most common cause of septic arthritis in sexually active young adults [3][7][8].

  5. Complement C5–C9 (terminal/MAC) deficiency predisposes to recurrent Neisseria infections — tested in immunodeficiency lectures and past papers [11].

  6. PID management: IM ceftriaxone single dose → PO doxycycline course → PO metronidazole course. This covers gonococcus, Chlamydia, and anaerobes — directly examined in 2024 MCQ Q89 [6][9].

  • Gonococcus vs Meningococcus: both are G−ve oxidase + diplococci. Discriminate by maltose (GC −, MC +), capsule (GC absent, MC present), and clinical context (STI vs meningitis/septicaemia).
  • Gonococcal arthritis vs Reactive arthritis (Reiter's): DGI arthritis is caused by direct joint invasion with positive joint culture; Reiter's is a sterile, reactive arthritis following enteric (Shigella, Salmonella, Campylobacter) or chlamydial infection with the classic triad of urethritis + conjunctivitis + arthritis. The 2019 MCQ Q9 vignette (diarrhoea → arthritis + urethral discharge + red eyes) = Reiter's, NOT gonococcal arthritis [10].
  • Gram stain in ♂ vs ♀: urethral swab Gram stain is sensitive in symptomatic males but NOT reliable in females — always use NAAT for females.
  • Ophthalmia neonatorum: N. gonorrhoeae causes hyperacute purulent conjunctivitis (within days of birth); C. trachomatis causes conjunctivitis slightly later (5–14 days). In adults, fulminant purulent conjunctivitis should prompt swab culture for Neisseria spp [5].

2024 Fourth Summative MCQ Q89 [9]

"A 23-year-old woman was admitted to gynaecology ward for lower abdominal pain and fever. She was sexually active and had 3 sexual partners in the past 6 months. Physical examination revealed lower abdominal tenderness. There was foul smelling vaginal discharge on speculum examination and cervical excitation was present on bimanual examination. Which of the following is the MOST APPROPRIATE management?"

Options: A. Single dose IM ceftriaxone + course of PO doxycycline and metronidazole; B. Single dose PO azithromycin; C. Single dose PO metronidazole; D. IV ceftriaxone + PO doxycycline.

Correct answer: A. This is classic PID (cervical excitation = chandelier sign). Outpatient PID regimen = IM ceftriaxone (covers GC) + PO doxycycline (covers Chlamydia) + PO metronidazole (covers anaerobes). Option D (IV route) is reserved for severe/admitted PID with systemic toxicity; the question asks "most appropriate" and the standard first-line outpatient regimen is A.

2019 Fourth Summative MCQ Q9 [10]

"A 36-year-old man presented with subacute onset pain and swelling of his right knee. Four weeks prior to this, he experienced bouts of diarrhoea which took over a week to settle. Further inquiry revealed painless urethral discharge and pain and redness of his eyes."

Correct answer: G (Reiter's syndrome / reactive arthritis). This is NOT gonococcal septic arthritis. The triad of arthritis + urethritis + conjunctivitis following enteric infection is classic reactive arthritis. Discriminator: gonococcal arthritis would present acutely with purulent joint or the DGI triad (tenosynovitis, dermatitis, polyarthralgia), and would not follow diarrhoea.

No other past paper questions directly examining N. gonorrhoeae were identified in the indexed past paper context.

[1] GC 105. Medically important microbes what every doctor should know.pdf [2] GC 101. Diagnosis of infections [Handouts].pdf [3] MBBS Final MB (Medicine) (Felix PY Lai).pdf [4] Maksim Medicine Notes.pdf [5] Ryan Ho Opthalmology.pdf [6] Block C - Vaginal discharge_ obstetric and gynaecological infections.pdf [7] Ryan Ho Rheumatology.pdf [8] Maksim Surgery Notes.pdf [9] Past papers: 2024 Fourth Summative MCQ.pdf [10] Past papers: 2019 Fourth Summative MCQ.pdf [11] Jerry's immunodeficiencies.pdf

Neisseria meningitidis

Encapsulated Gram-negative diplococcus that is a leading cause of bacterial meningitis and meningococcemia, transmitted via respiratory droplets.

Moraxella catarrhalis

Gram-negative, oxidase-positive diplococcus that is a common cause of otitis media in children and acute exacerbations of chronic obstructive pulmonary disease in adults.

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