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Gram-negativeCocci

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.

Organism Card

DomainMust know
Identity
  • G−ve diplococci; kidney-bean shaped [1]
  • Aerobic; classified with Neisseria among G−ve cocci [2][3]
  • Maltose non-fermenter (like N. gonorrhoeae; vs N. meningitidis which ferments maltose) [3]
  • Oral commensal of upper respiratory tract [1]
Lab discriminator
  • Oxidase +ve, catalase +ve
  • DNase +ve (distinguishes from Neisseria spp.)
  • Hockey-puck test: colonies can be pushed across agar plate intact
  • Maltose −ve, glucose −ve (does not ferment sugars like Neisseria)
  • Tributyrin hydrolysis +ve (vs Neisseria spp.)
Reservoir / transmission
  • Human nasopharyngeal commensal [1]
  • Colonisation rates highest in children (up to 75%)
  • Transmitted by respiratory droplets / direct contact
  • No specific Hong Kong–endemic exposure; ubiquitous worldwide
Key virulence
  • β-lactamase production → nearly universal (> 90%); penicillin/amoxicillin resistance [4]
  • Outer membrane proteins (OMPs) → adhesion to respiratory epithelium
  • LOS (lipo-oligosaccharide) → endotoxin-mediated inflammation
  • Complement resistance via surface proteins (UspA1/A2)
Clinical syndromes
  • Acute bacterial sinusitis (one of top 3 causes with S. pneumoniae, H. influenzae) [1]
  • Acute otitis media (AOM) — common paediatric pathogen [1][4]
  • Acute exacerbation of COPD (AECOPD) — important in chronic smokers [1]
  • Less common: pneumonia (elderly/immunocompromised), rarely bacteraemia
  • Triad of examinable syndromes: sinusitis + otitis media + COPD exacerbation
Diagnosis
  • Sputum Gram stain: G−ve diplococci (intracellular in PMNs)
  • Culture on blood or chocolate agar; grows at 35–37°C
  • Middle ear aspirate or sinus aspirate for definitive AOM/sinusitis diagnosis
  • Pitfall: normal flora in sputum — significance requires clinical correlation
Treatment
  • Amoxicillin-clavulanate: 1st-line for AOM / sinusitis (β-lactamase +ve strains) [4]
  • Alternatives: cefuroxime, cefpodoxime, ceftriaxone [4]
  • Macrolides (azithromycin/clarithromycin) for β-lactam allergy [4]
  • Fluoroquinolones active against M. catarrhalis [3]
  • Amoxicillin alone NOT reliable (β-lactamase producer) [4]
  • Carbapenems active but reserved for serious/resistant infections [3]
Prevention
  • No vaccine available
  • Reduce unnecessary antibiotic prescribing for URTIs (GC 106 teaching point) [5]
  • Standard droplet precautions; no special isolation or notification required
Classic traps
  • M. catarrhalis is maltose −ve like N. gonorrhoeae; N. meningitidis is maltose +ve [3]
  • Unlike Neisseria: DNase +ve, tributyrin +ve; does NOT cause STI or meningitis
  • Vignette discriminator: child with AOM or elderly COPD patient with purulent exacerbation → think M. catarrhalis when β-lactamase resistance is highlighted
  • Do NOT confuse with H. influenzae (G−ve coccobacillus, needs X+V factors)

Exam Intelligence

  1. GC 105 explicitly lists M. catarrhalis as a G−ve coccus that is an oral commensal, causes acute bacterial sinusitis, otitis media, and may be implicated in COPD exacerbations [1]. This is the highest-yield framing — memorise these three syndromes.

  2. GC 105 groups M. catarrhalis alongside N. gonorrhoeae and N. meningitidis as the three examinable G−ve cocci [1]. The key discriminator taught is maltose fermentation: N. meningitidis ferments maltose; N. gonorrhoeae and M. catarrhalis do not [3].

  3. Felix PY Lai senior notes confirm M. catarrhalis is classified under G−ve cocci, aerobic, maltose non-fermenter alongside N. gonorrhoeae in the bacterial taxonomy tree [3].

  4. Fluoroquinolones are active against M. catarrhalis — explicitly listed in the Felix PY Lai antibiotic table alongside Neisseria and H. influenzae [3]. This is a favourite MCQ distractor topic (antibiotic spectrum questions).

  5. AOM treatment must cover S. pneumoniae, H. influenzae, AND M. catarrhalis — amoxicillin-clavulanate for β-lactamase +ve strains; cephalosporins or macrolides as alternatives [4]. This is a paediatrics crossover point frequently examined.

  6. GC 106 emphasises antibiotic stewardship for URTIs [5] — M. catarrhalis-related sinusitis/otitis media is a common scenario where students must distinguish bacterial infection warranting antibiotics from viral URTI where antibiotics are inappropriate.

  • M. catarrhalis vs N. gonorrhoeae: Both are G−ve diplococci and maltose non-fermenters, but M. catarrhalis is a respiratory commensal causing sinusitis/otitis/COPD exacerbation, while N. gonorrhoeae is an STI pathogen. DNase and tributyrin tests differentiate them in the lab.
  • M. catarrhalis vs N. meningitidis: N. meningitidis ferments maltose and causes meningitis/meningococcaemia; M. catarrhalis does neither.
  • M. catarrhalis vs H. influenzae: Both cause AOM, sinusitis, and COPD exacerbations. H. influenzae is a G−ve coccobacillus requiring X and V factors on chocolate agar; M. catarrhalis is a true diplococcus growing on blood agar without special factors.
  • β-lactamase trap: Nearly all M. catarrhalis strains produce β-lactamase → amoxicillin alone is a classic wrong answer for M. catarrhalis treatment. Must add clavulanate or use a cephalosporin.
  • Exam vignette pattern: An elderly COPD patient with purulent sputum exacerbation or a child with recurrent AOM failing amoxicillin → suspect β-lactamase-producing M. catarrhalis; switch to amoxicillin-clavulanate.

No past paper questions directly testing Moraxella catarrhalis as the primary organism were identified in the indexed Fourth Summative MCQ, SAQ, or Minicase papers (2016–2025). M. catarrhalis is more likely to appear as a correct answer choice in questions about AOM aetiology, COPD exacerbation organisms, or antibiotic spectrum coverage. Students should be prepared for it to feature as one option in an EMQ/MCQ on respiratory pathogens or β-lactamase-producing organisms.

[1] Lecture slides: GC 105. Medically important microbes what every doctor should know.pdf — slide on Gram negative cocci
[2] Senior notes: Gen Clerk Anaes + Microbiology Summary.pdf — Gram −ve cocci table
[3] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf — G−ve bacteria classification (p16) and antibiotic tables (p22)
[4] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf — AOM aetiology (p116) and treatment (p120)
[5] Lecture slides: GC 106. Practical issues in antibotic use [Notes].pdf — antibiotic stewardship for URTIs

Neisseria gonorrhoeae

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

Escherichia coli

Gram-negative, facultatively anaerobic, rod-shaped bacterium of the family Enterobacteriaceae that is a normal gut commensal but includes pathogenic strains causing urinary tract infections, gastroenteritis, neonatal meningitis, and septicemia.

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