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Streptococcus agalactiae

Encapsulated beta-hemolytic Group B Streptococcus (Gram-positive coccus in chains) that is a leading cause of neonatal meningitis, sepsis, and pneumonia, as well as invasive infections in pregnant women and immunocompromised adults.

Organism Card

DomainMust know
Identity
  • Group B Streptococcus (GBS) = Streptococcus agalactiae [1][2][3]
  • Gram-positive cocci in chains
  • Lancefield group B antigen
  • Normal flora of lower female genital tract and GI tract [2]
Lab discriminator
  • β-haemolytic on blood agar
  • CAMP test positive (arrow-head haemolysis with S. aureus)
  • Hippurate hydrolysis positive (vs other β-haemolytic strep)
  • Bacitracin resistant (vs GAS which is bacitracin sensitive)
  • Catalase negative (vs staphylococci)
Reservoir / transmission
  • Normal flora of lower female genital tract → vertical transmission during delivery [2]
  • Colonises ~10–30% of pregnant women (vaginal/rectal)
  • Screen by low vaginal swab at 35–37 weeks gestation [2][7]
  • Also colonises GI tract → source for UTI in adults
Key virulence
  • Polysaccharide capsule (types Ia, Ib, II, III, V) → anti-phagocytic; type III strongly a/w neonatal meningitis
  • β-haemolysin/cytolysin → lung epithelial injury
  • C5a peptidase → impairs neutrophil recruitment
  • Sialic acid capsule → molecular mimicry, evades complement
Clinical syndromes
  • Neonatal sepsis (early-onset < 7 days; late-onset 7–90 days) [2][3][4]
  • Neonatal meningitis (top cause in neonates < 1 month, also 1–3 months) [4]
  • UTI (esp. in pregnant women, elderly, diabetics) [2][6]
  • Subacute bacterial endocarditis (penicillin-sensitive; friable vegetations) [2][5]
  • Chorioamnionitis, puerperal sepsis
  • Pneumonia in neonates and elderly
  • Meningitis in adults > 50 years and immunocompromised [4]
Diagnosis
  • Gram stain of CSF: Gram-positive cocci in chains in a neonate → think GBS [3][7]
  • Blood culture / CSF culture: definitive
  • Prenatal screening: low vaginal + rectal swab at 35–37 weeks for culture [2][7]
  • Rapid antigen detection (latex agglutination) on CSF — adjunct
  • Pitfall: culture may be negative if intrapartum antibiotics already given
Treatment
  • Penicillin G (benzylpenicillin) is first-line for all GBS infections [2][5][7]
  • Intrapartum antibiotic prophylaxis (IAP): IV benzylpenicillin given to GBS-colonised mothers in labour [7]
  • Ampicillin is alternative for IAP
  • If penicillin allergy: clindamycin or vancomycin (check susceptibility)
  • Meningitis Tx duration: 14–21 days [4]
  • IE: penicillin G ± gentamicin for synergy [5]
  • GBS remains universally penicillin-susceptible (no penicillin resistance reported)
Prevention
  • Universal antenatal GBS screening at 35–37 weeks gestation [2][7]
  • IAP with IV benzylpenicillin if screen positive, to prevent early-onset neonatal disease [7]
  • IAP also indicated if: previous infant with GBS disease, GBS bacteriuria in current pregnancy, unknown GBS status with risk factors (preterm < 37 wk, ROM ≥ 18 h, intrapartum fever ≥ 38°C)
  • No licensed GBS vaccine currently (research ongoing)
  • Not a notifiable disease in HK
Classic traps
  • GBS ≠ GAS: GBS causes neonatal disease; GAS causes rheumatic fever / scarlet fever / pharyngitis [2]
  • Day-2 neonate with fever + Gram-positive cocci in chains on CSF Gram stain → GBS, not pneumococcus or Enterococcus [3][7]
  • GBS is bacitracin-resistant and CAMP-positive; GAS is bacitracin-sensitive and CAMP-negative
  • GBS IE is treated with penicillin (like viridans strep); Enterococcal IE needs aminoglycoside addition due to intrinsic cephalosporin resistance [5]
  • UTI isolate of GBS in a woman of childbearing age → flag for antenatal implications

Exam Intelligence

  1. GBS is normal flora of the lower female genital tract → must be screened at 35–37 weeks gestation — this framing is directly from GC and senior teaching [2] and is the single most tested antenatal screening fact for GBS.

  2. Neonatal sepsis in the first week of life: GBS and E. coli are the two key organisms — AOS microbiology material explicitly lists these as the answer framework for "fever in a neonate < 7 days" [3].

  3. GBS causes subacute bacterial endocarditis and is penicillin-sensitive — senior cardiology notes group GBS with GAS and viridans strep as penicillin-responsive IE organisms (contrast with Enterococcus which requires aminoglycoside synergy) [5].

  4. Age-based meningitis aetiology: GBS is a top cause in neonates (< 1 month) and infants (1–3 months), re-emerges in adults > 50 years [4]. Know the table — examiners love age-stratified meningitis differentials.

  5. Meningitis treatment duration for GBS: 14–21 days (longer than meningococcus at ≥ 7 days, similar to Listeria) [4].

  6. Intrapartum antibiotic prophylaxis: IV benzylpenicillin is the drug of choice when the mother screens positive — tested in the 2025 MCQ paper directly [7].

  • GBS vs GAS confusion: GAS (S. pyogenes) → pharyngitis, scarlet fever, rheumatic fever, PSGN. GBS (S. agalactiae) → neonatal sepsis/meningitis, UTI, endocarditis. Examiners test this distinction frequently.
  • GBS vs S. pneumoniae in neonatal meningitis: Both are Gram-positive. However, pneumococcus is a lancet-shaped diplococcus (not chains), and pneumococcal meningitis is far more common in children > 3 months and adults. In a day-2 neonate, GBS is the answer.
  • GBS vs Enterococcus in CSF: Enterococcus faecalis is a distractor in the 2023 MCQ. Enterococci are rare causes of neonatal meningitis; GBS is overwhelmingly more common.
  • CAMP test: Classic lab discriminator — GBS produces CAMP factor that synergises with S. aureus β-toxin to produce arrowhead-shaped enhanced haemolysis. This is the definitive bench test separating GBS from other β-haemolytic strep.
  • GBS bacteriuria in pregnancy: Even asymptomatic GBS bacteriuria in pregnancy is an indication for IAP — it indicates heavy colonisation and high risk of vertical transmission.

2023 Fourth Summative MCQ Q83 [7]:

"A neonate (day 2 of life) develops high fever, lethargy and feeding difficulty. Sepsis workup is performed including lumbar puncture. Please refer to Gram stain image of cerebrospinal fluid below. Which of the following microorganisms is MOST LIKELY to be grown in culture from the cerebrospinal fluid? A. Enterococcus faecalis B. Group B streptococcus C. Staphylococcus aureus D. Streptococcus pneumoniae"

Correct answer: B. Group B streptococcus. Rationale: Day-2 neonate with sepsis + CSF Gram stain showing Gram-positive cocci in chains → early-onset GBS disease is the top differential. Enterococcus and S. aureus are uncommon causes of community-acquired neonatal meningitis. S. pneumoniae is predominantly a pathogen of children > 3 months. The same question stem also appears in the AOS Microbio material [3].

2025 Fourth Summative MCQ Q75 [8]:

"A 30-year-old pregnant woman was admitted to the labour ward at 38 weeks of gestation for rupture of membranes. This was her first pregnancy. She had her low vaginal swab screened positive for beta-haemolytic Gram-positive cocci at 35 weeks of gestation. Her pregnancy course was otherwise uneventful. She had no known drug allergy… Which of the following is the MOST APPROPRIATE action? A. Discharge her from hospital and advise her to return when she has abdominal pain B. Give intravenous benzylpenicillin after checking drug allergy record C. Give oral penicillin after checking drug allergy record D. Perform emergency Caesarean section immediately"

Correct answer: B. Give intravenous benzylpenicillin after checking drug allergy record. Rationale: GBS-positive screen at 35 weeks + rupture of membranes = indication for intrapartum antibiotic prophylaxis. IAP must be IV (not oral) benzylpenicillin. There is no indication for emergency LSCS. Discharge is inappropriate given GBS-positive status with ROM.

[1] Lecture slides: GC 105. Medically important microbes what every doctor should know.pdf [2] Senior notes: Block A - Fever and a murmur_ Valvular heart diseases; Infective endocarditis.pdf [3] AOS material: AOS - Microbio.pdf [4] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf [5] Senior notes: Ryan Ho Cardiology.pdf [6] Senior notes: Ryan Ho Urogenital.pdf [7] Past papers: 2023 Fourth Summative MCQ.pdf [8] Past papers: 2025 Fourth Summative MCQ.pdf

Streptococcus pneumoniae

Encapsulated, alpha-hemolytic, Gram-positive lancet-shaped diplococcus and the leading cause of community-acquired pneumonia, bacterial meningitis, and otitis media.

Enterococcus faecalis

Gram-positive, catalase-negative coccus found in chains, a leading cause of nosocomial urinary tract infections, endocarditis, and biliary tract infections, notable for intrinsic resistance to cephalosporins and low-level aminoglycosides.

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