Step 1 topics → Microbiology → Streptococcus agalactiae (Group B Strep)

Streptococcus agalactiae (Group B Strep)

Streptococcus agalactiae (Group B Strep, GBS) is a gram-positive, beta-hemolytic streptococcus and the leading cause of neonatal sepsis and meningitis in the first few months of life — and USMLE Step 1 tests it with a reliable trap: a GBS-positive mother scheduled for a planned cesarean section with intact membranes and no labor does NOT require intrapartum prophylaxis. The rationale is that vertical transmission risk occurs during labor and passage through the birth canal; without those conditions, there's no meaningful exposure to prevent. The exam tests GBS in two contexts: lab identification (the CAMP test) and clinical scenarios involving neonates and pregnant women.

The tricky part is that GBS shares features with other beta-hemolytic strep — it's also gram-positive, also catalase-negative, also on blood agar. What separates it is the CAMP test and its specific hemolysis pattern, plus its Lancefield group B antigen. Students who haven't locked in the CAMP test mechanics will misidentify GBS or confuse it with Group A Strep in a lab-based question. USMLE Step 1 will often give you a blood agar result or a one-line lab finding and expect you to work backwards to the organism.

The clinical side adds another layer of complexity: neonatal GBS disease is split into early-onset and late-onset presentations with completely different transmission sources, and prophylaxis decisions are not one-size-fits-all. Students frequently conflate these two presentations or over-apply the prophylaxis rule to C-section deliveries. Knowing the specific cutoffs and the logic behind them — not just the isolated facts — is what the exam rewards.

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Common misconceptions

Common mistake
Gap: Missing the CAMP test as the key lab identifier for Group B Strep
GBS is identified by a positive CAMP test, in which its CAMP factor enhances the beta-hemolysis of S. aureus on blood agar, producing a characteristic arrowhead pattern.
Many students know GBS is beta-hemolytic but don't know what makes it uniquely identifiable in the lab. The CAMP test is the key: GBS produces CAMP factor (a pore-forming protein) that acts synergistically with the sphingomyelinase (beta-toxin) of S. aureus, dramatically enhancing the zone of red blood cell lysis in a characteristic arrowhead or leaf shape. On Step 1, if a question gives you a blood agar plate with an arrowhead hemolysis pattern near a streak of S. aureus, that's your GBS tell — no other common streptococcus does this.
Common mistake
Wrong: Early-onset and late-onset GBS neonatal disease have the same source of infection.
Right: Early-onset GBS (within 7 days) is acquired vertically from maternal colonization during delivery, while late-onset GBS (7 days–3 months) is acquired from environmental or nosocomial sources after birth.
It's tempting to assume all neonatal GBS disease comes from the mother at delivery, but that's only true for early-onset disease (within the first 7 days). Early-onset GBS is classic vertical transmission — the neonate is colonized or infected as it passes through a GBS-colonized birth canal, presenting with sepsis, pneumonia, or meningitis in the first week. Late-onset disease (7 days to 3 months) has a different source entirely — environmental or nosocomial transmission after birth — and presents more often with meningitis. This distinction matters clinically and for exam questions that ask about source or prevention.
Common mistake
Wrong: All GBS-colonized mothers receive intrapartum antibiotic prophylaxis regardless of delivery route.
Right: Intrapartum penicillin prophylaxis is indicated for GBS-colonized mothers delivering vaginally; cesarean delivery without labor or membrane rupture does not require prophylaxis even with GBS colonization.
The rule isn't 'GBS-positive mother = prophylaxis.' It's 'GBS-positive mother delivering vaginally = prophylaxis.' The rationale is that the risk of vertical transmission occurs during labor and passage through the birth canal. A GBS-colonized mother undergoing a planned cesarean section without labor onset and without membrane rupture has no significant exposure risk to the neonate, so prophylaxis is not indicated. If a question tells you GBS colonization is present but asks about a scheduled C-section with intact membranes and no labor, the answer is no prophylaxis — resist the urge to over-apply the rule.
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What the exam tests

  1. Know the key lab features that distinguish GBS from other streptococci: beta-hemolysis on blood agar, positive CAMP test (arrowhead pattern with S. aureus), catalase-negative, bacitracin-resistant, and Lancefield group B antigen.
  2. Understand the CAMP test mechanism: GBS secretes CAMP factor, which synergizes with S. aureus beta-toxin to produce enhanced lysis and a characteristic arrowhead-shaped zone of hemolysis — this is the defining identification tool for GBS.
  3. Recognize the clinical presentations of neonatal GBS disease — early-onset (within 7 days: sepsis, pneumonia, meningitis; vertically acquired during delivery) versus late-onset (7 days to 3 months: meningitis predominates; acquired from environmental or nosocomial sources).
  4. Apply the correct intrapartum prophylaxis rules: GBS-colonized mothers delivering vaginally receive IV penicillin; cesarean delivery without active labor or membrane rupture does NOT require prophylaxis even if the mother is GBS-positive.

Can you avoid these mistakes?

A blood agar plate inoculated with a vaginal swab shows a zone of enhanced beta-hemolysis in a distinctive arrowhead pattern adjacent to a streak of S. aureus. The organism is catalase-negative and bacitracin-resistant. What organism is this, and what test confirmed its identity?
A 10-day-old previously healthy neonate presents with fever, irritability, and bulging fontanelle. CSF shows pleocytosis and gram-positive cocci in pairs and short chains. The mother's prenatal GBS screen was positive and she received intrapartum penicillin. What type of GBS disease is this, and where did the organism most likely come from?
A 36-week pregnant woman is found to be GBS-positive on routine rectovaginal culture at 35 weeks. She is scheduled for an elective cesarean section the following week with no current labor and intact membranes. Does she require intrapartum antibiotic prophylaxis? Explain your reasoning.
How do you distinguish GBS (S. agalactiae) from GAS (S. pyogenes) on lab testing? Name at least two distinguishing features beyond Lancefield grouping.

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