It’s frustrating when you know it’s “some kind of strep,” but the question writer baits you with every other gram-positive cocci buzzword in the book. Group B Strep (GBS) is a classic USMLE organism because it’s high-yield, clinically common, and easy to confuse with other “strep/staph/enterococcus” choices—unless you anchor to the vignette and confirm with a couple of decisive tests.
Tag: Microbiology > Gram-Positive Bacteria
The Q-Bank Vignette (Classic Setup)
A 2-day-old newborn develops respiratory distress, temperature instability, and poor feeding. Mom had no prenatal care. Blood culture grows gram-positive cocci in chains. The organism is β-hemolytic on blood agar and CAMP test positive.
What is the most likely pathogen?
How to Lock in the Correct Answer: Streptococcus agalactiae (Group B Strep)
Why GBS fits best
GBS is the classic cause of early-onset neonatal sepsis and pneumonia (typically within the first 24–48 hours, up to day 6). Key clues:
- Neonate + sepsis/pneumonia/meningitis
- Maternal colonization of the vaginal/rectal tract
- Gram-positive cocci in chains (Streptococcus)
- β-hemolysis
- CAMP positive
- Hippurate hydrolysis positive
- Group B Lancefield antigen
High-yield disease associations
- Early-onset disease (0–6 days): sepsis, pneumonia, respiratory distress
- Late-onset disease (7–89 days): meningitis is more common
- Pregnancy: can cause postpartum endometritis, chorioamnionitis
- Adults (esp. diabetics, elderly): skin/soft tissue infections, bacteremia
Step-Style Micro ID: GBS vs the Usual Suspects
| Organism | Gram stain / arrangement | Hemolysis | Key tests | Classic clinical clue |
|---|---|---|---|---|
| Strep agalactiae (GBS) | G+ cocci in chains | β | CAMP+, hippurate+ | Neonatal sepsis/pneumonia, maternal colonization |
| Strep pyogenes (GAS) | G+ cocci in chains | β | Bacitracin sensitive, PYR+ | Strep throat, impetigo, nec fasc, rheumatic fever |
| Strep pneumoniae | G+ lancet diplococci | α | Optochin sensitive, bile soluble | Lobar pneumonia, otitis media, meningitis |
| Viridans strep | G+ cocci in chains | α | Optochin resistant | Dental caries, endocarditis on damaged valves |
| Enterococcus (E. faecalis/faecium) | G+ cocci in chains | usually γ (can be α/β) | Bile esculin+, growth in 6.5% NaCl | UTIs, biliary infections, endocarditis; nosocomial |
The “Why Every Answer Choice Matters” Distractor Breakdown
Below is how question writers typically try to pull you off GBS—and how to shut each option down quickly.
Distractor 1: Streptococcus pyogenes (Group A Strep)
Why you might be tempted:
- Also gram-positive cocci in chains
- Also β-hemolytic
- “Strep” + “serious infection” feels plausible
Why it’s wrong here:
- GAS is not the classic cause of early-onset neonatal sepsis/pneumonia
- GAS is more tied to:
- Pharyngitis
- Impetigo/cellulitis
- Necrotizing fasciitis
- Toxic shock-like syndrome
- Rheumatic fever and post-strep GN
High-yield differentiator tests:
- GAS: Bacitracin sensitive, PYR positive
- GBS: CAMP positive, hippurate positive
Exam move: In a neonate with early sepsis + β-hemolytic strep → think GBS first unless the stem screams GAS soft-tissue catastrophe.
Distractor 2: Streptococcus pneumoniae
Why you might be tempted:
- Causes pneumonia and meningitis (and those are in the stem a lot)
Why it’s wrong here:
- Strep pneumo is α-hemolytic, not β
- Morphology: lancet-shaped diplococci, not chains
- Classic setting: older adults, asplenia, alcoholism, post-viral pneumonia; also otitis/sinusitis in kids
High-yield differentiator tests:
- Optochin sensitive
- Bile soluble
- Encapsulated (virulence; predisposes asplenic patients)
Exam move: If it’s neonatal and β-hemolytic → don’t drift to pneumococcus.
Distractor 3: Viridans group streptococci
Why you might be tempted:
- Another “strep” that lives in humans
- Can cause bacteremia/endocarditis
Why it’s wrong here:
- Viridans are α-hemolytic (or non-hemolytic), not β
- Classic diseases are about oral flora:
- Dental caries (S. mutans)
- Subacute endocarditis after dental work on damaged valves
- Brain abscess in some contexts
High-yield differentiator test:
- Optochin resistant (vs Strep pneumo sensitive)
Exam move: Viridans = mouth + valves + α-hemolysis. Not the neonatal early sepsis headline.
Distractor 4: Staphylococcus aureus
Why you might be tempted:
- Common cause of sepsis
- Gram-positive cocci (and question writers love S. aureus)
Why it’s wrong here:
- Arrangement is wrong: Staph forms clusters, not chains
- You’re told β-hemolytic strep with CAMP+—that’s not staph logic
High-yield ID pearls:
- Catalase positive (all staph)
- Coagulase positive (S. aureus)
- Often MRSA in hospital/community settings
- Causes: endocarditis (IVDU), osteomyelitis, skin abscesses, toxin-mediated disease (TSS, scalded skin, rapid-onset food poisoning)
Exam move: Chains → strep/enterococcus. Clusters → staph.
Distractor 5: Staphylococcus epidermidis
Why you might be tempted:
- Neonates and bacteremia can involve lines/devices
- “Coagulase-negative staph” appears in hospital vignettes
Why it’s wrong here:
- Not a classic cause of early-onset neonatal pneumonia/sepsis from maternal transmission
- Associated with:
- Prosthetic valves
- Prosthetic joints
- Indwelling catheters
- Also: clusters, not chains
High-yield ID pearl:
- Novobiocin sensitive
- Biofilm formation
Exam move: If the clue is device/prosthesis, think S. epidermidis. If it’s day-1 newborn + mom colonization, think GBS.
Distractor 6: Enterococcus faecalis / faecium
Why you might be tempted:
- Can appear as gram-positive cocci in chains
- Important nosocomial pathogens; can cause bacteremia
Why it’s wrong here:
- Not the classic “early-onset neonatal sepsis/pneumonia from vaginal colonization” answer (GBS is)
- Enterococcus is more associated with:
- UTIs
- Biliary tract infection
- Endocarditis
- Nosocomial infections, often resistant (VRE)
High-yield differentiator tests:
- Bile esculin positive
- Growth in 6.5% NaCl
- Often γ-hemolytic (but can vary)
Exam move: If the stem highlights UTI, instrumentation, or hospital resistance, Enterococcus climbs. For newborn early sepsis, GBS wins.
Must-Know GBS Facts (USMLE Gold)
Virulence & pathogenesis
- Polysaccharide capsule helps evade phagocytosis
- Colonizes GI and GU tract of adults → maternal carriage → newborn exposure during delivery
Screening & prevention (OB + Peds crossover)
- Universal screening at 36–37 weeks
- Intrapartum antibiotic prophylaxis for:
- Positive screen
- GBS bacteriuria during pregnancy
- Prior infant with invasive GBS disease
- Unknown status + risk factors (e.g., fever, prolonged rupture of membranes)
Drug of choice: Penicillin G (or ampicillin)
If penicillin allergy: depends on anaphylaxis risk and susceptibility (commonly cefazolin if low risk; alternatives like clindamycin require susceptibility testing).
Lab ID shortcuts
- β-hemolytic
- CAMP positive (enhanced hemolysis near S. aureus streak)
- Hippurate hydrolysis positive
- Bacitracin resistant (helps separate from GAS)
Rapid “One-Liner” for Test Day
GBS = β-hemolytic strep in chains, CAMP+, hippurate+, colonizes vagina → early neonatal sepsis/pneumonia (and late neonatal meningitis).