STEPTRACKERMICROBIOLOGY + IMMUNOLOGY | GRAM-POSITIVE COCCI | HIGH-YIELD REVIEW

Streptococci I — S pyogenes & S agalactiae

β-hemolytic chains: group A causes pharyngitis, skin infections and post-infectious immune disease; group B kills newborns.

FOCUS ON
Group A (S pyogenes): bacitracin S, PYR ⁺, M protein. Pyogenic (pharyngitis, impetigo, cellulitis, erysipelas, necrotizing fasciitis), toxigenic (scarlet fever, strep TSS), immunologic (rheumatic fever after pharyngitis only; PSGN after pharyngitis or skin). Penicillin prevents rheumatic fever but not PSGN. Group B (S agalactiae): bacitracin R, CAMP ⁺, hippurate ⁺ → neonatal sepsis, pneumonia, meningitis; screen at 35–37 weeks, intrapartum penicillin.
DISEASE
MECHANISM
FEATURES
CLUE · TREATMENT
Streptococcal pharyngitis
GAS invasion of tonsils
Fever, tonsillar exudate, tender anterior nodes, no cough (Centor)
Rapid antigen/culture · penicillin/amoxicillin (prevents RF)
Impetigo (non-bullous)
Superficial skin infection
Honey-crusted lesions around mouth/nose, children
Also S aureus · mupirocin
Erysipelas · cellulitis
Dermis/lymphatics · deeper dermis + fat
Erysipelas: raised, sharply demarcated, bright red · cellulitis: poorly demarcated
Hyaluronidase, streptokinase spread infection
Necrotizing fasciitis
Exotoxin B (cysteine protease), spread along fascia
Pain out of proportion, rapid spread, crepitus (polymicrobial), shock
Surgical emergency · debridement + penicillin + clindamycin (blocks toxin)
Scarlet fever
Erythrogenic toxin (SpeA, phage-encoded)
Sandpaper rash starting on trunk/neck, circumoral pallor, strawberry tongue, Pastia lines, desquamation
After pharyngitis
Streptococcal TSS
Superantigens
Shock + multiorgan failure, often with necrotizing fasciitis
Higher mortality than staph TSS
Acute rheumatic fever
Type II hypersensitivity: anti-M protein antibodies cross-react with heart, joints, brain
J♥NES: Joints (migratory polyarthritis), ♥ carditis, Nodules (subcutaneous), Erythema marginatum, Sydenham chorea
Only after pharyngitis · ↑ ASO/anti-DNase B · secondary penicillin prophylaxis
Post-streptococcal GN
Type III hypersensitivity (immune complexes, subepithelial humps)
Cola urine, edema, HTN 1–3 weeks after pharyngitis or 3–6 weeks after skin infection
↓ C3 · not prevented by antibiotics
PANDAS
Proposed autoimmune link
Abrupt OCD/tics after strep infection in children
Controversial
Group B — neonatal disease
Capsule; colonizes maternal vagina/rectum
Early onset (< 7 days): sepsis, pneumonia · late onset: meningitis
Most common cause of neonatal sepsis and meningitis
Group B — adults
—
Postpartum endometritis, diabetics' skin/soft tissue, elderly bacteremia
Penicillin
Group B prevention
Rectovaginal swab at 35–37 weeks
Positive, prior GBS baby, GBS bacteriuria, unknown status with risk → intrapartum IV penicillin/ampicillin
Clindamycin/vancomycin if allergic

Mechanism — from sore throat to damaged valve

GAS pharyngitis
→
Anti-M protein antibodies
→
Cross-react with cardiac myosinMolecular mimicry
→
Pancarditis → mitral stenosisAschoff bodies, Anitschkow cells

Skin strains are nephritogenic, not rheumatogenic — impetigo can cause PSGN but not rheumatic fever.

Rheumatic fever vs PSGN

FEATURE
RHEUMATIC FEVER
PSGN
Preceding infection
Pharyngitis only
Pharyngitis or skin
Hypersensitivity
Type II
Type III
Prevented by antibiotics
Yes
No
Key lab
ASO, ↑ ESR
↓ C3, hematuria
Long-term
Mitral stenosis
Usually resolves (kids)

How it's tested

COMMON CONFUSIONScarlet fever (GAS toxin after pharyngitis; strawberry tongue) vs Kawasaki disease (vasculitis; also strawberry tongue, plus conjunctivitis, coronary aneurysms).
THE TRAPClindamycin is added in necrotizing fasciitis to shut down toxin synthesis — penicillin alone works poorly when bacterial load is high (Eagle effect).
YOUR APPROACHβ-hemolytic chains → bacitracin → group A: pyogenic, toxigenic or immunologic? group B: newborn or postpartum?

If you see… think…

Sandpaper rash + strawberry tongue after sore throat → scarlet fever
Migratory arthritis, carditis, chorea after sore throat → rheumatic fever
Cola-colored urine 3 weeks after impetigo → PSGN
Honey-crusted facial lesions → impetigo
Pain out of proportion, rapidly spreading, shock → necrotizing fasciitis
Bright red, raised, sharply bordered facial plaque → erysipelas
Newborn sepsis in the first week → group B strep
Pregnant woman's swab at 36 weeks positive → intrapartum penicillin
β-hemolytic, bacitracin resistant, CAMP ⁺ → S agalactiae
Antibody marking recent GAS infection → ASO / anti-DNase B