Streptococcus

Classification and Overview

1. Hemolysis pattern on blood agar:
   • α-hemolysis (partial/green): S. pneumoniae, viridans streptococci
   • β-hemolysis (complete/clear): S. pyogenes (Group A), S. agalactiae (Group B)
   • γ-hemolysis (none): enterococci
2. Lancefield grouping (cell-wall carbohydrate antigens): Groups A through H, K through V

Streptococcus pyogenes (Group A Streptococcus, GAS)

Structure and Virulence

Epidemiology

• Transient colonization of the upper respiratory tract and skin
• Spreads by respiratory droplets (pharyngitis) or direct skin contact (pyoderma)
• Highest-risk groups:
  • Children 5-15 years: pharyngitis
  • Children 2-5 years with poor hygiene: pyoderma
  • Patients with prior pharyngitis: rheumatic fever, glomerulonephritis

Diseases

• Pharyngitis - most common bacterial cause; exudative tonsillitis, fever, cervical lymphadenopathy
• Scarlet fever - pharyngitis + SPE-mediated diffuse erythematous rash, strawberry tongue
• Impetigo/Pyoderma - superficial skin infection; honey-crusted lesions
• Erysipelas - acute skin infection with raised, sharply demarcated borders; systemic signs; predominantly in young children and the elderly, now more common on the legs
• Cellulitis - deeper skin + subcutaneous tissue; borders less defined than erysipelas
• Necrotizing fasciitis ("flesh-eating bacteria") - deep infection spreading along fascial planes; rapid destruction of fat and muscle; bullae, gangrene, multiorgan failure; requires urgent surgical debridement

Acute erysipelas of the leg with bullae formation

Necrotizing fasciitis - note purple bullae over calf before surgical exploration

• SPE superantigens activate massive T-cell cytokine release
• Fever, hypotension, multiorgan failure
• Bacteremia often detectable

Diagnosis

• Rapid antigen detection test (RADT) - for pharyngitis; quick, specific
• Throat culture on blood agar - gold standard for pharyngitis; β-hemolytic colonies, susceptible to bacitracin, PYR-positive
• ASO titer - elevated after pharyngitis; confirms rheumatic fever or glomerulonephritis from throat infection
• Anti-DNase B titer - elevated after both pharyngitis and pyoderma; used when glomerulonephritis follows skin infection (ASO may be negative)
• Culture from impetigo - scrape under crusted lesion; do not swab open draining pustules (risk of staphylococcal contamination)

Treatment

• Drug of choice: Oral penicillin V or amoxicillin (S. pyogenes has never developed penicillin resistance)
• Penicillin allergy: oral cephalosporin or macrolide (note increasing erythromycin/azithromycin resistance)
• Severe systemic infections: IV penicillin + clindamycin (clindamycin inhibits toxin synthesis)
• Necrotizing fasciitis: urgent surgical debridement + IV penicillin + clindamycin
• No vaccine currently available

Streptococcus agalactiae (Group B Streptococcus, GBS)

• Treatment: Penicillin G (drug of choice); empirical broad-spectrum coverage (cephalosporin + aminoglycoside) until identified
• Prevention: Intrapartum penicillin given at least 4 hours before delivery in colonized mothers; no vaccine available

Streptococcus pneumoniae (Pneumococcus)

Structure and Virulence

Morphology

Epidemiology and Diseases

• Disease arises from endogenous spread from colonized nasopharynx/oropharynx (person-to-person spread via droplets is rare)
• Colonization highest in young children and their contacts
• Risk factors: viral respiratory infection, functional asplenia, sickle cell disease, hematologic malignancy
• Diseases: Pneumonia, otitis media, sinusitis, meningitis, bacteremia/sepsis
  • Children and elderly at greatest risk for meningitis
  • Asplenic patients at risk for fulminant pneumococcal sepsis

Diagnosis

• Gram stain (highly sensitive) + culture on sheep blood agar
• Antigen tests for C polysaccharide: sensitive in CSF (meningitis), but NOT reliable in urine
• Nucleic acid-based tests preferred for meningitis (especially post-antibiotic)

Treatment and Prevention

• Empirical therapy for meningitis: vancomycin + ceftriaxone
• Susceptible isolates: cephalosporin, fluoroquinolone, or vancomycin as monotherapy
• Vaccines:
  • PCV13 (13-valent conjugated): Recommended for all children < 2 years and certain adults
  • PPSV23 (23-valent polysaccharide): Recommended for adults at risk (elderly, asplenic, immunocompromised)

Viridans Streptococci

• α-hemolytic or non-hemolytic; optochin-resistant (distinguishes from S. pneumoniae)
• Normal flora of the oropharynx, GI tract, and genitourinary tract
• Five groups: Mutans, Salivarius, Anginosus, Mitis, Sanguinis
• Key diseases:
  • Infective endocarditis - especially after dental procedures in patients with valvular abnormalities
  • Dental caries - S. mutans
  • Intraabdominal/brain abscesses - Anginosus group

Comparative Summary Table

Streptococcus - High-Yield Summary

Classification

S. pyogenes (Group A) - Most Tested

• M protein - antiphagocytic; class I M protein → Rheumatic fever
• Streptolysin O (SLO) - oxygen-labile; ASO titer measures antibodies
• Streptolysin S (SLS) - β-hemolysis on blood agar plates
• SPE (A,B,C) - superantigens → scarlet fever, STSS
• Hyaluronic acid capsule - antiphagocytic

• Pharyngitis → Rheumatic fever (throat only, NOT skin)
• Pyoderma/Impetigo → Glomerulonephritis (throat OR skin)
• Scarlet fever, Erysipelas, Cellulitis
• Necrotizing fasciitis (surgical emergency)

• Rapid antigen test (pharyngitis)
• ASO titer → RF or GN after pharyngitis
• Anti-DNase B → GN after skin infection

S. pneumoniae - Most Common Cause Of

• Community-acquired pneumonia
• Bacterial meningitis (adults)
• Otitis media (children)

S. agalactiae (Group B)

• Neonatal sepsis/meningitis/pneumonia
• UTI in pregnant women
• CAMP test positive
• Prevention: intrapartum penicillin (≥4 hrs before delivery)
• Treatment: Penicillin G

Key Distinctions (Exam Traps)