Primary Causative Organism of Erysipelas
The correct answer is A) Group A streptococcus (Streptococcus pyogenes), which is the primary causative organism of erysipelas.
Microbiology and Pathogenesis
Erysipelas is predominantly caused by Group A β-hemolytic streptococci (Streptococcus pyogenes) 1, 2. This is consistently documented across multiple high-quality guidelines from both the Infectious Diseases Society of America (2005) and the World Society of Emergency Surgery (2018).
Key Microbiological Facts:
- Most facial infections: Attributed to Group A Streptococcus (GAS) 1
- Lower extremity infections: Increasingly caused by non-GAS streptococci (Groups B, C, or G) 1, 2
- S. aureus role: Rarely causes erysipelas 1, 3; when present, typically associated with underlying abscesses or penetrating trauma
- MRSA involvement: Remains controversial and uncommon 1
Why Other Options Are Incorrect
Group C or G streptococcus (Option B): While these can cause lower extremity erysipelas, they are secondary causes, not primary 2. The evidence shows an "increasing percentage" of non-GAS infections, but GAS remains the predominant pathogen.
MSSA and MRSA (Options C and D): S. aureus "rarely causes erysipelas" according to multiple guidelines 1. The 2018 WSES consensus explicitly states this, and the role of MRSA "remains controversial" 1.
Erysipelothrix (Option E): This organism (E. rhusiopathiae) causes erysipeloid, a distinct occupational infection in meat handlers and fish workers—not erysipelas 2, 4. This is a common pitfall in nomenclature.
Clinical Implications
The streptococcal etiology explains why:
- Penicillin remains first-line therapy (A-I recommendation) 2
- 100% of strains remain susceptible to penicillin 2
- Anti-staphylococcal coverage is unnecessary for typical erysipelas 2
Common Diagnostic Pitfall
Do not confuse erysipelas with cellulitis. While both are streptococcal infections, erysipelas has well-demarcated borders, involves the upper dermis and superficial lymphatics, and is more specifically associated with S. pyogenes 2, 5. Cellulitis has poorly defined borders and may have mixed etiology including S. aureus when associated with abscesses or trauma.