Most Common Cause of Non-Purulent Cellulitis
The correct answer is B. S. Pyogenes (β-hemolytic Streptococcus). Non-purulent cellulitis is predominantly caused by β-hemolytic streptococci, particularly group A Streptococcus (S. pyogenes), not S. aureus. 1
Key Microbiological Evidence
Streptococci are the primary causative organisms in non-purulent cellulitis:
- β-hemolytic streptococci and methicillin-sensitive S. aureus are the most common pathogens in non-purulent, uncomplicated cellulitis, with streptococci being the predominant cause 2
- Streptococci cause diffuse, rapidly spreading infection, while staphylococcal cellulitis is typically more localized 1
- A serological study demonstrated that 69% of acute non-necrotising cellulitis cases showed evidence of streptococcal infection, and an additional 10 cases without serological evidence were successfully treated with penicillin alone 3
- In the 15% of cellulitis cases where organisms are identified, most are due to β-hemolytic Streptococcus and S. aureus 4
Critical Distinction: Purulent vs Non-Purulent
The presence or absence of purulence fundamentally changes the microbiology:
- Non-purulent cellulitis (without abscess or pus formation) is predominantly streptococcal 2, 3
- S. aureus, including MRSA, is more commonly associated with purulent infections such as abscesses and boils 1
- S. aureus rarely causes erysipelas (a form of superficial cellulitis) 1
Treatment Implications
First-line antibiotic selection should target streptococci:
- Treatment should begin promptly with agents effective against typical Gram-positive pathogens, especially streptococci 1
- Oral beta-lactams (penicillin, amoxicillin, cephalexin) are sufficient for non-purulent cellulitis in areas where CA-MRSA is not prevalent 2
- Even with rising rates of community-acquired MRSA, coverage for non-purulent cellulitis is generally not recommended 2
- Empiric MRSA coverage should only be considered for patients at specific risk (athletes, prisoners, IV drug users, prior MRSA exposure) or those who fail first-line therapy 1, 4
Common Pitfall to Avoid
Do not automatically cover for MRSA in non-purulent cellulitis. Despite the emergence and nationwide spread of CA-MRSA since 1999, population-based studies show that CA-MRSA is not a significant cause of non-purulent lower extremity cellulitis 5. The incidence of non-purulent cellulitis has actually decreased since the CA-MRSA era, suggesting these organisms play minimal role in this presentation 5.
Why Other Options Are Incorrect
- Option A (S. aureus): While S. aureus can cause cellulitis, it is typically more localized and is the predominant pathogen in purulent infections, not non-purulent cellulitis 1, 2
- Option C (P. aeruginosa): Not a typical cause of community-acquired non-purulent cellulitis
- Option D (P. multocida): Associated specifically with animal bites (cat or dog bites), not typical non-purulent cellulitis 6