Most Common Cause of Non-Purulent Cellulitis
The correct answer is B. S. Pyogenes (Streptococcus pyogenes), as β-hemolytic streptococci—particularly group A Streptococcus (S. pyogenes)—are the primary pathogens in typical non-purulent cellulitis. 1, 2, 3, 4
Pathogen Hierarchy in Non-Purulent Cellulitis
β-hemolytic streptococci, especially S. pyogenes, are the predominant cause:
- In the 15% of cellulitis cases where organisms are actually identified, most are due to β-hemolytic Streptococcus species, followed by methicillin-sensitive S. aureus 3
- Streptococci cause diffuse, rapidly spreading infection characteristic of non-purulent cellulitis 1
- The majority of non-purulent, uncomplicated cellulitis cases are caused by β-hemolytic streptococci or methicillin-sensitive S. aureus, with streptococci being the primary pathogen 4
S. aureus (MSSA) plays a secondary but important role:
- Staphylococcal cellulitis is typically more localized rather than the diffuse spreading pattern seen with streptococci 1
- When S. aureus causes cellulitis, it is usually methicillin-sensitive S. aureus (MSSA), not MRSA 3, 4
Critical Evidence Distinguishing the Pathogens
MRSA is NOT a common cause of typical non-purulent cellulitis:
- Despite the emergence and nationwide spread of community-acquired MRSA since 1999, the incidence of non-purulent lower extremity cellulitis actually decreased, suggesting CA-MRSA is not a significant cause 5
- MRSA coverage is unnecessary for typical non-purulent cellulitis, as beta-lactam monotherapy is successful in 96% of cases 2
- Even with rising rates of community-acquired MRSA, coverage for non-purulent cellulitis is generally not recommended 4
Molecular studies confirm streptococci as the presumed pathogen:
- While PCR and pyrosequencing studies have difficulty definitively identifying the causative organism in cellulitis, S. pyogenes and other streptococci remain the presumed pathogens based on clinical response to streptococcal-targeted therapy 6
- Methicillin-resistant S. aureus was not found by culture or PCR in cellulitis specimens, and S. pyogenes was not identified by molecular techniques, highlighting the difficulty in microbiological diagnosis but not negating clinical evidence 6
Why Other Options Are Incorrect
P. aeruginosa (Option C):
- Not a typical cause of non-purulent cellulitis in immunocompetent hosts
- Associated with specific exposures like hot tub folliculitis or penetrating trauma in aquatic environments
P. multocida (Option D):
- Specifically associated with animal bites (particularly cat and dog bites)
- Not a cause of typical non-purulent cellulitis without bite history 2
Clinical Implications for Treatment
This pathogen hierarchy directly informs antibiotic selection:
- Beta-lactam monotherapy (penicillin, amoxicillin, cephalexin, or dicloxacillin) targeting streptococci is the standard of care 2
- Treatment duration is 5 days if clinical improvement occurs 2
- MRSA coverage should only be added when specific risk factors are present (penetrating trauma, purulent drainage, injection drug use) 2, 3