Gram Stain and Culture of Pus or Exudate
For impetigo lesions, perform Gram stain and bacterial culture of the pus or exudate from the skin lesions 1.
When to Obtain Cultures
The IDSA guidelines explicitly recommend Gram stain and culture for impetigo and ecthyma lesions 1. While treatment without these studies is reasonable in typical cases (similar to the recommendation for carbuncles and abscesses), obtaining cultures provides critical information for:
- Identifying the causative organism (S. aureus vs. Group A Streptococcus vs. mixed infection)
- Determining antibiotic susceptibility, particularly for detecting MRSA
- Guiding targeted therapy when empiric treatment fails
Expected Microbiology
Understanding the typical bacterial patterns helps interpret culture results:
- S. aureus alone: Most common, found in approximately 46-60% of cases 2, 3
- Mixed S. aureus and Group A Streptococcus: Second most common, occurring in 25-61% of lesions 2, 3
- Group A Streptococcus alone: Less common, found in 9-21% of cases 2, 3
Important caveat: Early vesicular or pustular lesions more frequently yield pure streptococcal cultures, while later-stage lesions tend to show mixed growth 3. Streptococci persist longer in lesions than staphylococci when both are present 3.
Clinical Implications for Treatment
The culture results directly impact antibiotic selection per IDSA guidelines 1:
- If cultures yield streptococci alone: Oral penicillin is the recommended agent
- If S. aureus is present (most common scenario): Use dicloxacillin or cephalexin for methicillin-susceptible strains
- If MRSA is suspected or confirmed: Switch to doxycycline, clindamycin, or trimethoprim-sulfamethoxazole
Practical Considerations
Specimen collection technique: Swab the pus or exudate from beneath the crust after removing it, as this provides the highest bacterial yield. For bullous lesions, aspirate fluid from intact bullae when possible 4.
Common pitfall: Don't assume all impetigo is streptococcal—this outdated assumption can lead to treatment failure, as staphylococcal and mixed infections now predominate in most geographic areas 2, 3.