What is the most common pathogen causing impetigo?

Most Common Pathogen for Impetigo

Staphylococcus aureus is the most common effective pathogen causing impetigo, either alone or in combination with Streptococcus pyogenes. 1, 2

Pathogen Distribution by Impetigo Type

Nonbullous Impetigo (70% of cases)

• Caused by Staphylococcus aureus and/or Streptococcus pyogenes (Group A beta-hemolytic streptococci) 3, 4
• Presents with honey-colored crusts on erythematous base, typically on face and extremities 1
• Begins as papules that evolve into vesicles, then pustules that rupture and form characteristic crusts 3

Bullous Impetigo (30% of cases)

• Caused exclusively by toxin-producing strains of Staphylococcus aureus 3, 5
• Characterized by fragile, thin-roofed vesicopustules that rupture leaving crusted erosions 3
• More likely to affect intertriginous areas 1

Epidemiological Evidence

• In a study of 73 children with impetigo, 62% of cultures showed Staphylococcus aureus only, 19% showed both S. aureus and Group A beta-hemolytic streptococci, and only 8% showed Group A streptococci alone 6
• Staphylococcus aureus is explicitly indicated as a causative organism in the FDA drug label for mupirocin, a first-line topical treatment for impetigo 7
• The Infectious Diseases Society of America guidelines consistently identify S. aureus as the predominant pathogen in impetigo 3

Clinical Implications

• Treatment should cover both S. aureus and streptococci unless cultures yield streptococci alone 3
• For limited lesions, topical antibiotics like mupirocin or retapamulin are recommended first-line treatments 4, 8
• For extensive disease, oral antibiotics active against both pathogens should be used 3
• Dicloxacillin or first-generation cephalosporins (e.g., cephalexin) are recommended as they are effective against most staphylococcal isolates from impetigo 3
• In areas with high prevalence of methicillin-resistant S. aureus (MRSA), alternative agents such as clindamycin, doxycycline, or trimethoprim-sulfamethoxazole should be considered 3, 4

Special Considerations

• Increasing prevalence of antibiotic-resistant bacteria, particularly MRSA, may influence treatment decisions 1
• Penicillin alone is inadequate for treatment of impetigo due to the predominance of S. aureus, which is typically penicillinase-producing 6
• In a comparative study, treatment failure occurred in 24% of patients treated with penicillin V compared to 0% with cephalexin and 4% with erythromycin 6
• Systemic antimicrobials should be used during outbreaks of post-streptococcal glomerulonephritis to eliminate nephritogenic strains of S. pyogenes 3

Understanding the predominant role of S. aureus in impetigo is crucial for selecting appropriate empiric antimicrobial therapy and achieving optimal clinical outcomes.