Most Common Bacteria in Necrotizing Fasciitis
Necrotizing fasciitis is most commonly caused by either Streptococcus pyogenes (Group A Streptococcus) in monomicrobial infections or polymicrobial infections involving an average of 5 pathogens including aerobic and anaerobic organisms from bowel flora. 1
Monomicrobial Necrotizing Fasciitis (Type II)
The most common causative organisms in monomicrobial infections include:
- Streptococcus pyogenes (Group A Streptococcus) is the predominant pathogen, particularly in cases arising after varicella or trivial injuries such as minor scratches and insect bites 1
- Staphylococcus aureus (including MRSA) occurs less frequently as a primary organism but can occur simultaneously with streptococci 1
- Vibrio vulnificus in specific exposure contexts 1
- Aeromonas hydrophila in water-related exposures 1
- Anaerobic streptococci (Peptostreptococcus species) 1
Group A streptococcal necrotizing fasciitis carries mortality rates of 30-70% when accompanied by hypotension and organ failure 1. Nearly 50% of these patients have no visible portal of entry but develop deep infection at sites of nonpenetrating trauma such as bruises or muscle strains 1.
Polymicrobial Necrotizing Fasciitis (Type I)
Polymicrobial infections involve up to 15 different organisms with an average of 5 pathogens per wound, predominantly originating from bowel flora including coliforms and anaerobic bacteria. 1, 2
These infections are associated with four specific clinical settings:
- Perianal abscesses, penetrating abdominal trauma, or surgical procedures involving the bowel 1, 2
- Decubitus ulcers 1, 2
- Injection sites in illicit drug users 1, 2
- Spread from genital sites such as Bartholin abscess, episiotomy wounds, or minor vulvovaginal infections 1, 2
Clinical Implications for Diagnosis
Gram stain of deep tissue exudate provides early clues to the causative pathogen: 1
- Gram-positive cocci in chains suggest Streptococcus organisms (either Group A or anaerobic) 1
- Large gram-positive cocci in clumps suggest S. aureus, though this is unusual as a primary organism 1
Critical pitfall: Surface cultures of wounds are not valuable as they represent colonizing microbes rather than the underlying etiologic agent; tissue biopsies from deep tissues after thorough debridement or specimens obtained during operation provide definitive bacteriologic diagnosis 1. Blood cultures may be positive and aid in diagnosis 1.