What is the common cause of necrotizing fasciitis, also known as flesh-eating bacteria?

Common Causes of Necrotizing Fasciitis (Flesh-Eating Bacteria)

Necrotizing fasciitis can be either monomicrobial (caused by a single organism) or polymicrobial (caused by multiple organisms), with Group A Streptococcus (Streptococcus pyogenes) being the most common single pathogen, while polymicrobial infections typically involve mixed aerobic-anaerobic bacteria. 1

Monomicrobial Causes

Group A Streptococcus (S. pyogenes) is the most frequent single causative organism and carries mortality rates of 30-70% when associated with hypotension and organ failure 1. This pathogen is particularly associated with:

• Cases arising after varicella or trivial injuries (minor scratches, insect bites) 1
• Nearly 50% of cases with no identifiable portal of entry, developing at sites of nonpenetrating trauma like bruises or muscle strains 1
• Infections related to childbirth involving the uterus or episiotomy site 1

Other monomicrobial causes include 1:

• Vibrio vulnificus (typically from seawater or raw seafood exposure)
• Aeromonas hydrophila (freshwater exposure)
• Methicillin-resistant Staphylococcus aureus (MRSA) - increasingly recognized as a cause of community-acquired necrotizing fasciitis 1, 2
• Methicillin-susceptible S. aureus (MSSA) 2
• Anaerobic streptococci (Peptostreptococcus) 1

Polymicrobial Causes

Polymicrobial necrotizing fasciitis involves mixed aerobic and anaerobic bacteria and is most commonly associated with four clinical settings 1:

1. Perianal abscesses, penetrating abdominal trauma, or bowel surgery - involving enteric organisms 1
2. Decubitus ulcers - in patients with chronic wounds 1
3. Injection drug use sites - particularly in illicit drug users 1
4. Genital sources - including Bartholin abscess, episiotomy wounds, or vulvovaginal infections (Fournier gangrene) 1

Predisposing conditions for polymicrobial infections include 1:

• Diabetes mellitus
• Peripheral vascular disease
• Venous insufficiency with edema
• Spontaneous mucosal tears of the gastrointestinal or genitourinary tract

Gas Gangrene (Clostridial Myonecrosis)

Clostridial species cause a specific form of necrotizing infection with gas formation 1:

• Clostridium perfringens (most common)
• Clostridium septicum (more aerotolerant; associated with colonic lesions, diverticular disease, adenocarcinoma, or neutropenia)
• Clostridium histolyticum
• Clostridium novyi

These infections typically follow severe penetrating trauma or crush injuries with interrupted blood supply 1. Recent cases have been described in heroin users following intracutaneous injection of black tar heroin 1.

Key Clinical Pearls

The distinction between monomicrobial and polymicrobial infection is critical for empiric antibiotic selection 1. Approximately two-thirds of community-acquired cases occur in the lower extremities, with predisposing conditions including diabetes, arteriosclerotic vascular disease, venous insufficiency, ulcers, or injection drug use 1.

Severe pain may be the initial symptom with minimal cutaneous findings due to the deep fascial location of infection, particularly in Group A streptococcal cases 1. Gas in deep tissues is frequently found in both clostridial and polymicrobial infections 1.