Primary Causes of Abscesses
Abscesses are primarily caused by bacterial invasion of tissue, with the specific causative organisms determined by the anatomical location and route of inoculation. The pathogenesis involves localized collection of purulent material surrounded by inflammation and granulation tissue in response to infection 1.
Causative Organisms by Location
Skin and Soft Tissue Abscesses
- Staphylococcus aureus is the predominant pathogen in cutaneous abscesses, though it accounts for less than half of all cases 2
- Methicillin-resistant S. aureus (MRSA) is the most common organism in skin abscesses in contemporary practice 3
- Group A beta-hemolytic streptococci (Streptococcus pyogenes) are frequently isolated alongside staphylococci 4, 5
- Anaerobic bacteria are common in perineal abscesses and account for the majority of all cutaneous abscesses 2
Site-Specific Bacterial Flora
The organisms causing abscesses reflect the normal flora adjacent to the infection site 5:
- Head, neck, mouth, and finger abscesses: Group A beta-hemolytic streptococci, pigmented Prevotella and Porphyromonas species, and Fusobacterium species from oral cavity flora 5
- Intra-abdominal and lower extremity abscesses: Enteric Gram-negative bacilli and Bacteroides fragilis group from gastrointestinal flora 5
- Perianal and perirectal abscesses: Originate from obstructed anal crypt glands with mixed aerobic and anaerobic organisms 4
Brain Abscesses
- Oral cavity bacteria are the predominant cause, accounting for 59% of cases, including Streptococcus anginosus group, Fusobacterium species, and Aggregatibacter species 4
- Staphylococcus aureus and Gram-negative bacilli predominate in post-neurosurgical brain abscesses 4
- Opportunistic pathogens in severely immunocompromised patients include Nocardia species, fungi, parasites (Toxoplasma gondii), and Mycobacterium tuberculosis in endemic areas 4
Polymicrobial Nature
Most abscesses developing from introduction of normal flora into sterile body sites are polymicrobial 5. These mixed infections contain both aerobic and anaerobic organisms, requiring consideration of both components when antimicrobial therapy is indicated 5.
Predisposing Factors and Routes of Infection
For Brain Abscesses
Historical risk factors include 4:
- Head trauma
- Cyanotic congenital heart disease
- Chronic ear infections
Contemporary risk factors include 4:
- Dental infections
- Immunocompromise
- Neurosurgical procedures
For Cutaneous Abscesses
- Intravenous drug injection sites create direct inoculation routes for complex abscesses requiring broad-spectrum coverage 4
- Obstructed glands (anal crypt glands in perirectal abscesses) 4
- Breach of skin integrity allowing normal skin flora to invade deeper tissues 1
Clinical Pitfall
Do not assume all abscesses are caused by S. aureus alone—the anatomical location dictates the likely polymicrobial flora, and anaerobes are frequently involved, particularly in abscesses near mucosal surfaces 5, 2. This has direct implications for antibiotic selection when systemic therapy is required 4, 5.