Description

• Anaerobic bacteria are organisms capable of growing in a reduced oxygen environment, either exclusively (obligate anaerobes) or in addition to growing in air (facultative anaerobes).
• Anaerobic bacteria can cause invasive and serious infections.
• Anaerobic bacteria tend to participate with other anaerobic and aerobic flora in causing polymicrobial infections.

Epidemiology

• Although anaerobic bacteremia is less frequent in children than in adults, other anaerobic infections such as chronic sinusitis or chronic otitis media are common in children.
• Because of their fastidious nature, the ability of microbiology laboratories to identify anaerobic bacteria is highly dependent on proper collection and transport of culture specimens; hence, anaerobic bacteria are often missed and likely underreported.

Risk Factors

• Impaired host immunity
  • Malignancy
  • Splenic dysfunction
  • Hypogammaglobulinemia
• Presence of devitalized tissue
• Surgery, trauma
• Vascular insufficiency
• Poorly controlled diabetes
• Presence of foreign bodies
• Colitis

Pathophysiology

• Anaerobic infections commonly derive from the normal flora of the oropharynx, skin, intestines, or the female genital tract; thus, anaerobic infections occur when there is a loss of integrity of anatomic or epithelial barriers at these sites.
• Virulence factors include production of exotoxins (e.g., Clostridia spp.), endotoxins (e.g., Fusobacterium spp.), and presence of phagocyte-inhibiting capsules (e.g., Bacteroides spp.).

Etiology

The most common clinically-relevant anaerobes include the following:

• Gram-negative rods (Bacteroides, Prevotella, Porphyromonas, Fusobacterium)
• Gram-positive cocci (Peptostreptococcus, Peptococcus)
• Spore-forming gram-positive bacilli (Clostridia)
• Non–spore-forming gram-positive bacilli (Eubacterium, Bifidobacterium, Propionibacterium, Actinomyces, Lactobacillus)
• Gram-negative cocci (Veillonella, Acidaminococcus)
• Spirochetes (many of which are anaerobic)

Commonly Associated Conditions

• CNS infections:
  • Brain abscess due to bacteremia
  • Subdural empyema
  • Epidural abscess (most commonly due to complications from sinusitis)
• Head and neck infections:
  • Sinusitis (generally polymicrobial)
  • Chronic otitis media
  • Ludwig angina (infection of the submandibular space)
  • Cervical adenitis
  • Peritonsillar abscess
  • Dental abscess
  • Gingivitis
  • Actinomycosis of jaw
  • Lemierre disease
    • Septic thrombophlebitis of the internal jugular vein owing to anaerobic bacteremia
    • Most commonly with Fusobacterium spp., often resulting in pulmonary abscess formation and metastatic infection
• Pleuropulmonary infections:
  • Aspiration of infected amniotic or vaginal secretions in neonates
  • Aspiration of oral or gastrointestinal fluids in children (severe gingival or periodontal may be a risk factor)
  • Pneumonia, abscess formation due to aspirated foreign bodies
  • Actinomycosis
• Peritonitis/peritoneal abscess:
  • Appendiceal abscess
  • Perforated viscus
  • Postoperative complication
  • Trauma-related
  • Actinomycosis
• Cholangitis
  • Ascending infection may occur following biliary tract surgery (e.g., Kasai procedure).
• Soft tissue infections:
  • Paronychia
  • Pilonidal cyst
  • Hidradenitis suppurativa
  • Crepitant cellulitis
  • Necrotizing fasciitis
  • Gas gangrene (Clostridium spp.)
  • Infected decubitus ulcers (may result in contiguous osteomyelitis)
  • Penetrating wounds (may lead to tetanus)
• Infections of the female genital tract:
  • Endometritis
  • Salpingitis
  • Tubo-ovarian or adnexal abscess
  • Pelvic inflammatory diseases
  • Pelvic abscess
  • Bartholin gland, vulvar, or perineal abscess
  • Bacterial vaginosis
• Infected bite wounds
  • Anaerobes isolated from 50% of human or animal bites
• Bacteremia
  • Often associated with focal primary site of involvement (gastrointestinal disease, abscess)
• Neonatal infections:
  • Cellulitis at fetal monitoring sites
  • Aspiration pneumonia
  • Omphalitis
  • Conjunctivitis
  • Infant botulism