What anaerobic bacterium is likely to be involved in a sinus infection?

Anaerobic Bacteria in Sinus Infections

The predominant anaerobic bacteria involved in sinus infections are Prevotella species, anaerobic streptococci (Peptostreptococcus), and Fusobacterium species, which are particularly common in chronic sinusitis. 1

Microbiology of Sinus Infections: Acute vs. Chronic

Acute Sinusitis

• Primarily caused by aerobic bacteria:
  • Streptococcus pneumoniae
  • Haemophilus influenzae
  • Moraxella catarrhalis
• Anaerobes are uncommonly isolated in acute sinusitis 1

Chronic Sinusitis

• Typically polymicrobial with anaerobes predominating:
  • Prevotella species (31% of anaerobic isolates)
  • Anaerobic streptococci/Peptostreptococcus (22%)
  • Fusobacterium species (16%)
  • Other common aerobes include Staphylococcus aureus, H. influenzae, and Pseudomonas aeruginosa 1

Distribution of Anaerobes in Chronic Sinusitis

A large multicenter study found that anaerobic bacteria were recovered in up to 40% of chronic sinusitis cases 1. The specific distribution includes:

1. Prevotella species - Most common anaerobic isolate

   • Pigmented Prevotella are particularly prevalent
   • Often beta-lactamase producers

2. Peptostreptococcus species - Second most common

   • Found in approximately 22% of chronic sinusitis cases

3. Fusobacterium species - Third most common

   • Present in about 16% of chronic sinusitis cases

4. Bacteroides species - Also commonly isolated

5. Veillonella species - Less frequently isolated but still significant

Why Anaerobes Predominate in Chronic Sinusitis

Anaerobic bacteria emerge as pathogens as sinusitis becomes chronic due to:

1. Selective pressure from antimicrobial agents allowing resistant anaerobes to survive 2
2. Development of conditions favorable for anaerobic growth:
   • Reduced oxygen tension in the sinus cavity
   • Increased acidity within the sinus 2
   • Poor sinus ventilation and obstruction

Clinical Implications

• Anaerobes are rarely the initial cause of sinusitis (except in sinusitis of dental origin) but frequently superinfect and perpetuate chronic disease 3
• Many anaerobes (particularly Prevotella and Fusobacterium species) produce beta-lactamase, contributing to treatment failure with standard antibiotics 4
• The presence of these beta-lactamase-producing anaerobes can "shield" other susceptible bacteria from antibiotics 4

Diagnostic Considerations

• Direct sinus aspiration is the gold standard for identifying the causative organisms 1
• Middle meatal cultures correlate reasonably well with sinus aspirates in adults but have limitations 5
• Quantitative cultures should be performed with bacterial species recovered at ≥10³-10⁴ CFU/mL considered significant 1

Treatment Implications

For chronic sinusitis with suspected anaerobic involvement, the most effective antimicrobials are:

• Amoxicillin-clavulanate (provides coverage against beta-lactamase producers) 6, 4
• Clindamycin (excellent anaerobic coverage) 4
• Metronidazole combined with a penicillin 4

The FDA-approved indications for amoxicillin-clavulanate include sinusitis caused by beta-lactamase-producing isolates of H. influenzae and M. catarrhalis, though it also has activity against many anaerobes 6.

Special Considerations

• Sphenoid sinusitis shows a particularly high prevalence of anaerobes in chronic cases (43% anaerobes alone, 43% mixed with aerobes) 7
• Nosocomial sinusitis typically involves gram-negative enterics rather than anaerobes 1
• Immunocompromised patients may have different microbiology, including higher rates of Pseudomonas aeruginosa 8

Understanding the anaerobic microbiology of chronic sinusitis is crucial for selecting appropriate antimicrobial therapy that addresses all potential pathogens in these typically polymicrobial infections.