What are the most common microorganisms causing endocarditis?

Most Common Microorganisms Causing Endocarditis

Streptococci and staphylococci are the most common causative microorganisms of infective endocarditis (IE), collectively accounting for approximately 80% of all cases. 1

Microbiological Profile by Valve Type

Native Valve Endocarditis

• Streptococci:
  • Viridans streptococci (oral streptococci) including S. sanguis, S. mitis, S. salivarius, S. mutans
  • Streptococcus bovis (associated with gastrointestinal malignancies)
• Staphylococci:
  • Staphylococcus aureus (most virulent, associated with intravenous drug use and diabetes mellitus)
  • Coagulase-negative staphylococci (particularly in patients with underlying valvular abnormalities)

Prosthetic Valve Endocarditis (PVE)

• Early PVE (within first 2 months after surgery):

  • Staphylococcus epidermidis (predominant organism) 1
  • Staphylococcus aureus
  • Gram-negative bacilli
  • Fungi (particularly Candida species)

• Late PVE (after 2 months):

  • Similar profile to native valve endocarditis (streptococci and staphylococci) 1

Special Populations and Associated Organisms

• Intravenous Drug Users:

  • Staphylococcus aureus (80% of tricuspid valve infections) 1
  • Gram-negative bacilli
  • Fungi
  • Polymicrobial infections

• Patients with Diabetes Mellitus:

  • Staphylococcus aureus (frequently) 1

• Genitourinary/Gastrointestinal Procedures:

  • Enterococci (E. faecalis and E. faecium account for 90% of enterococcal endocarditis) 1
  • Streptococcus bovis (associated with colon cancer)

Less Common but Important Causative Organisms

• HACEK Group (3% of community-acquired IE) 2:

  • Haemophilus species
  • Actinobacillus actinomycetemcomitans
  • Cardiobacterium hominis
  • Eikenella corrodens
  • Kingella kingae
  • These organisms typically cause large vegetations (>1 cm) 1

• Fungi (rare in native valves, up to 15% in prosthetic valves) 2:

  • Candida albicans (most common fungal cause)
  • Aspergillus species

• Culture-Negative Endocarditis (approximately 10% of cases) 3:

  • Coxiella burnetii (Q fever)
  • Bartonella species
  • Brucella species
  • Chlamydia psittaci
  • Prior antibiotic use before blood cultures

Clinical Pearls

• Always obtain 3-5 sets of blood cultures before initiating antibiotics in suspected IE
• Staphylococcus lugdunensis, although a coagulase-negative staphylococcus, causes a more virulent form of IE with high rates of perivalvular extension 1
• The microbiology of IE has evolved over time, with a shift from predominantly streptococcal to staphylococcal infections in developed countries
• In developing countries with high rates of rheumatic heart disease, streptococci remain the predominant causative organisms 1
• Blood culture-negative endocarditis requires specialized testing techniques and close collaboration between clinicians and microbiologists

Pitfalls to Avoid

• Don't assume all coagulase-negative staphylococci are contaminants; they can cause serious native and prosthetic valve endocarditis
• Don't miss the opportunity to identify uncommon organisms by starting antibiotics before adequate blood cultures are obtained
• Don't overlook the possibility of fungal endocarditis in patients with prosthetic valves, prolonged hospitalization, indwelling catheters, or long-term antibiotic use
• Don't forget to consider nutritionally variant streptococci (like Abiotrophia defectiva) in culture-negative endocarditis, especially after dental procedures 4