What are the common pathogens causing infective endocarditis?

Common Pathogens Causing Infective Endocarditis

Staphylococcus aureus is currently the most common causative organism of infective endocarditis in most of the industrialized world, accounting for 38-60% of all cases. 1, 2

Distribution of Major Pathogens

• Staphylococci:

  • Staphylococcus aureus: Most frequent cause of both native valve and prosthetic valve IE (38-60% of cases) 1, 2
  • Coagulase-negative staphylococci: More common in prosthetic valve IE, particularly early-onset (within 2 months of surgery) 1, 2

• Streptococci:

  • Viridans group streptococci: 16-21% of cases, particularly common in community-acquired native valve IE 2, 3
  • Streptococcus bovis: Associated with gastrointestinal pathology, especially colorectal cancer 4

• Enterococci:

  • Account for approximately 8% of cases 2
  • Often associated with genitourinary or gastrointestinal procedures 2

• HACEK Group:

  • Haemophilus, Actinobacillus, Cardiobacterium, Eikenella, and Kingella species
  • Relatively rare causes (approximately 3% of cases) 4

• Fungi:

  • Candida species (13-14% of cases) 2
  • Aspergillus species
  • More common in immunosuppressed patients and those with cardiovascular devices 2

Pathogens in Specific Patient Populations

Prosthetic Valve Endocarditis (PVE)

• Early-onset PVE (within 2 months of surgery):

  • Coagulase-negative staphylococci (especially S. epidermidis) 2
  • S. aureus
  • Gram-negative bacilli
  • Fungi

• Late-onset PVE (after 2 months):

  • Similar distribution to native valve IE, with streptococci and staphylococci being common 2

Intravenous Drug Users

• S. aureus predominates (right-sided IE common, especially tricuspid valve) 1, 2
• Pseudomonas aeruginosa (13-14%) 2
• Polymicrobial infections (8.1%) 2

Pediatric Patients

• In newborns: S. aureus, coagulase-negative staphylococci, and Candida species 1
• Less frequently: Group B Streptococcus, enteric Gram-negative rods, and S. pneumoniae 1

Healthcare-Associated IE

• S. aureus predominates 1, 2
• Enterococci
• Coagulase-negative staphylococci

Culture-Negative Endocarditis

Culture-negative endocarditis accounts for 5-36% of all cases 2 and may be caused by:

• Prior antibiotic therapy (most common reason) 1
• Fastidious organisms:
  • Coxiella burnetii (Q fever) 1, 2
  • Bartonella species 1, 2
  • Tropheryma whipplei 1
  • Brucella species 2, 5
  • HACEK group organisms 1
  • Abiotrophia and Granulicatella species (nutritionally variant streptococci) 1

Changing Epidemiology

The epidemiology of IE has evolved significantly over recent decades:

• S. aureus has overtaken streptococci as the predominant causative organism 1, 3, 5
• Increasing incidence in elderly patients with degenerative valve disease 1
• Higher proportion of prosthetic valves and cardiac devices 1
• Decreasing proportion of rheumatic heart disease 1

Clinical Pearls

• Every case of S. aureus bacteremia should raise suspicion for IE, regardless of whether an obvious source is identified 6
• Geographic variations exist in the distribution of causative agents, emphasizing that a global one-size-fits-all approach is not appropriate 3
• The high number of microbiologically undiagnosed cases (26.6%) suggests the need to revisit diagnostic strategies 3
• Blood cultures should be obtained before starting antibiotic therapy to maximize the chance of pathogen identification 2

Understanding the common pathogens causing IE is essential for appropriate empiric antibiotic selection while awaiting culture results and for guiding diagnostic approaches in culture-negative cases.