Causative organisms

The microorganisms causing infective endocarditis include:

Infective agent and probability of being involved:

• viridans Streptococci: - cause about 50% of episodes; mostly subacute infective endocarditis - Strep. sanguis, Strep. mitis, Strep. mutans, Strep milleri
  
  
• enterococcal endocarditis - mainly with Enterococcus faecalis or Enterococcus faecium - about 10% of episodes - tends to occur in patients with lower gastrointestinal tract or genitourinary disease and may follow invasive procedure
  
  
• Staphylococcus aureus - cause about 20% of episodes - Staph. aureus, Staph. epidermidis; in IV drug abusers S. aureus is commonest causing 50-60% of episodes
  
  
• Fungi: 2-10% - Candida, Aspergillus, Histoplasma

In about 5% of patients with proven infective endocarditis, conventional blood cultures are negative. This may be due to infection with a difficult-to-culture or slow-growing organism e.g. gram-negative bacilli of the 'HACEK' group of oropharyngeal commensals, nutrionally variant streptococci and, rarely, Coxiella burnetti or Brucella species. Commonly culture-negativitity is a result of recent exposure to antimicrobial drugs.

Certain organisms are more common found in certain groups and these are illustrated in the linked pages below.

Reference:

Drugs and Therapeutics Bulletin (2002), 40 (4), 26-30.