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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

The three most common causes of bacterial endocarditis include streptococcus viridans, staphylococci and enterococcus.

Common Causes of Endocarditis

Many types of organism can cause infective endocarditis. These are generally isolated by blood culture, where the patient's blood is removed, and any growth is noted and identified. It is therefore important to draw blood cultures before initiating antibiotic therapy.

1. Alpha-haemolytic streptococci, that are present in the mouth will often be the organism isolated if a dental procedure caused the bacteraemia.
2. If the bacteraemia was introduced through the skin, such as contamination in surgery, during catheterization, or in an IV drug user, Staphylococcus aureus is common.
3. A third important cause of endocarditis is Enterococci. These bacteria enter the bloodstream as a consequence of abnormalities in the gastrointestinal or urinary tracts. Enterococci are increasingly recognized as causes of nosocomial or hospital-acquired endocarditis. This contrasts with alpha-haemolytic streptococci and Staphylococcus aureus which are causes of community-acquired endocarditis.

Less Common Causes of Endocarditis

Some organisms, when isolated, give valuable clues to the cause, as they tend to be specific.

• Candida albicans, a yeast, is associated with IV drug users and the immunocompromised. Fungal endocarditis accounts for 5% of cases of native endocarditis and 10% of cases of prosthetic valve endocarditis. A diagnosis of fungal endocarditis is difficult, because many patients are afebrile with a normal white blood cell count (WBC). The fungus is often difficult to culture, and blood cultures are typically negative. Fungal infections often result in large vegetations, systemic embolization, myocardial invasion, and are extremely resistant to medical therapy. Early surgical intervention is warranted because medical mortality approaches 100% Anti-fungal therapy for life is required.
• Pseudomonas species, which are very resilient organisms that thrive in water, may contaminate street drugs that have been contaminated with drinking water. P. aeruginosa can infect a child through foot punctures, and can cause both endocarditis and septic arthritis.
• Streptococcus bovis and Clostridium septicum, which are part of the natural flora of the bowel, are associated with colonic malignancies. When they present as the causative agent in endocarditis, it usually indicates that a colonoscopy should be performed due to worries regarding hematogenous spread of bacteria from the colon due to the neoplasm breaking down the barrier between the gut lumen and the blood vessels which drain the bowel.
• HACEK organisms are a group of bacteria that live on the dental gums, and can be seen with IV drug abusers who contaminate their needles with saliva. Patients may also have a history of poor dental hygiene, or pre-existing valvular disease.