Clinicians find it convenient to classify endocarditis in five categories:
1. Endocarditis that involves predominantly left-sided native valves (mitral and aortic) affected by disease, particularly of rheumatic, degenerative, or congenital origin.
2. Prosthetic valve endocarditis.
3. Endocarditis in intravenous drug users: the tricuspid valve is infected in more than 50% of cases, and the aortic in about 25%. More than 75% of patients have no known pre-existing valve lesions. The predominant organism is Staphylococcus aureus, but Pseudomonas and fungi cause severe valve infections. HIV positive patients may present with unusual organisms bar-tonella, salmonella, and listeria.
4. Nosocomial endocarditis is most often caused Staphy-lococcus and enterococci; infections that are associated with indwelling catheters and medical-surgical proce¬dures including hemodialysis and bone marrow transplant.
5. Culture negative endocarditis exhibits the usual bacterial organisms but they are masked by previous antibiotic therapy. Slow-growing penicillin-sensitive organisms with fastidious nutritional tastes may not be detected because special culture medium are required for their growth, isolation, and detection; also, Brucella, Chlamydia, and Coxiella may not be readily detected.

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