A 38-year-old man with a history of intravenous drug use presents with fever, new cardiac murmur, and positive blood cultures. Which is the most common causative organism in infective endocarditis in intravenous drug users?

Explanation

## Causative Organisms in Infective Endocarditis **Key Point:** Staphylococcus aureus is the most common organism in infective endocarditis among intravenous drug users, accounting for approximately 50–60% of cases in this population. ### Epidemiology by Risk Group | Risk Group | Most Common Organism | Key Features | |---|---|---| | **IV drug users** | *Staphylococcus aureus* | Acute presentation, often right-sided (tricuspid), aggressive course | | **Native valve (no IVDU)** | *Streptococcus viridans* | Subacute, left-sided (mitral/aortic), indolent | | **Prosthetic valve (early)** | *Staphylococcus epidermidis* | Nosocomial, device-related | | **Prosthetic valve (late)** | *Streptococcus viridans* | Similar to native valve | | **Dental procedures** | *Streptococcus viridans* | Alpha-hemolytic streptococci | **Clinical Pearl:** In IV drug users, *S. aureus* preferentially infects the tricuspid valve (right heart), leading to septic pulmonary emboli and a characteristic clinical picture of fever + pleuritic chest pain + nodular infiltrates on CXR. **High-Yield:** *S. aureus* endocarditis in IVDU is **acute and aggressive**, with rapid valve destruction and high mortality if untreated. Contrast this with *Streptococcus viridans* endocarditis, which is typically **subacute and indolent**. ### Why *S. aureus* Dominates in IVDU 1. Skin flora contamination during injection 2. High virulence and invasiveness 3. Ability to produce biofilms on valve surfaces 4. Direct inoculation of organisms into bloodstream **Mnemonic:** **IVDU = S. aureus** (remember: IV drug users get *Staph aureus*)