A 52-year-old woman with a prosthetic aortic valve (mechanical) inserted 8 years ago presents with fever and a new diastolic murmur. She is found to have infective endocarditis. Which organism is the most common cause of prosthetic valve endocarditis in this late presentation (>12 months post-implantation)?

Explanation

## Most Common Organism in Late Prosthetic Valve Endocarditis **Key Point:** Streptococcus viridans is the most common cause of late prosthetic valve endocarditis (>12 months post-implantation), accounting for approximately 50% of cases. Late PVE behaves like native valve IE. ### Timeline & Organism Patterns in Prosthetic Valve Endocarditis | Time Post-Op | Most Common Organism | Mechanism | Clinical Course | | --- | --- | --- | --- | | <2 months (Early PVE) | Coagulase-negative staph, S. aureus | Surgical contamination, healthcare-associated | Acute, fulminant | | 2–12 months (Intermediate) | Mixed (CoNS, S. aureus, viridans) | Transition period | Variable | | >12 months (Late PVE) | S. viridans, Enterococcus | Oral flora, community-acquired | Subacute, indolent | **High-Yield:** Late prosthetic valve endocarditis (>12 months) mirrors native valve IE in microbiology. Viridans streptococci predominate because the valve is now well-integrated and behaves like native tissue. ### Pathophysiology After 12 months, the prosthetic valve becomes endothelialized and is no longer susceptible to the nosocomial pathogens that cause early PVE. Instead: - Oral flora (S. viridans) seed the valve via dental procedures or poor hygiene - Presentation is subacute with gradual symptom onset - New diastolic murmur suggests aortic regurgitation from vegetation **Clinical Pearl:** The distinction between early and late PVE is critical for empiric antibiotic selection. Late PVE is treated like native valve IE (penicillin + gentamicin for viridans streptococci), whereas early PVE requires broader coverage (vancomycin + gentamicin + rifampin). **Mnemonic — "Late PVE = Like native valve IE"** [cite:Harrison 21e Ch 124]