## S. aureus IE vs. Viridans Streptococci IE ### Clinical and Microbiological Comparison **Key Point:** S. aureus causes acute, aggressive endocarditis with rapid valve destruction and septic emboli, often seeding previously normal valves. Viridans streptococci cause subacute endocarditis with insidious progression and a predilection for abnormal or damaged valves. ### Discriminating Features | Feature | S. aureus IE | Viridans Streptococci IE | Discriminator? | |---------|--------------|-------------------------|------------------| | **Onset** | Acute (days to weeks) | Insidious (weeks to months) | **Yes** | | **Valve involvement** | Normal valves (60–70%) | Pre-existing disease (>90%) | **Yes** | | **Progression** | Rapid, destructive | Slow, indolent | **Yes** | | **Septic emboli** | Frequent, large | Less common | **Yes** | | **Hemolysis** | Beta-hemolysis | Alpha-hemolysis | **Yes** | | **Coagulase** | Positive | Negative | **Yes** | | **Penicillin sensitivity** | Variable (MRSA resistant) | Sensitive | Partial | **High-Yield:** The **acute presentation with rapid valve destruction and septic emboli affecting normal valves** is the single best clinical discriminator. This reflects S. aureus's potent virulence factors (coagulase, alpha-toxin, Panton-Valentine leukocidin) and its ability to directly invade and damage healthy endocardium. **Clinical Pearl:** S. aureus IE is the leading cause of acute bacterial endocarditis in developed countries, especially in IV drug users and those with central lines. Viridans streptococci remain the most common cause of subacute IE in patients with underlying valvular disease (rheumatic heart disease, mitral valve prolapse). **Mnemonic:** **S. aureus = Acute, Aggressive, Abnormal valves (normal ones attacked)** vs. **Viridans = Subacute, Subtle, Sick valves (pre-existing disease)** [cite:Harrison 21e Ch 124]
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