## Microbiology of Septic Arthritis **Key Point:** Staphylococcus aureus is the most common pathogen in septic arthritis across all age groups and risk categories, accounting for 40–50% of cases in immunocompetent adults. ### Organism-Specific Epidemiology | Organism | Clinical Setting | Frequency | |----------|------------------|----------| | *Staphylococcus aureus* | Healthy adults, no recent instrumentation | 40–50% (most common) | | *Streptococcus pneumoniae* | Elderly, asplenic, immunocompromised | 10–15% | | *Neisseria gonorrhoeae* | Sexually active, disseminated gonococcal infection | 5–10% | | Gram-negative bacilli | IV drug users, immunocompromised, post-procedure | 10–20% | | *Haemophilus influenzae* | Unvaccinated children | 5–10% | **High-Yield:** In the absence of specific risk factors (IV drug use, recent arthrocentesis, immunosuppression, or disseminated gonococcal infection), *S. aureus* is the default answer. It is a skin commensal with high affinity for synovial tissue and can seed joints via bacteremia from minor skin breaks. **Clinical Pearl:** The rise of methicillin-resistant *S. aureus* (MRSA) in community settings means empiric coverage must include anti-staphylococcal agents (vancomycin or daptomycin) pending culture and susceptibility results. **Warning:** Do not confuse the most common organism overall with organism-specific risk groups. *N. gonorrhoeae* is common in sexually active young adults with disseminated gonococcal infection, but *S. aureus* remains more frequent even in that population. 
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