Which organism is the most common cause of septic arthritis in a healthy adult with no risk factors?

Explanation

## Microbiology of Septic Arthritis **Key Point:** Staphylococcus aureus is the single most common cause of bacterial septic arthritis across all age groups and risk categories in immunocompetent adults. ### Organism Frequency by Population | Population | Most Common Organism | Second Most Common | |---|---|---| | Healthy adult | S. aureus (40–50%) | Streptococcus spp. (15–20%) | | Neonate (0–3 months) | Group B Streptococcus, Gram-negative bacilli | S. aureus | | Child (3 months–5 years) | S. aureus | H. influenzae type b (if unvaccinated) | | Sexually active adult | N. gonorrhoeae | S. aureus | | IVDU | S. aureus (including MRSA) | Gram-negative bacilli, Candida | | Prosthetic joint | S. aureus, S. epidermidis | Gram-negative bacilli | **High-Yield:** S. aureus accounts for approximately 40–50% of all septic arthritis cases in immunocompetent hosts. It has a predilection for weight-bearing joints (knee, hip) and can seed joints via haematogenous spread or direct inoculation. **Clinical Pearl:** The prevalence of MRSA in septic arthritis is rising in many centres; empiric therapy must cover both methicillin-sensitive and methicillin-resistant strains until culture results are available. ### Why Other Organisms Are Less Common in Healthy Adults - **S. pneumoniae:** More common in elderly, asplenic, or immunocompromised patients; rare in young healthy adults. - **N. gonorrhoeae:** Causes septic arthritis primarily in sexually active young adults; presents with migratory polyarthralgia–arthritis syndrome. - **Gram-negative bacilli:** Predominate in neonates, elderly, immunocompromised, and post-surgical patients; uncommon in healthy young adults without risk factors.