Which organism is the most common cause of septic arthritis in healthy adults?

Explanation

## Most Common Causative Organism in Septic Arthritis **Key Point:** Staphylococcus aureus accounts for approximately 40–50% of all septic arthritis cases in healthy adults and remains the most frequent pathogen across all age groups and risk categories. ### Epidemiology by Host Status | Host Status | Most Common Organism | Frequency | |---|---|---| | Healthy adults | *Staphylococcus aureus* | 40–50% | | Neonates (< 3 months) | *S. aureus*, Group B Streptococcus, Gram-negative bacilli | Variable | | Children (3 months – 5 years) | *S. aureus*, *Streptococcus pyogenes* | 60–70% | | Immunocompromised | *S. aureus*, Gram-negative bacilli, fungi | Variable | | IVDU | *S. aureus* (including MRSA), Gram-negative bacilli | 60–80% | | Prosthetic joint | *S. aureus*, *S. epidermidis*, Gram-negative bacilli | Variable | **High-Yield:** *S. aureus* is the **default answer** for septic arthritis in any healthy adult unless the stem specifies a special risk factor (e.g., gonorrhea in sexually active young adults, gram-negative in elderly with UTI). ### Why *S. aureus* Dominates 1. **Ubiquitous colonization** — present on skin and mucous membranes of 20–30% of the population 2. **High virulence** — produces multiple toxins (alpha-toxin, Panton-Valentine leukocidin) and adhesins 3. **Tropism for joints** — readily invades synovial membrane via bacteremia or direct inoculation 4. **Ability to survive phagocytosis** — resists intracellular killing via protein A and other immune evasion mechanisms **Clinical Pearl:** Even in the pre-antibiotic era, *S. aureus* was the leading cause of suppurative arthritis. Its dominance has persisted despite decades of antibiotic use, reflecting its inherent pathogenic potential. ### MRSA Considerations **Warning:** In healthcare-associated septic arthritis (post-operative, prosthetic joints, or in patients with recent hospitalization), methicillin-resistant *S. aureus* (MRSA) prevalence may exceed 50%. Always consider MRSA coverage in empiric therapy for these populations. [cite:Robbins 10e Ch 26]