A 28-year-old woman presents with a comminuted fracture of the femoral shaft sustained 3 days ago. She now develops fever, tachycardia, and signs of systemic inflammatory response. Which is the most common causative organism in early post-traumatic osteomyelitis following an open fracture?

Explanation

## Post-Traumatic Osteomyelitis: Microbiology **Key Point:** *Staphylococcus aureus* is the most common causative organism in early post-traumatic osteomyelitis, regardless of open or closed fracture, accounting for 40–60% of cases. ### Epidemiology of Osteomyelitis by Timing and Context | Organism | Frequency | Context | Timing | | --- | --- | --- | --- | | *S. aureus* | 40–60% | All fractures, especially closed | Early (days–weeks) | | *S. epidermidis* | 10–20% | Prosthetic implants | Early–late | | *Pseudomonas aeruginosa* | 10–15% | Open fractures, contaminated wounds | Early–late | | *Clostridium perfringens* | Rare (<1%) | Severely contaminated soil, gas gangrene | Early (hours–days) | | *Mycobacterium tuberculosis* | Rare | Spinal TB, endemic areas | Late (months–years) | ### Why *S. aureus* Dominates 1. **Ubiquitous colonization** — Present on skin and mucous membranes of ~30% of population 2. **High virulence** — Produces multiple toxins (alpha-toxin, Panton-Valentine leukocidin) 3. **Bone tropism** — Expresses adhesins (fibronectin-binding protein) that promote osteoblast invasion 4. **Biofilm formation** — Adheres to bone and implanted hardware, evades antibiotics 5. **Early seeding** — Enters bone via hematogenous spread or direct inoculation at fracture site **High-Yield:** In **open fractures with heavy soil contamination**, *Pseudomonas* and anaerobes (including *Clostridium*) become more likely, but *S. aureus* remains the single most common organism overall. ### Clinical Pearl: MRSA Considerations - **Prevalence of MRSA** in post-traumatic osteomyelitis is rising (15–30% in many centers) - **Empiric coverage** for open fractures should include anti-staphylococcal agents (cephalosporin or vancomycin) pending culture - **Community-associated MRSA (CA-MRSA)** is increasingly seen in trauma patients ### Mnemonic: Early Post-Traumatic Bone Infection Organisms **SAPO** (in order of frequency): - **S**taphylococcus aureus (most common) - **A**naerobes (in contaminated wounds) - **P**seudomonas (open fractures) - **O**ther gram-negatives (Enterobacteriaceae) **Tip:** When a question asks for "most common organism" in post-traumatic osteomyelitis without specifying soil contamination or gas gangrene, the answer is *S. aureus* until proven otherwise. [cite:Robbins 10e Ch 20]