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?

Question

Accepted Answer

A. Staphylococcus aureus. ## 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]

Answer

B. Clostridium perfringens

Answer

C. Mycobacterium tuberculosis

Answer

D. Pseudomonas aeruginosa

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

Epidemiology of Osteomyelitis by Timing and Context

Why S. aureus Dominates

Clinical Pearl: MRSA Considerations

Mnemonic: Early Post-Traumatic Bone Infection Organisms

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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)