A 32-year-old man presents to the emergency department with a 3-day history of severe left knee pain, swelling, and inability to bear weight. He reports a puncture wound to the sole of his foot 10 days ago while walking barefoot on a beach. On examination, the left knee is warm, erythematous, and grossly swollen with severe pain on passive motion. Temperature is 38.5°C. Synovial fluid analysis shows WBC 85,000/μL (95% neutrophils), glucose 18 mg/dL, protein 6.2 g/dL, and Gram stain reveals Gram-negative rods. Blood cultures are pending. What is the most likely causative organism?

Explanation

## Clinical Presentation & Organism Identification This patient has acute septic arthritis with a clear epidemiological clue: **puncture wound to the foot in a marine/aquatic environment**. The synovial fluid findings (WBC >50,000/μL, low glucose, elevated protein, Gram-negative rods on Gram stain) are consistent with bacterial septic arthritis. ### Key Epidemiological Feature **High-Yield:** Puncture wounds to the foot sustained in **marine or aquatic environments** (seawater, beaches, ponds) are classically associated with **Pseudomonas aeruginosa** infection, particularly when the injury occurs through contaminated footwear or soil. ### Organism-Specific Features | Feature | Pseudomonas aeruginosa | S. aureus | S. pyogenes | N. gonorrhoeae | |---------|------------------------|-----------|-------------|----------------| | **Gram stain** | Gram-negative rod | Gram-positive cocci | Gram-positive cocci | Gram-negative diplococcus | | **Common source** | Water, soil, wounds | Skin, respiratory | Skin, respiratory | Urogenital | | **Synovial glucose** | Low | Low | Low | Variable | | **Acute septic arthritis** | Yes (especially post-trauma) | Most common | Yes | Rare (migratory polyarthritis) | ### Clinical Pearl **Clinical Pearl:** Pseudomonas aeruginosa has a predilection for **joint and bone infections following puncture wounds**, especially those contaminated with soil or water. It produces proteolytic enzymes and biofilms, making it particularly destructive to cartilage. ### Mnemonic for Puncture Wound Organisms **WATER = Waterborne organisms after puncture wounds** - **W**ater exposure → Pseudomonas - **A**cute trauma → Consider Staph/Strep - **T**etanus risk → Clostridium tetani (prophylaxis needed) - **E**nvironmental soil → Pseudomonas, anaerobes - **R**apid progression → Gram-negative rods ### Why Gram-Negative Rod Identification Matters The Gram stain showing **Gram-negative rods** immediately excludes S. aureus and S. pyogenes (both Gram-positive cocci) and N. gonorrhoeae (Gram-negative diplococcus, not rod). In the context of a puncture wound with aquatic exposure, Pseudomonas aeruginosa is the most likely Gram-negative rod pathogen.