Knee Septic Arthritis Caused by Coinfection with Rothia mucilaginosa and Erysipelothrix rhusiopathiae.

Introduction: Septic arthritis of the knee caused by the combination of Rothia mucilaginosa and Erysipelothrix rhusiopathiae is extremely rare. E. rhusiopathiae is a rare zoonotic pathogen that primarily affects individuals with occupational exposure to animals, while R. mucilaginosa can cause severe infections, particularly in immunocompromised patients. Case Presentation: A 59-year-old male underwent right knee arthroscopy in 2019 due to meniscal degeneration. Two weeks later, activity-related pain appeared. Magnetic resonance imaging showed proliferative synovitis, and joint aspiration revealed the presence of E. rhusiopathiae, which was treated with ciprofloxacin. As inflammation persisted, arthroscopic synovectomy was performed. Cultures revealed R. mucilaginosa and E. rhusiopathiae, prompting treatment with vancomycin and clindamycin. Despite repeated synovectomies, symptoms remained. After knee trauma in 2023, infection recurred. A two-stage total knee arthroplasty was performed in early 2024. At this time, another pathogen was isolated. At 12-month follow-up, the patient's function and alignment had improved significantly. Conclusions: The described case highlights the importance of anamnesis, early diagnostics, and knowledge about the possible resistances of rare pathogens to ensure appropriate treatment of the illness.