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

<b>Introduction</b>: Septic arthritis of the knee caused by the combination of <i>Rothia mucilaginosa</i> and <i>Erysipelothrix rhusiopathiae</i> is extremely rare. <i>E. rhusiopathiae</i> is a rare zoonotic pathogen that primarily affects individuals with occupational exposure to animals, while <i>R. mucilaginosa</i> can cause severe infections, particularly in immunocompromised patients. <b>Case Presentation</b>: 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 <i>E. rhusiopathiae</i>, which was treated with ciprofloxacin. As inflammation persisted, arthroscopic synovectomy was performed. Cultures revealed <i>R. mucilaginosa</i> and <i>E. rhusiopathiae</i>, 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. <b>Conclusions</b>: 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.