Aggregatibacter aphrophilus Septic Arthritis of the Knee With Negative Initial Cultures: A Case Report

Aggregatibacter aphrophilus is a fastidious Gram-negative coccobacillus of the HACEK group that colonizes the oral cavity and is best recognized as a cause of infective endocarditis.Septic arthritis caused by this organism is uncommon, particularly when involving the native knee.We report a 60-year-old man who presented with two weeks of progressive right knee pain and swelling.He was afebrile with a normal peripheral white blood cell count, although inflammatory markers were elevated.Synovial fluid analysis demonstrated 81,000 nucleated cells/µL with 88% polymorphonuclear leukocytes, but Gram stain and initial synovial fluid culture were negative.Arthrotomy with anterior synovectomy and drainage revealed approximately 50 mL of purulent fluid.Operative culture subsequently grew A. aphrophilus after five days of incubation, identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.Blood cultures remained negative, and transthoracic echocardiography showed no evidence of valvular vegetation.Dental evaluation demonstrated periodontal disease, although no definitive odontogenic source was identified.The patient was treated with surgical drainage followed by four weeks of intravenous ceftriaxone, with complete clinical recovery.This case highlights the diagnostic challenge of A. aphrophilus septic arthritis and the importance of operative cultures when clinical suspicion remains high despite negative initial microbiologic studies.