Septic Arthritis of the Knee Caused by Neisseria meningitidis Serogroup Y

Neisseria meningitidis is a notifiable pathogen in Japan when isolated from any sterile site. We describe a rare case of primary meningococcal arthritis without meningitis or bacteremia. A 48-year-old man presented with acute septic arthritis of his knee. A synovial fluid analysis using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry revealed gram-negative diplococci, identified as N. meningitidis. Further antimicrobial susceptibility testing showed the isolate was pan-susceptible, allowing de-escalation from ceftriaxone to ampicillin therapy. The patient recovered fully after surgical drainage and 4 weeks of antibiotic therapy. This case highlights the importance of considering invasive meningococcal disease upon the detection of gram-negative diplococci in synovial fluid.