Rare Infection with Precise Detection: Abiotrophia defectiva septic arthritis following anterior cruciate ligament reconstruction.

Abiotrophia defectiva, a nutritionally variant streptococcus, is an uncommon yet significant pathogen primarily associated with infective endocarditis and, more rarely, septic arthritis. We report a 23-year-old male patient who presented to a tertiary care centre in Rustaq, Oman, in 2024 with right knee pain and fever following an anterior cruciate ligament reconstruction. Standard cultures remained negative, but enrichment broth and matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry enabled prompt identification of A. defectiva. Subsequently, the patient was diagnosed with native knee septic arthritis due to A. defective. Following surgical debridement, the patient completed a 4-week course of targeted antibiotic therapy, resulting in full recovery and normalisation of inflammatory markers. This case highlights the value of enrichment media and advanced diagnostic techniques in detecting fastidious pathogens in postoperative joint infections.