Mycobacterium fortuitum: A Neglected Cause of Culture-Negative Prosthetic Valve Endocarditis and a Literature Review

Background/Objectives: Prosthetic valve endocarditis caused by non-tuberculous mycobacteria is a rare but serious condition and is often associated with delayed diagnosis due to initially negative routine blood cultures with late positivity after prolonged incubation. Mycobacterium fortuitum, a rapidly growing mycobacterium, is an uncommon cause of endocarditis but may result in significant morbidity if not promptly identified. Methods: We report a 67-year-old man with prior cardiac surgery who presented 18 months later with recurrent fever, weight loss, and renal dysfunction. Initial blood cultures, echocardiography, and standard imaging were non-diagnostic. Ongoing clinical suspicion prompted extended mycobacterial cultures with prolonged incubation and molecular identification performed at a reference laboratory, which revealed M. fortuitum. Results: Antimicrobial susceptibility testing demonstrated susceptibility to amikacin, ciprofloxacin, and clarithromycin, and treatment was initiated with an amikacin-based combination regimen. The patient showed marked clinical and laboratory improvement, including resolution of fever and stabilization of renal function. Conclusions: This case highlights the diagnostic and therapeutic challenges of M. fortuitum prosthetic valve endocarditis and underscores the limitations of routine diagnostic methods in culture-negative endocarditis. It also emphasizes the importance of prolonged incubation and targeted microbiological workflows in suspected cases.