We present a case of subacute endocarditis with the detection of Cellulosimicrobium cellulans after tricuspid valve replacement and pacemaker extraction without detection in blood cultures before pacemaker extraction despite multiple testing. This atypical pathogen has rarely been reported in cases of subacute endocarditis, particularly in non-immunocompromised patients. Treatment was initiated using fosfomycin and flucloxacillin and switched to vancomycin upon pathogen detection after pacemaker lead removal and tricuspid valve replacement. Interdisciplinary cooperation between cardiologists, cardiothoracic surgeons, and the infectious disease team enabled an accurate diagnosis and optimized antibiotic therapy before surgical intervention.