Prosthetic valve endocarditis due to non-toxigenic Corynebacterium diphtheriae: A case report.

Prosthetic valve endocarditis (PVE) caused by Corynebacterium diphtheriae is rare and may be associated with severe embolic complications and high mortality. In March 2026, a 61-year-old woman with a mechanical mitral valve receiving chronic acenocoumarol therapy was admitted with fever, malaise, and abdominal pain. Thoracoabdominal computed tomography revealed multiple splenic infarctions suggestive of systemic embolization. Transthoracic echocardiography demonstrated vegetations on the prosthetic mitral valve. Blood cultures yielded C. diphtheriae, identified by matrix-assisted laser desorption ionization-time of flight mass spectrometry. Whole-genome sequencing confirmed a non-toxigenic C. diphtheriae sequence type 542 lacking the tox gene. Antimicrobial susceptibility testing showed resistance to penicillin G and trimethoprim-sulfamethoxazole, with susceptibility to meropenem, erythromycin, clindamycin, tetracycline, linezolid, vancomycin and rifampicin, and susceptibility with increased exposure to cefotaxime and ciprofloxacin. Cerebral computed tomography angiography demonstrated complete occlusion of the right internal carotid artery without initial parenchymal lesions. Intravenous vancomycin and bisoprolol were initiated. Acenocoumarol was initially continued and subsequently switched to intravenous unfractionated heparin. On hospital day 3, the patient developed sudden neurological deterioration with coma, hemodynamic collapse, and respiratory failure. Brain imaging revealed massive intracerebral hemorrhage with intraventricular extension and cerebral herniation. Despite intensive supportive management, the patient died. This case highlights the severe invasive potential of non-toxigenic C. diphtheriae in PVE and the risk of devastating thromboembolic and hemorrhagic neurological complications. It also emphasizes the necessity of rapid microbiological identification, integration of genomic data, and multidisciplinary management, particularly regarding anticoagulation strategies in patients with mechanical prosthetic valves.