High-Grade Proteus mirabilis Bacteremia of Unclear Source Complicated by Infective Endocarditis and Embolic Stroke.

Infective endocarditis (IE) due to Proteus mirabilis is rare, particularly in the absence of an identifiable primary source. We report a case of a woman in her 50s with a history of remote IE and polysubstance use disorder who presented with progressive gait slowing and weakness over one month. Neuroimaging revealed an acute-to-subacute infarct in the right anterior cerebral artery territory. Further evaluation demonstrated mitral valve abnormalities with suspected vegetation, and subsequent blood cultures grew Proteus mirabilis in all four blood culture bottles. Notably, the initial blood cultures obtained during admission were negative despite subsequent high-grade bacteremia. Extensive evaluation failed to identify a primary genitourinary, gastrointestinal, or respiratory source of bacteremia. The patient was treated with a six-week course of IV ceftriaxone, resulting in microbiologic clearance and clinical improvement, with a residual mild neurologic deficit. Follow-up echocardiography demonstrated persistent valvular abnormalities, likely representing fibrotic sequelae. This case highlights a rare etiology of native valve endocarditis, the diagnostic challenges of subacute presentations, and the importance of comprehensive investigation in patients with stroke of unclear etiology.