Culture-Negative Subacute Lactobacillus Endocarditis Diagnosed by Microbial Cell-Free DNA Sequencing

Lactobacillus endocarditis is a rare infection generally occurring in patients with heart disease and immunosuppression. Although Lactobacillus is typically a benign part of the gastrointestinal and genitourinary flora, it can cause invasive infections.

We present a case of a 74-year-old patient with low-grade fever and a nonproductive cough following recent cardiac surgery. Transesophageal echocardiography revealed vegetations, suggestive of endocarditis, despite negative blood cultures. Due to high suspicion for endocarditis, advanced genetic testing identified Lactobacillus fermentum as the causative pathogen. The patient disclosed daily probiotic use, likely the infection source. Treatment with intravenous ampicillin resulted in significant symptom improvement.

This case underscores the importance of considering probiotics as a potential source of bacteremia in patients with negative cultures after surgery and highlights how metagenomic sequencing can identify pathogens and guide effective therapy in challenging cases.