Lacticaseibacillus paracasei bacteremia and mediastinitis secondary to esophageal invasion by left hilar pleomorphic carcinoma: case report

Lacticaseibacillus paracasei bacteremia associated with probiotic use is rare. We report a 71-year-old man with advanced lung cancer who developed L. paracasei bacteremia while ingesting a commercially available probiotic-fermented milk. Autopsy demonstrated direct tumor invasion and perforation of the mid-esophagus, providing a structural route for bacterial translocation from the gastrointestinal lumen into the bloodstream. Genetic analysis showed that the blood isolate was genetically identical to the probiotic strain contained in the ingested product. Bacteremia resolved with antibiotics, but the patient died of respiratory failure from progression of the malignancy. This case highlights that probiotic organisms, despite an established safety profile, can cause bacteremia when immunocompromise coexists with disruption of the gastrointestinal barrier.