First Case of Acute Pleural Empyema Caused by Mediterraneibacter gnavus.

Mediterraneibacter gnavus (M. gnavus) is an obligately anaerobic, gram-positive coccus that is a major constituent of the human gut microbiota. Although it has been implicated in inflammatory bowel disease and sporadic invasive infections in immunocompromised hosts, respiratory infections caused by M. gnavus have not been reported. We herein describe the first case of acute pleural empyema caused by M. gnavus in a 78-year-old man with multiple comorbidities, including diabetes mellitus, severe malnutrition and pre-existing loculated pleural effusion. Aerobic cultures of blood and pleural fluid were negative, whereas an anaerobic culture of pleural fluid yielded bacterial growth. The organism was initially identified as M. gnavus by matrix-assisted laser desorption ionisation time-of-flight mass spectrometry and subsequently confirmed by 16S rRNA gene sequencing. The patient was successfully treated with beta-lactam antibiotics and pleural drainage. This case highlights M. gnavus as a potential causative pathogen of pleural infections and underscores the importance of comprehensive microbiological evaluation, including anaerobic culture and advanced microbial identification techniques.