Red Complex Bacteria as a Hidden Cause of Chronic Lung Abscess: A Case Report

BACKGROUND Common oral pathogens such as Treponema denticola, Porphyromonas gingivalis, and Tannerella forsythia, which form biofilms in the periodontal pockets, are classified together as Red Complex bacteria in Socransky's subgingival cluster model and are key pathogenic bacteria in periodontitis. Oral pathogens play a critical role in pulmonary infections, particularly in the pathogenesis of lung abscesses. Lung abscesses caused by Red Complex bacteria have rarely been described; to our knowledge, few cases have been reported to date. CASE REPORT We present the first documented case of chronic lung abscess caused by Red Complex bacteria, with recurrent hemoptysis as the main symptom. The patient had chronic periodontitis and uncontrolled diabetes, and exhibited an indolent clinical course with consistently negative bacterial cultures. Chest computed tomography (CT) demonstrated a thick-walled cavity with minimal liquefactive changes in the right upper lobe. Red Complex bacteria in the bronchoalveolar lavage fluid and the lung tissue were identified by metagenomics next-generation sequencing (mNGS). The patient underwent intravenous penicillin therapy. The hemoptysis resolved completely coupled with improvement in clinical status and inflammatory markers. The chest CT demonstrated near-complete resolution of the lung abscess during follow-up. CONCLUSIONS Red Complex bacteria are rare pathogens in patients with periodontitis who develop culture-negative chronic lung abscesses accompanied by recurrent hemoptysis. This case report highlights this rare etiology and reveals a critical diagnostic limitation, as routine cultures frequently fail to detect these fastidious organisms. The incorporation of mNGS into the diagnostic algorithm for such cases can facilitate a definitive diagnosis and guide precise antimicrobial therapy.