Integrated Molecular Diagnostics Unmask Legionella Pneumophila and Fungal Coinfection in an Immunocompromised Host: A Case Study of Precision Antimicrobial Stewardship

Qianyun Deng,1,* Yujing Yang,1,* Siyi Gao,2 Mengdi Lu,1 Yunhu Zhao,1 Zixia Wang1 1Department of Laboratory Medicine, Guangdong Provincial People’s Hospital (Guangdong Academy of Medical Sciences), Southern Medical University, Guangzhou, Guangdong, 510000, People’s Republic of China; 2Inspection Section, Luonan County Chinese Medicine Hospital, Luonan, Shangluo, Shaanxi, 726100, People’s Republic of China*These authors contributed equally to this workCorrespondence: Yunhu Zhao, Email [email protected] Zixia Wang, Email [email protected]: Legionella pneumophila poses significant diagnostic challenges in immunocompromised hosts due to its fastidious growth requirements and nonspecific clinical presentation. Conventional culture methods have limited sensitivity (30– 80%), while molecular diagnostics require multi-platform validation to ensure reliability.Case Description: A 57-year-old woman with rheumatic heart disease, chronic renal failure, and immunosuppression presented with acute respiratory failure. Initial investigations revealed leukocytosis (19.03× 109/L), hyponatremia (127 mmol/L), elevated procalcitonin (42.55 ng/mL), and bilateral pulmonary infiltrates. Bronchoalveolar lavage fluid (BALF) analysis employed three molecular methods: isothermal amplification for screening (positive for L. pneumophila on ICU admission), digital PCR (dPCR, 4,455 copies/mL after 10-fold dilution) reconfirmed L. pneumophila infection, and metagenomic next-generation sequencing (mNGS; 384,661 Legionella reads alongside 3,474 Candida glabrata reads). Subsequent fungal β-D-glucan testing (674.8 pg/mL) and culture validated Candida glabrata coinfection. Antimicrobial therapy from targeted moxifloxacin/azithromycin to co-infection therapy with carbapenem escalated from Imipenem-cilastatin to sulbactam/cefoperazone for suspected gram-negative coinfection and fluconazole escalated to amphotericin B for resistant candidiasis guided by CRP/PCT trends.Conclusion: Integrated molecular diagnostics enable rapid pathogen identification in critically ill immunocompromised hosts. Multi-platform verification (isothermal amplification/dPCR/mNGS) overcomes technical limitations of single methods, while serial biomarker monitoring optimizes antimicrobial stewardship for mixed infections.Keywords: Legionella pneumophila, metagenomic next-generation sequencing, digital PCR, immunocompromised host, mixed infection