A Sequential Water Vapor Treatment of Shower Systems to Control Legionella Emissions in the Bioaerosols of Healthcare Settings

Legionnaires' disease (LD) causes severe pneumonia with 5-15% case fatality rates in hospitalized patients, with rising incidence across Europe, North America and Japan. While established disinfection protocols exist for building pipes, no large-scale solution addresses shower systems themselves, where point-of-use filters remain impractical for widespread implementation. We assessed an effective protocol to contain Legionella proliferation from terminal water outlets in healthcare facilities, nursing homes, and residences of patients at high-risk of contracting LD.We evaluated an innovative water-vapor protocol consisting of sequential water flushing followed by high-pressure vapor at 120C. Biofilms were grown under controlled conditions across five settings--one healthcare facility, one nursing home, and three private residences--targeting biofilms aged 5-104 weeks during peak LD season. Efficacy was assessed through scanning electron microscopy, flow cytometry, 16S rRNA sequencing, and functional pathway analysis (PICRUSt2), comparing results with conventional thermal disinfection (65C, 10 minutes) and untreated controls.Water-vapor treatment achieved complete Legionella eradication (100% removal to below detection limits) in all mature biofilms, surpassing conventional thermal disinfection. Treatment was the primary determinant of community structure (R2=0.285, p=0.001). Beyond pathogen elimination, treatment disrupted 70 metabolic pathways (ANCOM-BC, FDR<0.05), targeting biofilm structural integrity through lipopolysaccharide precursor suppression and core metabolism shutdown. Upregulation of Legionella-specific CMP-legionaminate biosynthesis indicated desperate repair attempts under lethal conditions, confirming metabolically impossible recovery and durable ecosystem modification.Water-vapor treatment provides dual mechanisms: complete pathogen eradication and durable ecosystem reset preventing recolonization, representing a paradigm shift from transient suppression to sustained control. This chemical-free intervention offers healthcare facilities a practical alternative to point-of-use filters, potentially breaking recurring contamination cycles. Large-scale validation in real-world healthcare settings is essential before widespread adoption for transforming Legionella prevention strategies.