Long-term control of Legionella pneumophila and Pseudomonas aeruginosa in dental unit waterlines following optimization of a hypochlorous acid-based water safety program.

Dental unit waterlines (DUWLs) are highly susceptible to biofilm formation and serve as reservoirs for opportunistic waterborne pathogens, particularly Legionella pneumophila and Pseudomonas aeruginosa. Effective long-term control of microbial contamination remains challenging due to the complex architecture of DUWLs, intermittent water flow, and the limited applicability of thermal disinfection. Routine microbiological surveillance at the University Dental Clinic Vienna identified recurrent contamination of multiple dental units with L. pneumophila and P. aeruginosa. To establish an effective and sustainable water management strategy, three disinfection protocols were evaluated under routine clinical conditions: (A) continuous application of electrolytically generated hypochlorous acid (HOCl), maintaining a free chlorine concentration of 0.3-0.6 mg/L, (B) increased HOCl concentration, and (C) HOCl combined with hydrogen peroxide (H2O2). Water samples were analyzed according to standardized ISO methods for detection of L. pneumophila, P. aeruginosa, and heterotrophic plate counts (HPC). Protocol A demonstrated reliable balance between microbiological efficacy, technical compatibility, and operational feasibility. Increased HOCl concentrations provided no relevant additional microbiological benefit and were associated with technical impairments, whereas the combined HOCl/H2O2 protocol showed less microbiological performance. Following standardization to Protocol A, all evaluated units were incorporated into a comprehensive water safety program consisting of continuous disinfection, automated flushing procedures, daily free-chlorine monitoring, and routine microbiological surveillance. During the 3-year follow-up period (2023-2025), sustained suppression of L. pneumophila was achieved across the dental unit network, with only isolated residual findings in a single unit. Monitoring further demonstrated that low heterotrophic plate counts did not consistently exclude the presence of opportunistic pathogens, supporting the need for targeted pathogen surveillance in addition to routine HPC monitoring. These findings demonstrate that a standardized HOCl-based water safety program in combination with automated flushing protocols can provide sustainable long-term control of microbial contamination in DUWLs and underscore the importance of pathogen-specific monitoring for a comprehensive microbiological risk assessment.