Comparative Culture-Based Evaluation of Dental Unit Waterline Biofilm Contamination

Background: DUWLs easily become colonized by microorganisms due to narrow tubing, intermittent flow, and stagnation allowing adhesion and biofilm formation to occur. Patients and dental health-care workers are potentially at risk of exposure to microorganisms during procedures if contaminated outlet water is present. Objectives: The aim of this study was to assess the microbial contamination, biofilm formation and waterline hygiene-related parameters in governmental and private dental clinics. Methods: This study was comparative cross sectional laboratory study carried out at Al-Hilla Surgical Teaching Hospital and clinics from 2/1/2026 to 30/5/2026. 60 dental units were involved in the study (30 governmental and 30 private dental units). Two hundred and forty (240) water samples were taken from air-water syringes, high-speed handpiece lines, cup fillers and source water/reservoir bottles. The culture-based heterotrophic plate counts were done and reported as CFUs/mL. Conventional microbiological and biochemical tests were used for the identification of isolates and selected bacteria were evaluated for the ability to form biofilms using the microtiter plate method. Results: The median heterotrophic counts were higher in governmental clinic than in the private clinic (1200 vs. 430 CFU/mL, p<0.001). The overall prevalence rate was 54.2% with more in governmental than in private clinics (70.0% vs. 38.3%, p<0.001). Lines using high speed handpieces were the most contaminated. Flushing decreased the median counts from 920 to 380CFU/mL with 35.0% still being unacceptable (p<0.001). Of the isolates tested, 55.0% had moderate or strong biofilm production and there was a positive correlation with CFU/mL (rho=0.58, p<0.001). The independent risk factors were lack of disinfection, lack of filtration, lack of flushing and the age of the unit (> 5 years). Conclusion: DUWL contamination was common, biofilm related, and associated with inadequate maintenance practice, therefore regular disinfection, filtration, flushing and microbiological monitoring practices were recommended.