Development and evaluation of a dynamic in vitro biofilm model of dental unit waterlines incorporating simulated back-suction

This study developed a dynamic in vitro dental unit waterlines (DUWLs) biofilm model incorporating simulated back-suction. The model consisted of 165-cm polyurethane tubing connected to a dynamic flow system and operated intermittently to simulate routine clinical use. Simulated back-suction was produced by injecting 0.1 mL of clinical oral fluid into the tubing after each operating cycle. Over 6 weeks, three model waterlines with simulated back-suction and three without simulated back-suction were compared with three clinical DUWLs from dental chair units. Planktonic bacterial counts in outlet water were measured longitudinally, and biofilm formation on the inner tubing surface was evaluated by heterotrophic plate counts and scanning electron microscopy at predefined time points. Outlet bacterial counts in clinical and back-suction model groups both exceeded 100 CFU/mL from day 3, with parallel trajectories (p > 0.05). In contrast, the model without back-suction exceeded the threshold only after day 9, with significantly lower counts. Biofilms in the back-suction model developed into dense, multilayered structures, whereas only sparse microcolonies formed otherwise. These findings support the clinical relevance of incorporating simulated back-suction into an in vitro DUWL biofilm model and its potential use as a platform for future studies of DUWL biofilm formation and control.