An in-vitro dynamic flow model for translational research into dental unit water system biofilms

Dental unit water systems (DUWS) provide an excellent environment for biofilm formation and can form a potential health risk for patients and staff. To control this biofilm formation, better understanding of the DUWS biofilm ecology is needed. Described is a newly developed in-vitro DUWS model which is easy to build, can be inoculated with different water sources and allows for sampling of both the effluent and biofilm. Unlike most models, a dynamic flow pattern, typical for a dental unit is used to provide water as a nutrient source. Microbial growth and composition were analyzed using heterotrophic plate counts (HPC) and 16S rDNA sequencing. Growth was reproducible in all models, reaching quasi-steady state at day 16 in the effluent (10⁵-10⁶ CFU∙mL^-1) and day 23 in the biofilm (10⁸ and 10⁷ CFU∙cm^-2) for non-potable and potable water, respectively. Principal component analysis of the microbial composition showed that biofilms originating from either non-potable or potable water were significantly different after 30 days of growth (n = 8, PERMANOVA, F = 35.6, p < .005). Treatment of the biofilms with 1000 ppm active chlorine showed a biological and statistical significant decrease in viable counts in the effluent phase to below the detection limit of 100 CFU∙mL^-1. The HPC returned to pre-treatment levels within 14 days. Using this model results in inoculum dependent biofilms with a higher bacterial density compared to previously described models. The relative ease in which samples can be taken allows for the monitoring of antimicrobial disinfection efficacy on the effluent, biofilm and matrix.