Microbial Growth in Dental Unit Waterlines and Biofluorescence Monitoring Using QLF Technology After Regular Disinfection

Background: Dental unit waterlines (DUWLs) are critical components of dental care. However, their small diameter and extended water stagnation time make them prone to microbial contamination and biofilm formation. This study evaluated microbial contamination following routine DUWL disinfection and investigated the applicability of quantitative light-induced fluorescence (QLF) technology as a device for monitoring changes in biofluorescence within DUWLs. Methods: This study evaluated microbial growth in DUWLs after routine disinfection with hydrogen peroxide every two weeks in a dental hygiene training laboratory. Water samples were collected from three devices (3-way syringes, ultrasonic scalers [US], and cup fillers) immediately after disinfection (baseline) and two weeks later. Colony-forming units (CFUs) were cultured on R2A agar and quantified. QLF technology, specifically QLF-Digital (QLF-D), was employed to measure biofluorescence intensity and contamination levels. Statistical analyses included the Kruskal-Wallis and paired t-tests. Results: Microbial counts met the contamination standards (＜100 CFU/ml) immediately after disinfection but increased significantly after two weeks. Among these devices, the US exhibited the highest contamination and biofluorescence levels, highlighting their vulnerability to microbial growth. The red fluorescence intensity of the microbial pellets also increased significantly over two weeks, correlating with the microbial contamination levels. Conclusion: These findings suggest that although regular disinfection effectively reduces initial contamination, its efficacy diminishes over time, especially for high-risk devices. Furthermore, QLF-D is useful for monitoring microbial growth and biofilm formation, indicating its potential for routine DUWL contamination assessments. Enhanced protocols are recommended to improve the long-term waterline quality for high-risk devices.