치과 유니트 수관 소독 후 세균 오염 추적 분석 : 치과 유니트 수관 소독 후 세균 오염 추적 분석 = Longitudinal Analysis of Bacterial Recontamination after Disinfection of Dental Unit Waterlines

다국어 초록 (Multilingual Abstract)

Ph.D. Thesis

Longitudinal Analysis of Bacterial Recontamination after Disinfection of Dental Unit Waterlines

Eun-Kyoung Lee
Department of Public Health
Graduate School
Yeungnam University

（Supervised by Professor Joon Sakong）

Abstract

Dental unit waterlines are narrow-bore tubing systems that supply water to dental handpieces, ultrasonic scalers, and air-water syringes. Their narrow lumen and complex internal structure provide favorable conditions for biofilm formation, allowing microbial contamination to persist over time. The Centers for Disease Control and Prevention （CDC） recommends that bacterial contamination in dental unit water should not exceed 500 CFU/mL, whereas the Korean Dental Association （KDA） recommends a more stringent limit of 100 CFU/mL. Although these microbial standards are widely accepted, they provide little guidance on appropriate disinfection intervals. Consequently, many dental clinics continue to use the same disinfection schedule for all dental chairs despite differences in contamination patterns. The present study investigated longitudinal changes in bacterial contamination following dental unit waterline disinfection and evaluated whether repeated microbial monitoring could provide a practical basis for establishing chair-specific disinfection intervals.
A prospective longitudinal observational study was conducted through a pilot investigation in 2019 and a follow-up investigation in 2026. The pilot study monitored three dental chairs for 10 weeks after disinfection. Findings from the pilot investigation were used to refine the design of the 2026 study, which included baseline sampling before disinfection, immediate post-disinfection sampling, serial follow-up measurements, and an additional observation period after repeated disinfection. Water samples were collected from multiple outlets of each dental chair, cultured on R2A agar, incubated at 28°C for seven days, and quantified as colony-forming units per milliliter （CFU/mL）. Changes in bacterial counts were evaluated over time, and the time points at which the KDA and CDC microbial standards were exceeded were identified.
In the 2019 pilot investigation, bacterial counts decreased to 0 CFU/mL immediately after disinfection in all three dental chairs and then increased gradually throughout the follow-up period. Two chairs exceeded the KDA standard at week 5, whereas the third chair exceeded the same threshold at week 6. Depending on the chair, bacterial counts exceeded the CDC standard between weeks 6 and 8.
Despite identical models, installation years, and maintenance conditions, the three dental chairs showed substantial differences in baseline bacterial contamination during the 2026 investigation. Baseline bacterial counts ranged from 36 to 11,000 CFU/mL, representing a 305-fold difference among the chairs. Disinfection markedly reduced bacterial counts in every chair; however, subsequent bacterial regrowth followed different patterns. One chair exceeded the KDA standard within two weeks and reached 9,500 CFU/mL by week 4, whereas the remaining chairs exhibited distinct recontamination profiles. After an additional round of disinfection, bacterial counts declined again in all chairs, although differences in bacterial regrowth remained evident throughout the second follow-up period.
Overall, the findings suggest that bacterial recontamination progresses at different rates among dental chairs, even under comparable equipment and maintenance conditions. These results indicate that applying a uniform disinfection interval to every dental chair may not be sufficient to maintain microbial water quality within recommended standards. Instead, repeated microbial monitoring after disinfection offers a practical basis for determining an appropriate interval for each chair. A chair-specific management strategy based on longitudinal microbial monitoring may improve long-term dental unit waterline management and contribute to more effective infection control in routine dental practice.

목차 (Table of Contents)

• 서 론 1
• 대상 및 방법 5
• 성 적 13
• 고 찰 22
• 참 고 문 헌 29
• 영 문 초 록 37