Density and diversity of bacterial and fungal communities in dental unit waterlines (DUWL) subjected to disinfectants. Density and diversity of bacterial and fungal communities in dental unit waterlines (DUWL) subjected to disinfectants

Chemical disinfection processes are widely advocated to reduce the level of microbial contamination in dental unit waterlines (DUWL). However, their efficacy on the microbiological quality of DUWL output water has been poorly examined in real conditions after long-term treatments. In this study, the microbial community in three 8-12 year-old DUWL treated with Calbenium© or Oxygenal 6©, two disinfectants commonly used by French dentists, were investigated during routine general dental practice. DUWL were collected from the tap water supplying units to the output exposure point of the turbine handpiece following a stagnation period, and immediately after the last patient of the sampling day. Microbial abundance was estimated using quantitative PCR targeting the ribosomal genes, in complement to plate count. For the first time in this context, both bacterial and fungal diversities were investigated using 454 high-throughput pyrosequencing. Results showed that the unit itself is the principal source of microbial contamination and, that water stagnation, DU maintenance practices and quality of water supplying DU were critical to both density and composition of the microbial community in output water. Despite flushing process associated to disinfectant, an increase of total microbial density and shifts in both bacterial and fungal community composition were observed as water circulated in the DU. The microbial contamination remained relevant in the three studied DUWL output water. Moreover, the occurrence of potentially pathogenic bacteria and fungi such as Stenotrophomonas, Propionibacterium, Legionella, Pseudomonas, Halomonas, Shewanella and Candida suggest variable susceptibilities of microbial genera to treatment and a potential infectious risk due to exposure for both patients and dental staff. Interestingly our results suggest also that disinfectant shock before a prolonged stagnation period may be displayed as recommendation of reliably improving the quality of DUWL output water, provided that the microbial quality of water supplying DUWL is consistent with standard guidelines. Failure or reduction of disinfectant efficacy after DUWL long-term application may also be attributed partially to the selection of disinfectant-resistant microorganisms forming biofilm inside tubing. Through these three distinct case studies, our results demonstrated the necessity of considering density but also diversity of both bacterial and fungal communities for the evaluation of DUWL disinfectant efficacy. This study also highlighted the need for regular water quality control of DUWL from public and private offices and European standards for DUWL output water.