Emergence of Cupriavidus metallidurans in Silver Ion-Disinfected Dental Unit Water.

Objectives: This study determined the microbiological quality of dental unit water continuously treated with a silver ion-based disinfectant.
Methods: Aseptically-collected water samples from air-water syringes were obtained, and neutralized for residual chlorine with sodium thiosulfate, from 52 dental operatory chairs where attached bottles provided municipal water treated with a 0.78% silver ion-based disinfectant (Citrisil daily blue and twice-monthly shock tablets, Sterisil, Inc.). Similar samples were longitudinally obtained from seven other dental units after chair tubing was additionally disinfected with a 70% bleach solution for 24 hours. All samples were evaluated for aerobic mesophilic heterotrophic bacteria using nationally-accepted analytical procedures detailed in Standard Methods for the Examination of Water and Wastewater, 23rd Edition (2017), with sample aliquots spread-plated onto R₂A agar and aerobically incubated at 28°C for 7 days. Predominant cultivable isolates were identified using matrix-assisted laser desorption-ionization time-of-flight mass spectrometry and Bruker MALDI Biotyper analytic software.
Results: Aerobic mesophilic heterotrophic bacteria averaged 1,075 ± 387 (SD) CFU/ml in silver ion-disinfected dental unit water samples, with 50 of 52 (96.2%) dental units revealing elevated counts > 500 CFU/ml. All dental operatory chairs additionally subjected to bleach disinfection developed > 500 CFU/ml of aerobic mesophilic heterotrophic bacteria in output water within two months. Cupriavidus metallidurans was the most predominant species in water samples yielding > 500 CFU/ml, and exhibited in vitro resistance to the silver ion-based disinfectant in modified Kirby-Bauer inhibition testing. Other species isolated in low numbers were Acidovorax temperans, Sphingobium yanoikuyae, Methylobacterium rhodesianum, and Micrococcus luteus.
Conclusions: Dental unit water continuously subjected to a silver ion-based disinfectant yielded high numbers of Cupriavidus metallidurans, a water chlorination-tolerant organism resistant to the antimicrobial effects of silver and other heavy metals, and implicated as a potential pathogen in opportunistic human infections. These findings raised questions about the effectiveness of silver ion-based disinfectants in dental unit waterlines.