Assessment of Microbial Contamination in Stagnant Water from Dental Unit Reservoirs

Water stagnation in dental unit reservoirs may be associated with increased microbial contamination and potential exposure to opportunistic microorganisms, particularly among medically vulnerable patients and dental healthcare professionals. This study aimed to assess the microbiological quality of stagnant water collected from reservoirs of dental units at a tertiary academic school of dentistry by determining bacterial and coliform counts and detecting and morphologically identifying filamentous fungi. Although 152 dental units were available across the five clinics, only 106 reservoirs contained stagnant water at the time of sampling and could therefore be included in the microbiological analysis. Water samples were analysed using Petrifilm™ AC (total aerobic bacteria), EC (Escherichia coli and total coliforms), and YM (yeasts and moulds) plates, with results expressed as colony-forming units per millilitre (CFU/mL). Filamentous fungi were identified based on macroscopic and microscopic morphological characteristics following subculture onto Potato Dextrose Agar. Fifteen samples showed estimated aerobic bacterial counts exceeding 500 CFU/mL, three tested positive for total coliforms, and filamentous fungal growth was detected in nine samples. The fungal genera identified included Aspergillus, Absidia, Cunninghamella, and Trichophyton. Notably, Trichophyton rubrum was identified in one stagnant water sample from a dental unit reservoir. To the best of our knowledge, this finding has not previously been reported in dental unit water systems, although it should be interpreted with caution because the identification was based exclusively on morphological characteristics and was not confirmed by molecular methods. These findings demonstrate that microbial contamination may persist in stagnant water from dental unit reservoirs despite routine chemical disinfection protocol and reinforce the importance of continuous microbiological monitoring and effective management of dental unit water systems.