Studies have demonstrated that dental unit waterlines (ie, narrow-bore plastic tubing that carries water to the high speed handpiece, air/water syringe and ultrasonic scaler) can become colonized with microorganisms, including bacteria, fungi and protozoa. Protected by a polysaccharide slime layer known as glycocalyx, these microorganisms colonize and replicate on the interior surfaces of the waterline tubing and form a biofilm, which serves as a reservoir that can amplify the number of free floating (ie, planktonic) microorganisms in water used for dental treatment. Although oral flora and human pathogens (eg, Pseudomonas aeruginosa, Lengionella species and nontuberculous Mycobacterium species) have been isolated from dental water systems, the majority of organisms recovered from dental waterlines are common heterotrophic water bacteria. These exhibit limited pathogenic potential for immunocompetent persons.