Dental unit waterlines: biofilms, disinfection and recurrence

Transmission of microbial pathogens to patients from biofilm within dental unit waterlines, or DUWLs, is a concern. To reduce the risk of toxicity to dental patients when water coolants are used, numerous chemical agents have been tested. In a series of trials, the authors investigated the recurrence of microbial growth after treating DUWLs with sodium hypochlorite (bleach), or B; glutaraldehyde, or G; or isopropanol 15.3 percent, or I.

The authors excised tubing sections from dental units in a general clinic. The tubing sections were evaluated at baseline and after overnight treatment. Effluent water samples and biofilm samples from tubing sections also were evaluated, by culture, at baseline and after treatment with the chemical agents. Biofilm within the tubing was examined by scanning electron microscopy, or SEM, and the authors identified bacterial isolates using standard techniques. The authors performed minimum inhibitory concentration tests on identified isolates pre- and posttreatment and compared the results to determine possible differences in resistance.

In baseline evaluations, the authors determined that the effluent and biofilm matrix harbored an average of 1 x 10(5) colony-forming units, or CFU, per square centimeter and 1 x 10(4) CFU/cm2 recoverable microorganisms, respectively. A single overnight treatment of the DUWLs with B, G or I rendered effluent and biofilm samples that were free of recoverable bacteria. The number of viable bacteria in the effluent and the biofilm of B- or I-treated DUWLs returned to pretreatment levels by day six and day 15, respectively. DUWLs treated with G showed evidence of bacterial recurrence in the effluent and the biofilm to pretreatment levels by day three. The authors compared recurrence of biofilm and effluent posttreatment with untreated control tubing. The lower recurrence of viable bacteria in both biofilm and effluent samples for tubing treated with B and I was significant (P < or = .05). No evidence of resistance to the agents was noted during the study. Multiple treatments held the bacterial population to below recoverable levels but failed to remove the biofilm matrix, as evidenced by SEM.

B, G and I eliminated recoverable bacteria after treatment and inhibited their recurrence in DUWL. Recolonization rates varied by agent.

The residual effect of these agents raises concerns about the slow release of potentially toxic substances from the residual biofilm matrix. These agents reduce microorganisms in effluent water but do little to destroy the biofilm matrix in the DUWL, even with periodic treatments. Bacterial populations in the dental unit water rapidly recolonize the DUWL. Chemical agents or agents that potentially could be trapped in the matrix can represent an additional risk to the patient.