Background. Transmission of microbial pathogens to patients from biofilm wi
thin 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 t
he recurrence of microbial growth after treating DUWLs with sodium hypochlo
rite (bleach), or B; glutaraldehyde, or G; or isopropanol 15.3 percent, or
I.
Methods. 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 electro
n microscopy, or SEM, and the authors identified bacterial isolates using s
tandard techniques. The authors performed minimum inhibitory concentration
tests on identified isolates pre- and posttreatment and compared the result
s to determine possible differences in resistance.
Results. In baseline evaluations, the authors determined that the effluent
and biofilm matrix harbored an average of 1 x 10(5) colony-forming units, o
r CFU, per square centimeter and 1 x 10(4) CFU/cm(2) recoverable microorgan
isms, respectively. A single overnight treatment of DUWLs with B, G or I re
ndered 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-t
reated DUWLs returned to pretreatment levels by day six and day 15, respect
ively. DUWLs treated with G showed evidence of bacterial recurrence in the
effluent and the biofilm to pretreatment levels by day three. The authors c
ompared recurrence of biofilm and effluent posttreatment with untreated con
trol tubing. The lower recurrence of viable bacteria in both biofilm and ef
fluent samples for tubing treated with B and I was significant (P less than
or equal to .05). No evidence of resistance to the agents was noted during
the study. Multiple treatments held the bacterial population to below reco
verable levels but failed to remove the biofilm matrix, as evidenced by SEM
.
Conclusions. B, G and I eliminated recoverable bacteria after treatment and
inhibited their recurrence in DUWL. Recolonization rates varied by agent.
Clinical Implications. The residual effect of these agents raises concerns
about the slow release of potentially toxic substances from the residual bi
ofilm matrix. These agents reduce microorganisms in effluent water but do l
ittle to destroy the biofilm matrix in the DUWL, even with periodic treatme
nts. Bacterial populations in the dental unit water rapidly recolonize the
DUWL. Chemical agents or agents that potentially could be trapped in the ma
trix can represent an additional risk to the patient.