Water effluents from DUWL are highly populated in routine dental practice by heterotrophic bacteria principally originating from municipal water piped to the dental unit. Human pathogens, sucked back into the lines during dental procedures due to inadequate anti-retraction on dental units, have also been reported in some studies [3, 18, 19]. Both oral pathogens and heterotrophic bacteria can be responsible for severe diseases, and immune-compromised patients in particular may be at high risk .
As far as heterotrophic bacteria are concerned, the results of this study confirmed that DUWL are highly contaminated when dental units in use for several months receive no decontaminating treatment. According to our data, the great amount of DUWL contamination did not greatly differ from a dental unit to another and none of the water samples collected at the beginning of randomly selected working days reached CFU/ml values below the limit imposed by ADA for the year 2000 .
Interesting results have been obtained by testing the water samples against peracetic acid delivered by the chemical solution. After 5 minute of contact no growth of heterotrophic bacteria was found in any of 60 water samples tested. These results are in agreement with other reports showing that preformed peracetic acid possess a very rapid and broad spectrum microbicidal activity together with a very good activity against biofilm in waterlines used for haemodialysis [8–12].
However, despite these interesting properties, preformed peracetic acid has not been so far utilized in any study in dentistry with the aim to control DUWL contamination. Effectively, as delivered, preformed peracetic acid is unstable, potentially explosive, highly acidic and as a consequence highly corrosive. These properties make products containing preformed peracetic acid difficult to formulate for long term storage stability and difficult to handle and transport so limiting the use of this product in dentistry [9–11].
In the recent years, TAED with peroxygen source at near neutral pH has been clamed to provide a nonhazardous means of generating peracetic acid in situ, in the absence of the preformed peracetic acid side-effects .
Preliminary data from a previous study showed the relevant biocidal "in vitro" activity of the test formulation against human pathogens including spores, and data from the present study underline its great efficacy against heterotrophic bacteria both when tested "in vitro" and when flushed into DUWL .
In fact, DUWL flushing with the chemical solution left standing into DUWL for 5 minutes provided a good control of DUWL in all dental units studied, the mean CFU/ml values being well lower than the limit imposed from ADA, and values higher than 2.3 Log10 CFU/mL being only detected in 6 out of 60 samples.
These data are in agreement with other studies which have obtained similar results, but adopting different disinfecting procedure [21–23], consisting of introducing chemicals into water systems either continuously or intermittently during working pauses [24–28].
Although the mentioned treatments offer less potential for recolonization of waterlines since they keep into contact the chemicals with DUWL for longer periods of time, most of these agents can not probably ensure a rapid killing of viruses and bacteria eventually sucked back during dental procedures, since they are used at very low concentration in the continuous waterline supply or during working pauses .
Instead, we may speculate that intermittent between-patient treatment regimens using potentially biocidal concentrations of germicide, besides keeping low level of heterotrophic bacterial counts during dental procedures, could be also effective in eliminating oral pathogens eventually aspirated from patients under dental treatment and spread out during next procedures.
For this purpose the chemical formulation tested in this study might be very useful when used between patients in combination with dental units which incorporate the capacity to disinfect DUWL by the automatic flushing of lines.