An evaluation of sampling and laboratory procedures for determination of heterotrophic plate counts in dental unit waterlines.

BACKGROUND The high numbers of heterotrophic microorganisms that have been cultured from dental unit waterlines (DUWs) have raised concern that this water may exceed suggested limits for heterotrophic plate counts (HPCs). The main purpose of this investigation was to examine HPC variability in DUWs and to examine in detail the effect of laboratory processing of water samples on HPC values. METHODS Water samples were collected from dental offices either at the beginning of or during the clinic day and were transported to the laboratory, where they were analyzed. RESULTS Measuring HPC levels within an office would involve testing all units, because significant differences were found between units connected to the same municipal water supply. Within a unit, the average microbial count from high-speed lines was approximately twice the average count from air/water lines. The laboratory processing of water samples significantly affected the numbers of heterotrophic microorganisms that were recovered. Incubation temperature, time and media, as well as neutralization of residual chlorine, all had significant effects on the HPC values. However, no significant differences in microbial counts were found between samples plated with the spread plate method on R2A agar and those plated with the pour plate method with Plate Count Agar. CONCLUSIONS Dental organizations have suggested target limits in terms of numbers of heterotrophic microorganisms recovered in water from dental units, but standards for laboratory handling must be established as well. A protocol for sample collection and laboratory handling is proposed.