Analysis of the Microbial Community in Dental Clinical Wastewater

Objective: The microbial community in dental clinical wastewater is of concern to public health because potentially toxic organic mercury is released from disposed amalgam by microbial metabolic activity. In order to better understand the impact of dental wastewater on public health, we analyzed bacterial species and their relative abundance in several dental wastewater units. Methods: Wastewater samples were collected from seven different general dentistry clinics. Nutrient agar was used to isolate single bacterial colonies after aerobic or anaerobic incubation for 1 to 7 days. Single colony types were characterized by colonial morphology, Gram-strain status, catalase activity and their relative abundance in each dental wastewater sample. The bacterial isolates were analyzed for resistance against inorganic (mercury nitrate) and organic (phenylmercury acetate) mercuries. Chromosomal DNA was isolated from each isolate. The 16S rRNA gene was amplified with standard primers derived from Escherichia coli, and the nucleotide sequence was determined. The sequences were analyzed by BLAST and aligned by Clustal W. A phylogenetic tree was constructed with the TreeExplore program. Results: Dental wastewater samples obtained from different clinics varied in type and number of dominant bacterial species, although some species were found in multiple clinics. Microbial species and their relative abundance in various samples were Bacillus thuringiencsis (3-20%), Bacillus cereus (10-40%), Aeromonas hydrophila (34%), Citrobacter freundii (30-63%), Citrobacter amalonaticus (34%), Serratia marcescens (29-82%), and Pseudomonas sp. (41%). Most isolates were sensitive to the two mercury salts tested. Conclusions: Dental wasterwater microbial communities varied significantly from clinic to clinic. Further investigation of these communities is needed to better understand the impact of microbial ecology in dental wastewater on the environment and public health. Supported by NIH grant DE014777.