We have used heterotrophic plate counts, together with live-dead direct staining and denaturing gradient gel electrophoresis (DGGE), to characterize the eubacterial communities that had formed as biofilms within domestic sink drain outlets. Laboratory microcosms of these environments were established using excised biofilms from two separate drain biofilm samples to inoculate constant-depth film fermentors (CDFFs). Drain biofilms harbored 9.8 to 11.3 log 10 cells of viable enteric species and pseudomonads/g, while CDFF-grown biofilms harbored 10.6 to 11.4 log 10 cells/g. Since live-dead direct staining revealed various efficiencies of recovery by culture, samples were analyzed by DGGE, utilizing primers specific for the V2-V3 region of eubacterial 16S rDNA. These analyses showed that the major PCR amplicons from in situ material were represented in the microcosms and maintained there over extended periods. Sequencing of amplicons resolved by DGGE revealed that the biofilms were dominated by a small number of genera, which were also isolated by culture. One drain sample harbored the protozoan Colpoda maupasi, together with rhabtidid nematodes and bdelloid rotifers. The microcosm enables the maintenance of stable drain-type bacterial communities and represents a useful tool for the modeling of this ecosystem.Clinical epidemiologists have long recognized the potential of sink drains in hospital wards to harbor pathogens. Several studies have identified sink drains within medical-surgical intensive-care wards (19,22,30,35) and cystic fibrosis units (24) as possible sources of infection. Despite the increased information relating to the occurrence of bacterial biofilms and their reported involvement in the biofouling of domestic drains (7), there are few reports in the literature concerning the ecology and microbiology of this environment. The persistence (1) and significance (10) of biofilms in virtually all environments is widely acknowledged. Studies in the home (14, 34) have identified the potential health risks of microbial contamination. Scott et al. (34) identified possible pathogens in the kitchen, toilet, and bathrooms in Ͼ200 homes in the United Kingdom. More recent studies (3,8,9) have demonstrated that homes represent an environment into which bacterial, viral, and fungal pathogens are continuously introduced in association with food, people, and pets. Studies by Cogan et al. (8,9) showed that detergent-based cleaning was relatively ineffective in controlling the spread of salmonella and campylobacter to kitchen surfaces during the preparation of contaminated poultry. Despite such concerns, there have been few investigations into the bacterial composition of biofilms within domestic sink drains. As with hospital drains, the pipe work presents a variety of solid surfaces that are suitable substrates for biofilm formation (7, 26). Biofilm has been implicated in a high proportion of slow-running drains in the United States (7). Domestic drains are subject to intermittent wetting, periodic feeding with a plet...