Studies to determine how a microbe can persist in a foreign environment are essential in understanding water contamination by infectious agents. Annular reactors were designed, constructed, and used as a laboratory-based model to study naturally-occurring biofilms. Untreated groundwater was used a s the bulk liquid, and a foreign microbe with a green fluorescent protein (GFP) marker was introduced. It was demonstrated that under oligotrophic conditions Escherichia coli O157:H7 86-24 did not grow planktonically. The microbe demonstrated an ability to integrate into the existing biofilm. Various environmentally-relevant concentrations of nitrogen and phosphate were also used to detect any effects on its persistence. The results suggest that the persistence of the E. coli O157:H7 86-24 was enhanced when the bulk fluid was amended to contain 100 ppm nitrate, and hindered when phosphate was added. To utilize available molecular tools, polymerase chain reaction (PCR) coupled with denaturing high performance liquid chromatography (DHPLC) were used. It was found that the E. coli O157:H7-specific primers were not as reliable in detecting E.coli O157 within the biofilm when compared to detection using the GFP marker. PCR using 16S rRNA primers were also used to gain insight into the microbial diversity of the biofilm.