Retention of a model pathogen in a porous media biofilm
Bauman, Wesley James
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The inadvertent or deliberate introduction of bacterial pathogens into drinking water systems can lead to serious public health consequences. As a result, rapid sampling opportunities within distribution systems are needed that can provide information on the source, species and fate of introduced pathogens. In this study, a porous media biofilm reactor was used to investigate the ability of an established mixed-species drinking water biofilm to immobilize cyan-labeled Escherichia coli 0157:H7 as a model pathogen. Test reactors were colonized with biofilm for two or three weeks at 0.5 mg/l C, resulting in the formation of thin and thick biofilms, respectively. Colonized reactors were then injected with slug doses of approximately 1 x 109 cfu E. coli O157:H7. Plate counts were able to successfully close a mass balance on E. coli O157:H7 around the reactor and were used measure the fractions of inocula immobilized within reactors. Compared with control reactors (0.22%), reactors colonized for two or three weeks immobilized significantly more cells (0.75% and 9.37% respectively).For E. coli O157:H7 passing through the reactor, retardation with respect to the bulk fluid was measured. Retardation factors (Rf) indicated that cells traveling through colonized reactors were significantly retarded compared to those traveling through clean control reactors, resulting in a prolonged washout of cells. Quantitative PCR (qPCR) and direct microscopic counts were also used to enumerate E. coli O157:H7 cells. Threshold cycle (CT) values from qPCR typically underestimated the plate counts for effluent samples and were highly inconsistent with respect to enumerating cells entrained in biofilm or attached to reactor surfaces. Possible inhibition by biofilm-associated substances was investigated. Direct microscopic counts were not possible when homogenized biofilm was present and otherwise consistently overestimated plate counts by an average of 0.6 orders of magnitude. This data shows that engineered porous media systems colonized with biofilm may be an effective tool for immobilizing pathogens from bulk flow in drinking water distribution systems.